Environments: Add support for include URLs (#29026)

* Preliminary support for include URLs in spack.yaml (environment) files

This commit adds support in environments for external configuration files obtained from a URL with a preference for grabbing raw text from GitHub and gitlab for efficient downloads of the relevant files. The URL can also be a link to a directory that contains multiple configuration files.

Remote configuration files are retrieved and cached for the environment. Configuration files with the same name will not be overwritten once cached.

lib/spack/docs/conf.py

@@ -201,6 +201,7 @@ def setup(sphinx):
     ("py:class", "unittest.case.TestCase"),
     ("py:class", "_frozen_importlib_external.SourceFileLoader"),
     ("py:class", "clingo.Control"),
+    ("py:class", "six.moves.urllib.parse.ParseResult"),
     # Spack classes that are private and we don't want to expose
     ("py:class", "spack.provider_index._IndexBase"),
     ("py:class", "spack.repo._PrependFileLoader"),

lib/spack/docs/environments.rst

@@ -478,14 +478,21 @@ them to the Environment.
    spack:
      include:
      - relative/path/to/config.yaml
+     - https://github.com/path/to/raw/config/compilers.yaml
      - /absolute/path/to/packages.yaml
 
-Environments can include files with either relative or absolute
+Environments can include files or URLs. File paths can be relative or
-paths. Inline configurations take precedence over included
+absolute. URLs include the path to the text for individual files or
-configurations, so you don't have to change shared configuration files
+can be the path to a directory containing configuration files.
-to make small changes to an individual Environment. Included configs
+
-listed earlier will have higher precedence, as the included configs are
+^^^^^^^^^^^^^^^^^^^^^^^^
-applied in reverse order.
+Configuration precedence
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Inline configurations take precedence over included configurations, so
+you don't have to change shared configuration files to make small changes
+to an individual environment. Included configurations listed earlier will
+have higher precedence, as the included configs are applied in reverse order.
 
 -------------------------------
 Manually Editing the Specs List

lib/spack/spack/binary_distribution.py

@@ -26,7 +26,6 @@
 import spack.cmd
 import spack.config as config
 import spack.database as spack_db
-import spack.fetch_strategy as fs
 import spack.hooks
 import spack.hooks.sbang
 import spack.mirror
@@ -1231,7 +1230,7 @@ def try_fetch(url_to_fetch):
 
     try:
         stage.fetch()
-    except fs.FetchError:
+    except web_util.FetchError:
         stage.destroy()
         return None
 
@@ -1954,7 +1953,7 @@ def get_keys(install=False, trust=False, force=False, mirrors=None):
                 if not os.path.exists(stage.save_filename):
                     try:
                         stage.fetch()
-                    except fs.FetchError:
+                    except web_util.FetchError:
                         continue
 
             tty.debug("Found key {0}".format(fingerprint))
@@ -2106,7 +2105,7 @@ def _download_buildcache_entry(mirror_root, descriptions):
             try:
                 stage.fetch()
                 break
-            except fs.FetchError as e:
+            except web_util.FetchError as e:
                 tty.debug(e)
         else:
             if fail_if_missing:

lib/spack/spack/cmd/buildcache.py

@@ -15,7 +15,6 @@
 import spack.cmd.common.arguments as arguments
 import spack.config
 import spack.environment as ev
-import spack.fetch_strategy as fs
 import spack.hash_types as ht
 import spack.mirror
 import spack.relocate
@@ -711,7 +710,7 @@ def sync_fn(args):
                 stage.create()
                 stage.fetch()
                 web_util.push_to_url(local_path, dest_url, keep_original=True)
-            except fs.FetchError as e:
+            except web_util.FetchError as e:
                 tty.debug("spack buildcache unable to sync {0}".format(rel_path))
                 tty.debug(e)
             finally:

lib/spack/spack/config.py

@@ -64,6 +64,7 @@
 
 # Hacked yaml for configuration files preserves line numbers.
 import spack.util.spack_yaml as syaml
+import spack.util.web as web_util
 from spack.error import SpackError
 from spack.util.cpus import cpus_available
 
@@ -994,18 +995,19 @@ def read_config_file(filename, schema=None):
     # schema when it's not necessary) while allowing us to validate against a
     # known schema when the top-level key could be incorrect.
 
-    # Ignore nonexisting files.
     if not os.path.exists(filename):
+        # Ignore nonexistent files.
+        tty.debug("Skipping nonexistent config path {0}".format(filename))
         return None
 
     elif not os.path.isfile(filename):
         raise ConfigFileError("Invalid configuration. %s exists but is not a file." % filename)
 
     elif not os.access(filename, os.R_OK):
-        raise ConfigFileError("Config file is not readable: %s" % filename)
+        raise ConfigFileError("Config file is not readable: {0}".format(filename))
 
     try:
-        tty.debug("Reading config file %s" % filename)
+        tty.debug("Reading config from file {0}".format(filename))
         with open(filename) as f:
             data = syaml.load_config(f)
 
@@ -1304,6 +1306,95 @@ def _config_from(scopes_or_paths):
     return configuration
 
 
+def raw_github_gitlab_url(url):
+    """Transform a github URL to the raw form to avoid undesirable html.
+
+    Args:
+        url: url to be converted to raw form
+
+    Returns: (str) raw github/gitlab url or the original url
+    """
+    # Note we rely on GitHub to redirect the 'raw' URL returned here to the
+    # actual URL under https://raw.githubusercontent.com/ with '/blob'
+    # removed and or, '/blame' if needed.
+    if "github" in url or "gitlab" in url:
+        return url.replace("/blob/", "/raw/")
+
+    return url
+
+
+def collect_urls(base_url):
+    """Return a list of configuration URLs.
+
+    Arguments:
+        base_url (str): URL for a configuration (yaml) file or a directory
+            containing yaml file(s)
+
+    Returns: (list) list of configuration file(s) or empty list if none
+    """
+    if not base_url:
+        return []
+
+    extension = ".yaml"
+
+    if base_url.endswith(extension):
+        return [base_url]
+
+    # Collect configuration URLs if the base_url is a "directory".
+    _, links = web_util.spider(base_url, 0)
+    return [link for link in links if link.endswith(extension)]
+
+
+def fetch_remote_configs(url, dest_dir, skip_existing=True):
+    """Retrieve configuration file(s) at the specified URL.
+
+    Arguments:
+        url (str): URL for a configuration (yaml) file or a directory containing
+            yaml file(s)
+        dest_dir (str): destination directory
+        skip_existing (bool): Skip files that already exist in dest_dir if
+            ``True``; otherwise, replace those files
+
+    Returns: (str) path to the corresponding file if URL is or contains a
+       single file and it is the only file in the destination directory or
+       the root (dest_dir) directory if multiple configuration files exist
+       or are retrieved.
+    """
+
+    def _fetch_file(url):
+        raw = raw_github_gitlab_url(url)
+        tty.debug("Reading config from url {0}".format(raw))
+        return web_util.fetch_url_text(raw, dest_dir=dest_dir)
+
+    if not url:
+        raise ConfigFileError("Cannot retrieve configuration without a URL")
+
+    # Return the local path to the cached configuration file OR to the
+    # directory containing the cached configuration files.
+    config_links = collect_urls(url)
+    existing_files = os.listdir(dest_dir) if os.path.isdir(dest_dir) else []
+
+    paths = []
+    for config_url in config_links:
+        basename = os.path.basename(config_url)
+        if skip_existing and basename in existing_files:
+            tty.warn(
+                "Will not fetch configuration from {0} since a version already"
+                "exists in {1}".format(config_url, dest_dir)
+            )
+            path = os.path.join(dest_dir, basename)
+        else:
+            path = _fetch_file(config_url)
+
+        if path:
+            paths.append(path)
+
+    if paths:
+        return dest_dir if len(paths) > 1 else paths[0]
+
+    raise ConfigFileError("Cannot retrieve configuration (yaml) from {0}".format(url))
+
+
 class ConfigError(SpackError):
     """Superclass for all Spack config related errors."""
 

