performance: speed up existence checks in packages (#23661)

Spack doesn't require users to manually index their repos; it reindexes the indexes automatically when things change. To determine when to do this, it has to `stat()` all package files in each repository to make sure that indexes up to date with packages. We currently index virtual providers, patches by sha256, and tags on packages.

When this was originally implemented, we ran the checker all the time, at startup, but that was slow (see #7587). But we didn't go far enough -- it still consults the checker and does all the stat operations just to see if a package exists (`Repo.exists()`).  That might've been a wash in 2018, but as the number of packages has grown, it's gotten slower -- checking 5k packages is expensive and users see this for small operations.  It's a win now to make `Repo.exists()` check files directly.

**Fix:**

This PR does a number of things to speed up `spack load`, `spack info`, and other commands:

- [x] Make `Repo.exists()` check files directly again with `os.path.exists()` (this is the big one)
- [x] Refactor `Spec.satisfies()` so that a checking for virtual packages only happens if needed
      (avoids some calls to exists())
- [x] Avoid calling `Repo.exists(spec)` in `Repo.get()`. `Repo.get()` will ultimately try to load
      a `package.py` file anyway; we can let the failure to load it indicate that the package doesn't
      exist, and avoid another call to exists().
- [x] Fix up some comments in spec parsing
- [x] Call `UnknownPackageError` more consistently in `repo.py`

lib/spack/spack/repo.py

@@ -918,8 +918,12 @@ def _read_config(self):
     @autospec
     def get(self, spec):
         """Returns the package associated with the supplied spec."""
-        if not self.exists(spec.name):
-            raise UnknownPackageError(spec.name)
+        # NOTE: we only check whether the package is None here, not whether it
+        # actually exists, because we have to load it anyway, and that ends up
+        # checking for existence. We avoid constructing FastPackageChecker,
+        # which will stat all packages.
+        if spec.name is None:
+            raise UnknownPackageError(None, self)
 
         if spec.namespace and spec.namespace != self.namespace:
             raise UnknownPackageError(spec.name, self.namespace)
@@ -1064,7 +1068,16 @@ def all_package_classes(self):
 
     def exists(self, pkg_name):
         """Whether a package with the supplied name exists."""
-        return pkg_name in self._pkg_checker
+        if pkg_name is None:
+            return False
+
+        # if the FastPackageChecker is already constructed, use it
+        if self._fast_package_checker:
+            return pkg_name in self._pkg_checker
+
+        # if not, check for the package.py file
+        path = self.filename_for_package_name(pkg_name)
+        return os.path.exists(path)
 
     def last_mtime(self):
         """Time a package file in this repo was last updated."""
@@ -1331,7 +1344,7 @@ def __init__(self, name, repo=None):
         long_msg = None
         if name:
             if repo:
-                msg = "Package '{0}' not found in repository '{1}'"
+                msg = "Package '{0}' not found in repository '{1.root}'"
                 msg = msg.format(name, repo)
             else:
                 msg = "Package '{0}' not found.".format(name)

lib/spack/spack/spec.py

@@ -3193,25 +3193,23 @@ def satisfies(self, other, deps=True, strict=False, strict_deps=False):
         if other.concrete:
             return self.concrete and self.dag_hash() == other.dag_hash()
 
-        # A concrete provider can satisfy a virtual dependency.
-        if not self.virtual and other.virtual:
-            try:
-                pkg = spack.repo.get(self.fullname)
-            except spack.repo.UnknownEntityError:
-                # If we can't get package info on this spec, don't treat
-                # it as a provider of this vdep.
-                return False
-
-            if pkg.provides(other.name):
-                for provided, when_specs in pkg.provided.items():
-                    if any(self.satisfies(when_spec, deps=False, strict=strict)
-                           for when_spec in when_specs):
-                        if provided.satisfies(other):
-                            return True
-            return False
-
-        # Otherwise, first thing we care about is whether the name matches
+        # If the names are different, we need to consider virtuals
         if self.name != other.name and self.name and other.name:
+            # A concrete provider can satisfy a virtual dependency.
+            if not self.virtual and other.virtual:
+                try:
+                    pkg = spack.repo.get(self.fullname)
+                except spack.repo.UnknownEntityError:
+                    # If we can't get package info on this spec, don't treat
+                    # it as a provider of this vdep.
+                    return False
+
+                if pkg.provides(other.name):
+                    for provided, when_specs in pkg.provided.items():
+                        if any(self.satisfies(when, deps=False, strict=strict)
+                               for when in when_specs):
+                            if provided.satisfies(other):
+                                return True
             return False
 
         # namespaces either match, or other doesn't require one.
@@ -4639,7 +4637,8 @@ def spec(self, name):
                     break
 
             elif self.accept(HASH):
-                # Get spec by hash and confirm it matches what we already have
+                # Get spec by hash and confirm it matches any constraints we
+                # already read in
                 hash_spec = self.spec_by_hash()
                 if hash_spec.satisfies(spec):
                     spec._dup(hash_spec)

lib/spack/spack/test/cmd/pkg.py

@@ -129,7 +129,8 @@ def test_pkg_add(mock_pkg_git_repo):
         finally:
             shutil.rmtree('pkg-e')
             # Removing a package mid-run disrupts Spack's caching
-            spack.repo.path.repos[0]._fast_package_checker.invalidate()
+            if spack.repo.path.repos[0]._fast_package_checker:
+                spack.repo.path.repos[0]._fast_package_checker.invalidate()
 
     with pytest.raises(spack.main.SpackCommandError):
         pkg('add', 'does-not-exist')