lib/spack/spack/environment/environment.py

@@ -17,7 +17,6 @@
 
 import llnl.util.filesystem as fs
 import llnl.util.tty as tty
-from llnl.util.filesystem import rename
 from llnl.util.lang import dedupe
 from llnl.util.symlink import symlink
 
@@ -593,7 +592,7 @@ def regenerate(self, concretized_root_specs):
             symlink(new_root, tmp_symlink_name)
 
             # mv symlink atomically over root symlink to old_root
-            rename(tmp_symlink_name, self.root)
+            fs.rename(tmp_symlink_name, self.root)
         except Exception as e:
             # Clean up new view and temporary symlink on any failure.
             try:
@@ -897,6 +896,11 @@ def repos_path(self):
     def log_path(self):
         return os.path.join(self.path, env_subdir_name, "logs")
 
+    @property
+    def config_stage_dir(self):
+        """Directory for any staged configuration file(s)."""
+        return os.path.join(self.env_subdir_path, "config")
+
     @property
     def view_path_default(self):
         # default path for environment views
@@ -928,6 +932,47 @@ def included_config_scopes(self):
             # allow paths to contain spack config/environment variables, etc.
             config_path = substitute_path_variables(config_path)
 
+            # strip file URL prefix, if needed, to avoid unnecessary remote
+            # config processing for local files
+            config_path = config_path.replace("file://", "")
+
+            if not os.path.exists(config_path):
+                # Stage any remote configuration file(s)
+                if spack.util.url.is_url_format(config_path):
+                    staged_configs = (
+                        os.listdir(self.config_stage_dir)
+                        if os.path.exists(self.config_stage_dir)
+                        else []
+                    )
+                    basename = os.path.basename(config_path)
+                    if basename in staged_configs:
+                        # Do NOT re-stage configuration files over existing
+                        # ones with the same name since there is a risk of
+                        # losing changes (e.g., from 'spack config update').
+                        tty.warn(
+                            "Will not re-stage configuration from {0} to avoid "
+                            "losing changes to the already staged file of the "
+                            "same name.".format(config_path)
+                        )
+
+                        # Recognize the configuration stage directory
+                        # is flattened to ensure a single copy of each
+                        # configuration file.
+                        config_path = self.config_stage_dir
+                        if basename.endswith(".yaml"):
+                            config_path = os.path.join(config_path, basename)
+                    else:
+                        staged_path = spack.config.fetch_remote_configs(
+                            config_path,
+                            self.config_stage_dir,
+                            skip_existing=True,
+                        )
+                        if not staged_path:
+                            raise SpackEnvironmentError(
+                                "Unable to fetch remote configuration {0}".format(config_path)
+                            )
+                        config_path = staged_path
+
             # treat relative paths as relative to the environment
             if not os.path.isabs(config_path):
                 config_path = os.path.join(self.path, config_path)
@@ -936,10 +981,14 @@ def included_config_scopes(self):
             if os.path.isdir(config_path):
                 # directories are treated as regular ConfigScopes
                 config_name = "env:%s:%s" % (self.name, os.path.basename(config_path))
+                tty.debug("Creating ConfigScope {0} for '{1}'".format(config_name, config_path))
                 scope = spack.config.ConfigScope(config_name, config_path)
             elif os.path.exists(config_path):
                 # files are assumed to be SingleFileScopes
                 config_name = "env:%s:%s" % (self.name, config_path)
+                tty.debug(
+                    "Creating SingleFileScope {0} for '{1}'".format(config_name, config_path)
+                )
                 scope = spack.config.SingleFileScope(
                     config_name, config_path, spack.schema.merged.schema
                 )

lib/spack/spack/fetch_strategy.py

@@ -52,7 +52,7 @@
 import spack.util.crypto as crypto
 import spack.util.pattern as pattern
 import spack.util.url as url_util
-import spack.util.web
+import spack.util.web as web_util
 import spack.version
 from spack.util.compression import decompressor_for, extension
 from spack.util.executable import CommandNotFoundError, which
@@ -337,7 +337,7 @@ def fetch(self):
         url = None
         errors = []
         for url in self.candidate_urls:
-            if not self._existing_url(url):
+            if not web_util.url_exists(url, self.curl):
                 continue
 
             try:
@@ -352,30 +352,6 @@ def fetch(self):
         if not self.archive_file:
             raise FailedDownloadError(url)
 
-    def _existing_url(self, url):
-        tty.debug("Checking existence of {0}".format(url))
-
-        if spack.config.get("config:url_fetch_method") == "curl":
-            curl = self.curl
-            # Telling curl to fetch the first byte (-r 0-0) is supposed to be
-            # portable.
-            curl_args = ["--stderr", "-", "-s", "-f", "-r", "0-0", url]
-            if not spack.config.get("config:verify_ssl"):
-                curl_args.append("-k")
-            _ = curl(*curl_args, fail_on_error=False, output=os.devnull)
-            return curl.returncode == 0
-        else:
-            # Telling urllib to check if url is accessible
-            try:
-                url, headers, response = spack.util.web.read_from_url(url)
-            except spack.util.web.SpackWebError as werr:
-                msg = "Urllib fetch failed to verify url\
-                      {0}\n with error {1}".format(
-                    url, werr
-                )
-                raise FailedDownloadError(url, msg)
-            return response.getcode() is None or response.getcode() == 200
-
     def _fetch_from_url(self, url):
         if spack.config.get("config:url_fetch_method") == "curl":
             return self._fetch_curl(url)
@@ -397,8 +373,8 @@ def _fetch_urllib(self, url):
 
         # Run urllib but grab the mime type from the http headers
         try:
-            url, headers, response = spack.util.web.read_from_url(url)
+            url, headers, response = web_util.read_from_url(url)
-        except spack.util.web.SpackWebError as e:
+        except web_util.SpackWebError as e:
             # clean up archive on failure.
             if self.archive_file:
                 os.remove(self.archive_file)
@@ -433,38 +409,19 @@ def _fetch_curl(self, url):
         else:
             save_args = ["-O"]
 
-        curl_args = save_args + [
+        timeout = 0
-            "-f",  # fail on >400 errors
+        cookie_args = []
-            "-D",
-            "-",  # print out HTML headers
-            "-L",  # resolve 3xx redirects
-            url,
-        ]
-
-        if not spack.config.get("config:verify_ssl"):
-            curl_args.append("-k")
-
-        if sys.stdout.isatty() and tty.msg_enabled():
-            curl_args.append("-#")  # status bar when using a tty
-        else:
-            curl_args.append("-sS")  # show errors if fail
-
-        connect_timeout = spack.config.get("config:connect_timeout", 10)
-
         if self.extra_options:
             cookie = self.extra_options.get("cookie")
             if cookie:
-                curl_args.append("-j")  # junk cookies
+                cookie_args.append("-j")  # junk cookies
-                curl_args.append("-b")  # specify cookie
+                cookie_args.append("-b")  # specify cookie
-                curl_args.append(cookie)
+                cookie_args.append(cookie)
 
             timeout = self.extra_options.get("timeout")
-            if timeout:
-                connect_timeout = max(connect_timeout, int(timeout))
 
-        if connect_timeout > 0:
+        base_args = web_util.base_curl_fetch_args(url, timeout)
-            # Timeout if can't establish a connection after n sec.
+        curl_args = save_args + base_args + cookie_args
-            curl_args.extend(["--connect-timeout", str(connect_timeout)])
 
         # Run curl but grab the mime type from the http headers
         curl = self.curl
@@ -479,26 +436,10 @@ def _fetch_curl(self, url):
             if partial_file and os.path.lexists(partial_file):
                 os.remove(partial_file)
 
-            if curl.returncode == 22:
+            try:
-                # This is a 404.  Curl will print the error.
+                web_util.check_curl_code(curl.returncode)
-                raise FailedDownloadError(url, "URL %s was not found!" % url)
+            except web_util.FetchError as err:
-
+                raise spack.fetch_strategy.FailedDownloadError(url, str(err))
-            elif curl.returncode == 60:
-                # This is a certificate error.  Suggest spack -k
-                raise FailedDownloadError(
-                    url,
-                    "Curl was unable to fetch due to invalid certificate. "
-                    "This is either an attack, or your cluster's SSL "
-                    "configuration is bad.  If you believe your SSL "
-                    "configuration is bad, you can try running spack -k, "
-                    "which will not check SSL certificates."
-                    "Use this at your own risk.",
-                )
-
-            else:
-                # This is some other curl error.  Curl will print the
-                # error, but print a spack message too
-                raise FailedDownloadError(url, "Curl failed with error %d" % curl.returncode)
 
         self._check_headers(headers)
 
@@ -556,7 +497,7 @@ def archive(self, destination):
         if not self.archive_file:
             raise NoArchiveFileError("Cannot call archive() before fetching.")
 
-        spack.util.web.push_to_url(self.archive_file, destination, keep_original=True)
+        web_util.push_to_url(self.archive_file, destination, keep_original=True)
 
     @_needs_stage
     def check(self):
@@ -1385,19 +1326,19 @@ def fetch(self):
 
         parsed_url = url_util.parse(self.url)
         if parsed_url.scheme != "s3":
-            raise FetchError("S3FetchStrategy can only fetch from s3:// urls.")
+            raise web_util.FetchError("S3FetchStrategy can only fetch from s3:// urls.")
 
         tty.debug("Fetching {0}".format(self.url))
 
         basename = os.path.basename(parsed_url.path)
 
         with working_dir(self.stage.path):
-            _, headers, stream = spack.util.web.read_from_url(self.url)
+            _, headers, stream = web_util.read_from_url(self.url)
 
             with open(basename, "wb") as f:
                 shutil.copyfileobj(stream, f)
 
-            content_type = spack.util.web.get_header(headers, "Content-type")
+            content_type = web_util.get_header(headers, "Content-type")
 
         if content_type == "text/html":
             warn_content_type_mismatch(self.archive_file or "the archive")
@@ -1426,15 +1367,13 @@ def __init__(self, *args, **kwargs):
 
     @_needs_stage
     def fetch(self):
-        import spack.util.web as web_util
-
         if self.archive_file:
             tty.debug("Already downloaded {0}".format(self.archive_file))
             return
 
         parsed_url = url_util.parse(self.url)
         if parsed_url.scheme != "gs":
-            raise FetchError("GCSFetchStrategy can only fetch from gs:// urls.")
+            raise web_util.FetchError("GCSFetchStrategy can only fetch from gs:// urls.")
 
         tty.debug("Fetching {0}".format(self.url))
 
@@ -1489,7 +1428,7 @@ def from_kwargs(**kwargs):
             on attribute names (e.g., ``git``, ``hg``, etc.)
 
     Raises:
-        FetchError: If no ``fetch_strategy`` matches the args.
+        spack.util.web.FetchError: If no ``fetch_strategy`` matches the args.
     """
     for fetcher in all_strategies:
         if fetcher.matches(kwargs):
@@ -1586,7 +1525,7 @@ def for_package_version(pkg, version):
     # if it's a commit, we must use a GitFetchStrategy
     if isinstance(version, spack.version.GitVersion):
         if not hasattr(pkg, "git"):
-            raise FetchError(
+            raise web_util.FetchError(
                 "Cannot fetch git version for %s. Package has no 'git' attribute" % pkg.name
             )
         # Populate the version with comparisons to other commits
@@ -1731,15 +1670,11 @@ def destroy(self):
         shutil.rmtree(self.root, ignore_errors=True)
 
 
-class FetchError(spack.error.SpackError):
+class NoCacheError(web_util.FetchError):
-    """Superclass for fetcher errors."""
-
-
-class NoCacheError(FetchError):
     """Raised when there is no cached archive for a package."""
 
 
-class FailedDownloadError(FetchError):
+class FailedDownloadError(web_util.FetchError):
     """Raised when a download fails."""
 
     def __init__(self, url, msg=""):
@@ -1747,23 +1682,23 @@ def __init__(self, url, msg=""):
         self.url = url
 
 
-class NoArchiveFileError(FetchError):
+class NoArchiveFileError(web_util.FetchError):
-    """ "Raised when an archive file is expected but none exists."""
+    """Raised when an archive file is expected but none exists."""
 
 
-class NoDigestError(FetchError):
+class NoDigestError(web_util.FetchError):
     """Raised after attempt to checksum when URL has no digest."""
 
 
-class ExtrapolationError(FetchError):
+class ExtrapolationError(web_util.FetchError):
     """Raised when we can't extrapolate a version for a package."""
 
 
-class FetcherConflict(FetchError):
+class FetcherConflict(web_util.FetchError):
     """Raised for packages with invalid fetch attributes."""
 
 
-class InvalidArgsError(FetchError):
+class InvalidArgsError(web_util.FetchError):
     """Raised when a version can't be deduced from a set of arguments."""
 
     def __init__(self, pkg=None, version=None, **args):
@@ -1776,11 +1711,11 @@ def __init__(self, pkg=None, version=None, **args):
         super(InvalidArgsError, self).__init__(msg, long_msg)
 
 
-class ChecksumError(FetchError):
+class ChecksumError(web_util.FetchError):
     """Raised when archive fails to checksum."""
 
 
-class NoStageError(FetchError):
+class NoStageError(web_util.FetchError):
     """Raised when fetch operations are called before set_stage()."""
 
     def __init__(self, method):

lib/spack/spack/package_base.py

@@ -63,6 +63,7 @@
 from spack.util.executable import ProcessError, which
 from spack.util.package_hash import package_hash
 from spack.util.prefix import Prefix
+from spack.util.web import FetchError
 from spack.version import GitVersion, Version, VersionBase
 
 if sys.version_info[0] >= 3:
@@ -3022,13 +3023,6 @@ def possible_dependencies(*pkg_or_spec, **kwargs):
     return visited
 
 
-class FetchError(spack.error.SpackError):
-    """Raised when something goes wrong during fetch."""
-
-    def __init__(self, message, long_msg=None):
-        super(FetchError, self).__init__(message, long_msg)
-
-
 class PackageStillNeededError(InstallError):
     """Raised when package is still needed by another on uninstall."""
 

lib/spack/spack/stage.py

@@ -42,6 +42,7 @@
 import spack.util.pattern as pattern
 import spack.util.url as url_util
 from spack.util.crypto import bit_length, prefix_bits
+from spack.util.web import FetchError
 
 # The well-known stage source subdirectory name.
 _source_path_subdir = "spack-src"
@@ -529,7 +530,7 @@ def print_errors(errors):
 
             self.fetcher = self.default_fetcher
             default_msg = "All fetchers failed for {0}".format(self.name)
-            raise fs.FetchError(err_msg or default_msg, None)
+            raise FetchError(err_msg or default_msg, None)
 
         print_errors(errors)
 

lib/spack/spack/test/bootstrap.py

@@ -134,7 +134,7 @@ def test_config_yaml_is_preserved_during_bootstrap(mutable_config):
 
 
 @pytest.mark.regression("26548")
-def test_custom_store_in_environment(mutable_config, tmpdir):
+def test_bootstrap_custom_store_in_environment(mutable_config, tmpdir):
     # Test that the custom store in an environment is taken into account
     # during bootstrapping
     spack_yaml = tmpdir.join("spack.yaml")

lib/spack/spack/test/cmd/env.py

@@ -29,6 +29,7 @@
 from spack.util.executable import Executable
 from spack.util.mock_package import MockPackageMultiRepo
 from spack.util.path import substitute_path_variables
+from spack.util.web import FetchError
 from spack.version import Version
 
 # TODO-27021
@@ -656,7 +657,7 @@ def test_env_with_config():
     assert any(x.satisfies("mpileaks@2.2") for x in e._get_environment_specs())
 
 
-def test_with_config_bad_include(capfd):
+def test_with_config_bad_include():
     env_name = "test_bad_include"
     test_config = """\
 spack:
@@ -667,15 +668,14 @@ def test_with_config_bad_include(capfd):
     _env_create(env_name, StringIO(test_config))
 
     e = ev.read(env_name)
-    with pytest.raises(SystemExit):
+    with pytest.raises(spack.config.ConfigFileError) as exc:
         with e:
             e.concretize()
 
-    out, err = capfd.readouterr()
+    err = str(exc)
+    assert "not retrieve configuration" in err
+    assert os.path.join("no", "such", "directory") in err
 
-    assert "missing include" in err
-    assert "/no/such/directory" in err
-    assert os.path.join("no", "such", "file.yaml") in err
     assert ev.active_environment() is None
 
 
@@ -721,91 +721,126 @@ def test_env_with_include_config_files_same_basename():
     assert environment_specs[1].satisfies("mpileaks@2.2")
 
 
-def test_env_with_included_config_file():
+@pytest.fixture(scope="function")
-    test_config = """\
+def packages_file(tmpdir):
-env:
+    """Return the path to the packages configuration file."""
-  include:
+    raw_yaml = """
-  - ./included-config.yaml
-  specs:
-  - mpileaks
-"""
-    _env_create("test", StringIO(test_config))
-    e = ev.read("test")
-
-    with open(os.path.join(e.path, "included-config.yaml"), "w") as f:
-        f.write(
-            """\
 packages:
   mpileaks:
     version: [2.2]
 """
+    filename = tmpdir.ensure("testconfig", "packages.yaml")
+    filename.write(raw_yaml)
+    yield filename
+
+
+def mpileaks_env_config(include_path):
+    """Return the contents of an environment that includes the provided
+    path and lists mpileaks as the sole spec."""
+    return """\
+env:
+  include:
+  - {0}
+  specs:
+  - mpileaks
+""".format(
+        include_path
     )
 
+
+def test_env_with_included_config_file(packages_file):
+    """Test inclusion of a relative packages configuration file added to an
+    existing environment."""
+    include_filename = "included-config.yaml"
+    test_config = mpileaks_env_config(os.path.join(".", include_filename))
+
+    _env_create("test", StringIO(test_config))
+    e = ev.read("test")
+
+    included_path = os.path.join(e.path, include_filename)
+    fs.rename(packages_file.strpath, included_path)
+
     with e:
         e.concretize()
 
     assert any(x.satisfies("mpileaks@2.2") for x in e._get_environment_specs())
 
 
-def test_env_with_included_config_scope():
+def test_env_with_included_config_file_url(tmpdir, mutable_empty_config, packages_file):
+    """Test configuration inclusion of a file whose path is a URL before
+    the environment is concretized."""
+
+    spack_yaml = tmpdir.join("spack.yaml")
+    with spack_yaml.open("w") as f:
+        f.write("spack:\n  include:\n    - file://{0}\n".format(packages_file))
+
+    env = ev.Environment(tmpdir.strpath)
+    ev.activate(env)
+    scopes = env.included_config_scopes()
+    assert len(scopes) == 1
+
+    cfg = spack.config.get("packages")
+    assert cfg["mpileaks"]["version"] == [2.2]
+
+
+def test_env_with_included_config_missing_file(tmpdir, mutable_empty_config):
+    """Test inclusion of a missing configuration file raises FetchError
+    noting missing file."""
+
+    spack_yaml = tmpdir.join("spack.yaml")
+    missing_file = tmpdir.join("packages.yaml")
+    with spack_yaml.open("w") as f:
+        f.write("spack:\n  include:\n    - {0}\n".format(missing_file.strpath))
+
+    env = ev.Environment(tmpdir.strpath)
+    with pytest.raises(FetchError, match="No such file or directory"):
+        ev.activate(env)
+
+
+def test_env_with_included_config_scope(tmpdir, packages_file):
+    """Test inclusion of a package file from the environment's configuration
+    stage directory. This test is intended to represent a case where a remote
+    file has already been staged."""
     config_scope_path = os.path.join(ev.root("test"), "config")
-    test_config = (
-        """\
-env:
-  include:
-  - %s
-  specs:
-  - mpileaks
-"""
-        % config_scope_path
-    )
 
+    # Configure the environment to include file(s) from the environment's
+    # remote configuration stage directory.
+    test_config = mpileaks_env_config(config_scope_path)
+
+    # Create the environment
     _env_create("test", StringIO(test_config))
 
     e = ev.read("test")
 
+    # Copy the packages.yaml file to the environment configuration
+    # directory so it is picked up during concretization. (Using
+    # copy instead of rename in case the fixture scope changes.)
     fs.mkdirp(config_scope_path)
-    with open(os.path.join(config_scope_path, "packages.yaml"), "w") as f:
+    include_filename = os.path.basename(packages_file.strpath)
-        f.write(
+    included_path = os.path.join(config_scope_path, include_filename)
-            """\
+    fs.copy(packages_file.strpath, included_path)
-packages:
-  mpileaks:
-    version: [2.2]
-"""
-        )
 
+    # Ensure the concretized environment reflects contents of the
+    # packages.yaml file.
     with e:
         e.concretize()
 
     assert any(x.satisfies("mpileaks@2.2") for x in e._get_environment_specs())
 
 
-def test_env_with_included_config_var_path():
+def test_env_with_included_config_var_path(packages_file):
+    """Test inclusion of a package configuration file with path variables
+    "staged" in the environment's configuration stage directory."""
     config_var_path = os.path.join("$tempdir", "included-config.yaml")
-    test_config = (
+    test_config = mpileaks_env_config(config_var_path)
-        """\
-env:
-  include:
-  - %s
-  specs:
-  - mpileaks
-"""
-        % config_var_path
-    )
 
     _env_create("test", StringIO(test_config))
     e = ev.read("test")
 
     config_real_path = substitute_path_variables(config_var_path)
     fs.mkdirp(os.path.dirname(config_real_path))
-    with open(config_real_path, "w") as f:
+    fs.rename(packages_file.strpath, config_real_path)
-        f.write(
+    assert os.path.exists(config_real_path)
-            """\
-packages:
-  mpileaks:
-    version: [2.2]
-"""
-        )
 
     with e:
         e.concretize()
@@ -3030,3 +3065,24 @@ def test_unify_when_possible_works_around_conflicts():
     assert len([x for x in e.all_specs() if x.satisfies("mpileaks+opt")]) == 1
     assert len([x for x in e.all_specs() if x.satisfies("mpileaks~opt")]) == 1
     assert len([x for x in e.all_specs() if x.satisfies("mpich")]) == 1
+
+
+def test_env_include_packages_url(
+    tmpdir, mutable_empty_config, mock_spider_configs, mock_curl_configs
+):
+    """Test inclusion of a (GitHub) URL."""
+    develop_url = "https://github.com/fake/fake/blob/develop/"
+    default_packages = develop_url + "etc/fake/defaults/packages.yaml"
+    spack_yaml = tmpdir.join("spack.yaml")
+    with spack_yaml.open("w") as f:
+        f.write("spack:\n  include:\n    - {0}\n".format(default_packages))
+    assert os.path.isfile(spack_yaml.strpath)
+
+    with spack.config.override("config:url_fetch_method", "curl"):
+        env = ev.Environment(tmpdir.strpath)
+        ev.activate(env)
+        scopes = env.included_config_scopes()
+        assert len(scopes) == 1
+
+        cfg = spack.config.get("packages")
+        assert "openmpi" in cfg["all"]["providers"]["mpi"]

lib/spack/spack/test/cmd/spec.py

@@ -12,8 +12,8 @@
 import spack.error
 import spack.spec
 import spack.store
-from spack.fetch_strategy import FetchError
 from spack.main import SpackCommand, SpackCommandError
+from spack.util.web import FetchError
 
 pytestmark = pytest.mark.usefixtures("config", "mutable_mock_repo")
 

lib/spack/spack/test/config.py

@@ -12,7 +12,8 @@
 import pytest
 from six import StringIO
 
-from llnl.util.filesystem import getuid, mkdirp, touch
+import llnl.util.tty as tty
+from llnl.util.filesystem import getuid, join_path, mkdirp, touch, touchp
 
 import spack.config
 import spack.environment as ev
@@ -828,9 +829,9 @@ def test_bad_config_section(mock_low_high_config):
         spack.config.get("foobar")
 
 
-@pytest.mark.skipif(sys.platform == "win32", reason="Not supported on Windows (yet)")
+@pytest.mark.skipif(sys.platform == "win32", reason="chmod not supported on Windows")
 @pytest.mark.skipif(getuid() == 0, reason="user is root")
-def test_bad_command_line_scopes(tmpdir, mock_low_high_config):
+def test_bad_command_line_scopes(tmpdir, config):
     cfg = spack.config.Configuration()
 
     with tmpdir.as_cwd():
@@ -1265,3 +1266,168 @@ def test_user_cache_path_is_overridable(working_env):
 def test_user_cache_path_is_default_when_env_var_is_empty(working_env):
     os.environ["SPACK_USER_CACHE_PATH"] = ""
     assert os.path.expanduser("~%s.spack" % os.sep) == spack.paths._get_user_cache_path()
+
+
+github_url = "https://github.com/fake/fake/{0}/develop"
+gitlab_url = "https://gitlab.fake.io/user/repo/-/blob/config/defaults"
+
+
+@pytest.mark.parametrize(
+    "url,isfile",
+    [
+        (github_url.format("tree"), False),
+        ("{0}/README.md".format(github_url.format("blob")), True),
+        ("{0}/etc/fake/defaults/packages.yaml".format(github_url.format("blob")), True),
+        (gitlab_url, False),
+        (None, False),
+    ],
+)
+def test_config_collect_urls(mutable_empty_config, mock_spider_configs, url, isfile):
+    with spack.config.override("config:url_fetch_method", "curl"):
+        urls = spack.config.collect_urls(url)
+        if url:
+            if isfile:
+                expected = 1 if url.endswith(".yaml") else 0
+                assert len(urls) == expected
+            else:
+                # Expect multiple configuration files for a "directory"
+                assert len(urls) > 1
+        else:
+            assert not urls
+
+
+@pytest.mark.parametrize(
+    "url,isfile,fail",
+    [
+        (github_url.format("tree"), False, False),
+        (gitlab_url, False, False),
+        ("{0}/README.md".format(github_url.format("blob")), True, True),
+        ("{0}/compilers.yaml".format(gitlab_url), True, False),
+        (None, False, True),
+    ],
+)
+def test_config_fetch_remote_configs(
+    tmpdir,
+    mutable_empty_config,
+    mock_collect_urls,
+    mock_curl_configs,
+    url,
+    isfile,
+    fail,
+):
+    def _has_content(filename):
+        # The first element of all configuration files for this test happen to
+        # be the basename of the file so this check leverages that feature. If
+        # that changes, then this check will need to change accordingly.
+        element = "{0}:".format(os.path.splitext(os.path.basename(filename))[0])
+        with open(filename, "r") as fd:
+            for line in fd:
+                if element in line:
+                    return True
+        tty.debug("Expected {0} in '{1}'".format(element, filename))
+        return False
+
+    dest_dir = join_path(tmpdir.strpath, "defaults")
+    if fail:
+        msg = "Cannot retrieve configuration"
+        with spack.config.override("config:url_fetch_method", "curl"):
+            with pytest.raises(spack.config.ConfigFileError, match=msg):
+                spack.config.fetch_remote_configs(url, dest_dir)
+    else:
+        with spack.config.override("config:url_fetch_method", "curl"):
+            path = spack.config.fetch_remote_configs(url, dest_dir)
+            assert os.path.exists(path)
+            if isfile:
+                # Ensure correct file is "fetched"
+                assert os.path.basename(path) == os.path.basename(url)
+                # Ensure contents of the file has expected config element
+                assert _has_content(path)
+            else:
+                for filename in os.listdir(path):
+                    assert _has_content(join_path(path, filename))
+
+
+@pytest.fixture(scope="function")
+def mock_collect_urls(mock_config_data, monkeypatch):
+    """Mock the collection of URLs to avoid mocking spider."""
+
+    _, config_files = mock_config_data
+
+    def _collect(base_url):
+        if not base_url:
+            return []
+
+        ext = os.path.splitext(base_url)[1]
+        if ext:
+            return [base_url] if ext == ".yaml" else []
+
+        return [join_path(base_url, f) for f in config_files]
+
+    monkeypatch.setattr(spack.config, "collect_urls", _collect)
+
+    yield
+
+
+@pytest.mark.parametrize(
+    "url,skip",
+    [
+        (github_url.format("tree"), True),
+        ("{0}/compilers.yaml".format(gitlab_url), True),
+    ],
+)
+def test_config_fetch_remote_configs_skip(
+    tmpdir, mutable_empty_config, mock_collect_urls, mock_curl_configs, url, skip
+):
+    """Ensure skip fetching remote config file if it already exists when
+    required and not skipping if replacing it."""
+
+    def check_contents(filename, expected):
+        with open(filename, "r") as fd:
+            lines = fd.readlines()
+            if expected:
+                assert lines[0] == "compilers:"
+            else:
+                assert not lines
+
+    dest_dir = join_path(tmpdir.strpath, "defaults")
+    filename = "compilers.yaml"
+
+    # Create a stage directory with an empty configuration file
+    path = join_path(dest_dir, filename)
+    touchp(path)
+
+    # Do NOT replace the existing cached configuration file if skipping
+    expected = None if skip else "compilers:"
+
+    with spack.config.override("config:url_fetch_method", "curl"):
+        path = spack.config.fetch_remote_configs(url, dest_dir, skip)
+        result_filename = path if path.endswith(".yaml") else join_path(path, filename)
+        check_contents(result_filename, expected)
+
+
+def test_config_file_dir_failure(tmpdir, mutable_empty_config):
+    with pytest.raises(spack.config.ConfigFileError, match="not a file"):
+        spack.config.read_config_file(tmpdir.strpath)
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="chmod not supported on Windows")
+def test_config_file_read_perms_failure(tmpdir, mutable_empty_config):
+    """Test reading a configuration file without permissions to ensure
+    ConfigFileError is raised."""
+    filename = join_path(tmpdir.strpath, "test.yaml")
+    touch(filename)
+    os.chmod(filename, 0o200)
+
+    with pytest.raises(spack.config.ConfigFileError, match="not readable"):
+        spack.config.read_config_file(filename)
+
+
+def test_config_file_read_invalid_yaml(tmpdir, mutable_empty_config):
+    """Test reading a configuration file with invalid (unparseable) YAML
+    raises a ConfigFileError."""
+    filename = join_path(tmpdir.strpath, "test.yaml")
+    with open(filename, "w") as f:
+        f.write("spack:\nview")
+
+    with pytest.raises(spack.config.ConfigFileError, match="parsing yaml"):
+        spack.config.read_config_file(filename)

lib/spack/spack/test/conftest.py

@@ -26,6 +26,7 @@
 import archspec.cpu.schema
 
 import llnl.util.lang
+import llnl.util.tty as tty
 from llnl.util.filesystem import mkdirp, remove_linked_tree, working_dir
 
 import spack.binary_distribution
@@ -47,8 +48,9 @@
 import spack.util.executable
 import spack.util.gpg
 import spack.util.spack_yaml as syaml
-from spack.fetch_strategy import FetchError, FetchStrategyComposite, URLFetchStrategy
+from spack.fetch_strategy import FetchStrategyComposite, URLFetchStrategy
 from spack.util.pattern import Bunch
+from spack.util.web import FetchError
 
 is_windows = sys.platform == "win32"
 
@@ -1684,3 +1686,82 @@ def noncyclical_dir_structure(tmpdir):
         with open(j("file_3"), "wb"):
             pass
     yield d
+
+
+@pytest.fixture(scope="function")
+def mock_config_data():
+    config_data_dir = os.path.join(spack.paths.test_path, "data", "config")
+    return config_data_dir, os.listdir(config_data_dir)
+
+
+@pytest.fixture(scope="function")
+def mock_curl_configs(mock_config_data, monkeypatch):
+    """
+    Mock curl-based retrieval of configuration files from the web by grabbing
+    them from the test data configuration directory.
+
+    Fetches a single (configuration) file if the name matches one in the test
+    data directory.
+    """
+    config_data_dir, config_files = mock_config_data
+
+    class MockCurl(object):
+        def __init__(self):
+            self.returncode = None
+
+        def __call__(self, *args, **kwargs):
+            url = [a for a in args if a.startswith("http")][0]
+            basename = os.path.basename(url)
+            if os.path.splitext(url)[1]:
+                if basename in config_files:
+                    filename = os.path.join(config_data_dir, basename)
+
+                    with open(filename, "r") as f:
+                        lines = f.readlines()
+                        write_file(os.path.basename(filename), "".join(lines))
+
+                    self.returncode = 0
+                else:
+                    # This is a "404" and is technically only returned if -f
+                    # flag is provided to curl.
+                    tty.msg("curl: (22) The requested URL returned error: 404")
+                    self.returncode = 22
+
+    def mock_curl(*args):
+        return MockCurl()
+
+    monkeypatch.setattr(spack.util.web, "_curl", mock_curl)
+
+    yield
+
+
+@pytest.fixture(scope="function")
+def mock_spider_configs(mock_config_data, monkeypatch):
+    """
+    Mock retrieval of configuration file URLs from the web by grabbing
+    them from the test data configuration directory.
+    """
+    config_data_dir, config_files = mock_config_data
+
+    def _spider(*args, **kwargs):
+        root_urls = args[0]
+        if not root_urls:
+            return [], set()
+
+        root_urls = [root_urls] if isinstance(root_urls, str) else root_urls
+
+        # Any URL with an extension will be treated like a file; otherwise,
+        # it is considered a directory/folder and we'll grab all available
+        # files.
+        urls = []
+        for url in root_urls:
+            if os.path.splitext(url)[1]:
+                urls.append(url)
+            else:
+                urls.extend([os.path.join(url, f) for f in config_files])
+
+        return [], set(urls)
+
+    monkeypatch.setattr(spack.util.web, "spider", _spider)
+
+    yield

lib/spack/spack/test/gcs_fetch.py

@@ -10,6 +10,7 @@
 import spack.config
 import spack.fetch_strategy
 import spack.stage
+from spack.util.web import FetchError
 
 
 @pytest.mark.parametrize("_fetch_method", ["curl", "urllib"])
@@ -32,7 +33,7 @@ def test_gcsfetchstrategy_bad_url(tmpdir, _fetch_method):
         with spack.stage.Stage(fetcher, path=testpath) as stage:
             assert stage is not None
             assert fetcher.archive_file is None
-            with pytest.raises(spack.fetch_strategy.FetchError):
+            with pytest.raises(FetchError):
                 fetcher.fetch()
 
 

lib/spack/spack/test/packaging.py

@@ -573,7 +573,7 @@ def _instr(pkg):
         monkeypatch.setattr(spack.package_base.PackageBase, "download_instr", _instr)
 
     expected = pkg.download_instr if manual else "All fetchers failed"
-    with pytest.raises(spack.fetch_strategy.FetchError, match=expected):
+    with pytest.raises(spack.util.web.FetchError, match=expected):
         pkg.do_fetch()
 
 

lib/spack/spack/test/s3_fetch.py

@@ -10,6 +10,7 @@
 import spack.config as spack_config
 import spack.fetch_strategy as spack_fs
 import spack.stage as spack_stage
+from spack.util.web import FetchError
 
 
 @pytest.mark.parametrize("_fetch_method", ["curl", "urllib"])
@@ -32,7 +33,7 @@ def test_s3fetchstrategy_bad_url(tmpdir, _fetch_method):
         with spack_stage.Stage(fetcher, path=testpath) as stage:
             assert stage is not None
             assert fetcher.archive_file is None
-            with pytest.raises(spack_fs.FetchError):
+            with pytest.raises(FetchError):
                 fetcher.fetch()
 
 

lib/spack/spack/test/stage.py

@@ -22,6 +22,7 @@
 from spack.resource import Resource
 from spack.stage import DIYStage, ResourceStage, Stage, StageComposite
 from spack.util.path import canonicalize_path
+from spack.util.web import FetchError
 
 # The following values are used for common fetch and stage mocking fixtures:
 _archive_base = "test-files"
@@ -525,7 +526,7 @@ def test_no_search_mirror_only(self, failing_fetch_strategy, failing_search_fn):
         with stage:
             try:
                 stage.fetch(mirror_only=True)
-            except spack.fetch_strategy.FetchError:
+            except FetchError:
                 pass
         check_destroy(stage, self.stage_name)
 
@@ -540,7 +541,7 @@ def test_search_if_default_fails(self, failing_fetch_strategy, search_fn, err_ms
         stage = Stage(failing_fetch_strategy, name=self.stage_name, search_fn=search_fn)
 
         with stage:
-            with pytest.raises(spack.fetch_strategy.FetchError, match=expected):
+            with pytest.raises(FetchError, match=expected):
                 stage.fetch(mirror_only=False, err_msg=err_msg)
 
         check_destroy(stage, self.stage_name)

lib/spack/spack/test/url_fetch.py

@@ -17,6 +17,7 @@
 import spack.repo
 import spack.util.crypto as crypto
 import spack.util.executable
+import spack.util.web as web_util
 from spack.spec import Spec
 from spack.stage import Stage
 from spack.util.executable import which
@@ -328,9 +329,8 @@ def _which(*args, **kwargs):
         err_msg = err_fmt.format(args[0])
         raise spack.util.executable.CommandNotFoundError(err_msg)
 
-    # Patching the 'which' symbol imported by fetch_strategy works
+    # Patching the 'which' symbol imported by fetch_strategy needed due
-    # since it is too late in import processing to patch the defining
+    # to 'from spack.util.executable import which' in this module.
-    # (spack.util.executable) module's symbol.
     monkeypatch.setattr(fs, "which", _which)
 
     testpath = str(tmpdir)
@@ -342,3 +342,79 @@ def _which(*args, **kwargs):
             with Stage(fetcher, path=testpath) as stage:
                 out = stage.fetch()
             assert err_fmt.format("curl") in out
+
+
+def test_url_fetch_text_without_url(tmpdir):
+    with pytest.raises(web_util.FetchError, match="URL is required"):
+        web_util.fetch_url_text(None)
+
+
+def test_url_fetch_text_curl_failures(tmpdir, monkeypatch):
+    """Check fetch_url_text if URL's curl is missing."""
+    err_fmt = "No such command {0}"
+
+    def _which(*args, **kwargs):
+        err_msg = err_fmt.format(args[0])
+        raise spack.util.executable.CommandNotFoundError(err_msg)
+
+    # Patching the 'which' symbol imported by spack.util.web needed due
+    # to 'from spack.util.executable import which' in this module.
+    monkeypatch.setattr(spack.util.web, "which", _which)
+
+    with spack.config.override("config:url_fetch_method", "curl"):
+        with pytest.raises(web_util.FetchError, match="Missing required curl"):
+            web_util.fetch_url_text("https://github.com/")
+
+
+def test_url_check_curl_errors():
+    """Check that standard curl error returncodes raise expected errors."""
+    # Check returncode 22 (i.e., 404)
+    with pytest.raises(web_util.FetchError, match="not found"):
+        web_util.check_curl_code(22)
+
+    # Check returncode 60 (certificate error)
+    with pytest.raises(web_util.FetchError, match="invalid certificate"):
+        web_util.check_curl_code(60)
+
+
+def test_url_missing_curl(tmpdir, monkeypatch):
+    """Check url_exists failures if URL's curl is missing."""
+    err_fmt = "No such command {0}"
+
+    def _which(*args, **kwargs):
+        err_msg = err_fmt.format(args[0])
+        raise spack.util.executable.CommandNotFoundError(err_msg)
+
+    # Patching the 'which' symbol imported by spack.util.web needed due
+    # to 'from spack.util.executable import which' in this module.
+    monkeypatch.setattr(spack.util.web, "which", _which)
+
+    with spack.config.override("config:url_fetch_method", "curl"):
+        with pytest.raises(web_util.FetchError, match="Missing required curl"):
+            web_util.url_exists("https://github.com/")
+
+
+def test_url_fetch_text_urllib_bad_returncode(tmpdir, monkeypatch):
+    class response(object):
+        def getcode(self):
+            return 404
+
+    def _read_from_url(*args, **kwargs):
+        return None, None, response()
+
+    monkeypatch.setattr(spack.util.web, "read_from_url", _read_from_url)
+
+    with spack.config.override("config:url_fetch_method", "urllib"):
+        with pytest.raises(web_util.FetchError, match="failed with error code"):
+            web_util.fetch_url_text("https://github.com/")
+
+
+def test_url_fetch_text_urllib_web_error(tmpdir, monkeypatch):
+    def _raise_web_error(*args, **kwargs):
+        raise web_util.SpackWebError("bad url")
+
+    monkeypatch.setattr(spack.util.web, "read_from_url", _raise_web_error)
+
+    with spack.config.override("config:url_fetch_method", "urllib"):
+        with pytest.raises(web_util.FetchError, match="fetch failed to verify"):
+            web_util.fetch_url_text("https://github.com/")

lib/spack/spack/util/path.py

@@ -306,7 +306,14 @@ def add_padding(path, length):
 
 
 def canonicalize_path(path):
-    """Same as substitute_path_variables, but also take absolute path."""
+    """Same as substitute_path_variables, but also take absolute path.
+
+    Arguments:
+        path (str): path being converted as needed
+
+    Returns:
+        (str): An absolute path with path variable substitution
+    """
     # Get file in which path was written in case we need to make it absolute
     # relative to that path.
     filename = None

lib/spack/spack/util/url.py

@@ -12,7 +12,7 @@
 import re
 import sys
 
-import six.moves.urllib.parse as urllib_parse
+import six.moves.urllib.parse
 from six import string_types
 
 from spack.util.path import (
@@ -67,11 +67,18 @@ def local_file_path(url):
 def parse(url, scheme="file"):
     """Parse a url.
 
-    For file:// URLs, the netloc and path components are concatenated and
+    Path variable substitution is performed on file URLs as needed. The
-    passed through spack.util.path.canoncalize_path().
+    variables are documented at
+    https://spack.readthedocs.io/en/latest/configuration.html#spack-specific-variables.
 
-    Otherwise, the returned value is the same as urllib's urlparse() with
+    Arguments:
-    allow_fragments=False.
+        url (str): URL to be parsed
+        scheme (str): associated URL scheme
+    Returns:
+        (six.moves.urllib.parse.ParseResult): For file scheme URLs, the
+        netloc and path components are concatenated and passed through
+        spack.util.path.canoncalize_path().  Otherwise, the returned value
+        is the same as urllib's urlparse() with allow_fragments=False.
     """
     # guarantee a value passed in is of proper url format. Guarantee
     # allows for easier string manipulation accross platforms
@@ -79,7 +86,11 @@ def parse(url, scheme="file"):
         require_url_format(url)
         url = escape_file_url(url)
     url_obj = (
-        urllib_parse.urlparse(url, scheme=scheme, allow_fragments=False)
+        six.moves.urllib.parse.urlparse(
+            url,
+            scheme=scheme,
+            allow_fragments=False,
+        )
         if isinstance(url, string_types)
         else url
     )
@@ -108,8 +119,13 @@ def parse(url, scheme="file"):
     if sys.platform == "win32":
         path = convert_to_posix_path(path)
 
-    return urllib_parse.ParseResult(
+    return six.moves.urllib.parse.ParseResult(
-        scheme=scheme, netloc=netloc, path=path, params=params, query=query, fragment=None
+        scheme=scheme,
+        netloc=netloc,
+        path=path,
+        params=params,
+        query=query,
+        fragment=None,
     )
 
 
@@ -188,7 +204,11 @@ def join(base_url, path, *extra, **kwargs):
     last_abs_component = None
     scheme = ""
     for i in range(n - 1, -1, -1):
-        obj = urllib_parse.urlparse(paths[i], scheme="", allow_fragments=False)
+        obj = six.moves.urllib.parse.urlparse(
+            paths[i],
+            scheme="",
+            allow_fragments=False,
+        )
 
         scheme = obj.scheme
 
@@ -198,7 +218,11 @@ def join(base_url, path, *extra, **kwargs):
                 # Without a scheme, we have to go back looking for the
                 # next-last component that specifies a scheme.
                 for j in range(i - 1, -1, -1):
-                    obj = urllib_parse.urlparse(paths[j], scheme="", allow_fragments=False)
+                    obj = six.moves.urllib.parse.urlparse(
+                        paths[j],
+                        scheme="",
+                        allow_fragments=False,
+                    )
 
                     if obj.scheme:
                         paths[i] = "{SM}://{NL}{PATH}".format(
@@ -214,13 +238,17 @@ def join(base_url, path, *extra, **kwargs):
     if last_abs_component is not None:
         paths = paths[last_abs_component:]
         if len(paths) == 1:
-            result = urllib_parse.urlparse(paths[0], scheme="file", allow_fragments=False)
+            result = six.moves.urllib.parse.urlparse(
+                paths[0],
+                scheme="file",
+                allow_fragments=False,
+            )
 
             # another subtlety: If the last argument to join() is an absolute
             # file:// URL component with a relative path, the relative path
             # needs to be resolved.
             if result.scheme == "file" and result.netloc:
-                result = urllib_parse.ParseResult(
+                result = six.moves.urllib.parse.ParseResult(
                     scheme=result.scheme,
                     netloc="",
                     path=posixpath.abspath(result.netloc + result.path),
@@ -278,8 +306,13 @@ def _join(base_url, path, *extra, **kwargs):
         base_path = convert_to_posix_path(base_path)
 
     return format(
-        urllib_parse.ParseResult(
+        six.moves.urllib.parse.ParseResult(
-            scheme=scheme, netloc=netloc, path=base_path, params=params, query=query, fragment=None
+            scheme=scheme,
+            netloc=netloc,
+            path=base_path,
+            params=params,
+            query=query,
+            fragment=None,
         )
     )
 
@@ -337,9 +370,12 @@ def parse_git_url(url):
     return (scheme, user, hostname, port, path)
 
 
+def is_url_format(url):
+    return re.search(r"^(file://|http://|https://|ftp://|s3://|gs://|ssh://|git://|/)", url)
+
+
 def require_url_format(url):
-    ut = re.search(r"^(file://|http://|https://|ftp://|s3://|gs://|ssh://|git://|/)", url)
+    if not is_url_format(url):
-    if not ut:
         raise ValueError("Invalid url format from url: %s" % url)
 
 

lib/spack/spack/util/web.py

@@ -22,7 +22,7 @@
 
 import llnl.util.lang
 import llnl.util.tty as tty
-from llnl.util.filesystem import mkdirp, rename
+from llnl.util.filesystem import mkdirp, rename, working_dir
 
 import spack
 import spack.config
@@ -33,6 +33,7 @@
 import spack.util.s3 as s3_util
 import spack.util.url as url_util
 from spack.util.compression import ALLOWED_ARCHIVE_TYPES
+from spack.util.executable import CommandNotFoundError, which
 from spack.util.path import convert_to_posix_path
 
 #: User-Agent used in Request objects
@@ -224,30 +225,222 @@ def push_to_url(local_file_path, remote_path, keep_original=True, extra_args=Non
         )
 
 
-def url_exists(url):
+def base_curl_fetch_args(url, timeout=0):
-    url = url_util.parse(url)
+    """Return the basic fetch arguments typically used in calls to curl.
-    local_path = url_util.local_file_path(url)
+
+    The arguments include those for ensuring behaviors such as failing on
+    errors for codes over 400, printing HTML headers, resolving 3xx redirects,
+    status or failure handling, and connection timeouts.
+
+    It also uses the following configuration option to set an additional
+    argument as needed:
+
+        * config:connect_timeout (int): connection timeout
+        * config:verify_ssl (str): Perform SSL verification
+
+    Arguments:
+        url (str): URL whose contents will be fetched
+        timeout (int): Connection timeout, which is only used if higher than
+            config:connect_timeout
+
+    Returns (list): list of argument strings
+    """
+    curl_args = [
+        "-f",  # fail on >400 errors
+        "-D",
+        "-",  # "-D -" prints out HTML headers
+        "-L",  # resolve 3xx redirects
+        url,
+    ]
+    if not spack.config.get("config:verify_ssl"):
+        curl_args.append("-k")
+
+    if sys.stdout.isatty() and tty.msg_enabled():
+        curl_args.append("-#")  # status bar when using a tty
+    else:
+        curl_args.append("-sS")  # show errors if fail
+
+    connect_timeout = spack.config.get("config:connect_timeout", 10)
+    if timeout:
+        connect_timeout = max(int(connect_timeout), int(timeout))
+    if connect_timeout > 0:
+        curl_args.extend(["--connect-timeout", str(connect_timeout)])
+
+    return curl_args
+
+
+def check_curl_code(returncode):
+    """Check standard return code failures for provided arguments.
+
+    Arguments:
+        returncode (int): curl return code
+
+    Raises FetchError if the curl returncode indicates failure
+    """
+    if returncode != 0:
+        if returncode == 22:
+            # This is a 404. Curl will print the error.
+            raise FetchError("URL was not found!")
+
+        if returncode == 60:
+            # This is a certificate error.  Suggest spack -k
+            raise FetchError(
+                "Curl was unable to fetch due to invalid certificate. "
+                "This is either an attack, or your cluster's SSL "
+                "configuration is bad.  If you believe your SSL "
+                "configuration is bad, you can try running spack -k, "
+                "which will not check SSL certificates."
+                "Use this at your own risk."
+            )
+
+        raise FetchError("Curl failed with error {0}".format(returncode))
+
+
+def _curl(curl=None):
+    if not curl:
+        try:
+            curl = which("curl", required=True)
+        except CommandNotFoundError as exc:
+            tty.error(str(exc))
+            raise FetchError("Missing required curl fetch method")
+    return curl
+
+
+def fetch_url_text(url, curl=None, dest_dir="."):
+    """Retrieves text-only URL content using the configured fetch method.
+    It determines the fetch method from:
+
+        * config:url_fetch_method (str): fetch method to use (e.g., 'curl')
+
+    If the method is `curl`, it also uses the following configuration
+    options:
+
+        * config:connect_timeout (int): connection time out
+        * config:verify_ssl (str): Perform SSL verification
+
+    Arguments:
+        url (str): URL whose contents are to be fetched
+        curl (spack.util.executable.Executable or None): (optional) curl
+            executable if curl is the configured fetch method
+        dest_dir (str): (optional) destination directory for fetched text
+            file
+
+    Returns (str or None): path to the fetched file
+
+    Raises FetchError if the curl returncode indicates failure
+    """
+    if not url:
+        raise FetchError("A URL is required to fetch its text")
+
+    tty.debug("Fetching text at {0}".format(url))
+
+    filename = os.path.basename(url)
+    path = os.path.join(dest_dir, filename)
+
+    fetch_method = spack.config.get("config:url_fetch_method")
+    tty.debug("Using '{0}' to fetch {1} into {2}".format(fetch_method, url, path))
+    if fetch_method == "curl":
+        curl_exe = _curl(curl)
+        if not curl_exe:
+            raise FetchError("Missing required fetch method (curl)")
+
+        curl_args = ["-O"]
+        curl_args.extend(base_curl_fetch_args(url))
+
+        # Curl automatically downloads file contents as filename
+        with working_dir(dest_dir, create=True):
+            _ = curl_exe(*curl_args, fail_on_error=False, output=os.devnull)
+            check_curl_code(curl_exe.returncode)
+
+        return path
+
+    else:
+        try:
+            _, _, response = read_from_url(url)
+
+            returncode = response.getcode()
+            if returncode and returncode != 200:
+                raise FetchError("Urllib failed with error code {0}".format(returncode))
+
+            output = codecs.getreader("utf-8")(response).read()
+            if output:
+                with working_dir(dest_dir, create=True):
+                    with open(filename, "w") as f:
+                        f.write(output)
+
+                return path
+
+        except SpackWebError as err:
+            raise FetchError("Urllib fetch failed to verify url: {0}".format(str(err)))
+
+    return None
+
+
+def url_exists(url, curl=None):
+    """Determines whether url exists.
+
+    A scheme-specific process is used for Google Storage (`gs`) and Amazon
+    Simple Storage Service (`s3`) URLs; otherwise, the configured fetch
+    method defined by `config:url_fetch_method` is used.
+
+    If the method is `curl`, it also uses the following configuration option:
+
+        * config:verify_ssl (str): Perform SSL verification
+
+    Otherwise, `urllib` will be used.
+
+    Arguments:
+        url (str): URL whose existence is being checked
+        curl (spack.util.executable.Executable or None): (optional) curl
+            executable if curl is the configured fetch method
+
+    Returns (bool): True if it exists; False otherwise.
+    """
+    tty.debug("Checking existence of {0}".format(url))
+    url_result = url_util.parse(url)
+
+    # Check if a local file
+    local_path = url_util.local_file_path(url_result)
     if local_path:
         return os.path.exists(local_path)
 
-    if url.scheme == "s3":
+    # Check if Amazon Simple Storage Service (S3) .. urllib-based fetch
-        # Check for URL specific connection information
+    if url_result.scheme == "s3":
-        s3 = s3_util.create_s3_session(url, connection=s3_util.get_mirror_connection(url))
+        # Check for URL-specific connection information
+        s3 = s3_util.create_s3_session(
+            url_result, connection=s3_util.get_mirror_connection(url_result)
+        )  # noqa: E501
 
         try:
-            s3.get_object(Bucket=url.netloc, Key=url.path.lstrip("/"))
+            s3.get_object(Bucket=url_result.netloc, Key=url_result.path.lstrip("/"))
             return True
         except s3.ClientError as err:
             if err.response["Error"]["Code"] == "NoSuchKey":
                 return False
             raise err
 
-    elif url.scheme == "gs":
+    # Check if Google Storage .. urllib-based fetch
-        gcs = gcs_util.GCSBlob(url)
+    if url_result.scheme == "gs":
+        gcs = gcs_util.GCSBlob(url_result)
         return gcs.exists()
 
-    # otherwise, just try to "read" from the URL, and assume that *any*
+    # Otherwise, use the configured fetch method
-    # non-throwing response contains the resource represented by the URL
+    if spack.config.get("config:url_fetch_method") == "curl":
+        curl_exe = _curl(curl)
+        if not curl_exe:
+            return False
+
+        # Telling curl to fetch the first byte (-r 0-0) is supposed to be
+        # portable.
+        curl_args = ["--stderr", "-", "-s", "-f", "-r", "0-0", url]
+        if not spack.config.get("config:verify_ssl"):
+            curl_args.append("-k")
+        _ = curl_exe(*curl_args, fail_on_error=False, output=os.devnull)
+        return curl_exe.returncode == 0
+
+    # If we get here, then the only other fetch method option is urllib.
+    # So try to "read" from the URL and assume that *any* non-throwing
+    #  response contains the resource represented by the URL.
     try:
         read_from_url(url)
         return True
@@ -716,6 +909,10 @@ def unfuzz(header):
         raise
 
 
+class FetchError(spack.error.SpackError):
+    """Superclass for fetch-related errors."""
+
+
 class SpackWebError(spack.error.SpackError):
     """Superclass for Spack web spidering errors."""