Creating versions from urls doesn't modify class attributes (#15452)

fixes #15449

Before this PR a call to pkg.url_for_version was modifying
class attributes determining different results for subsequents
calls and an error when the urls was empty.

lib/spack/spack/fetch_strategy.py

@@ -1239,7 +1239,7 @@ def _from_merged_attrs(fetcher, pkg, version):
         # TODO: refactor this logic into its own method or function
         # TODO: to avoid duplication
         mirrors = [spack.url.substitute_version(u, version)
-                   for u in getattr(pkg, 'urls', [])]
+                   for u in getattr(pkg, 'urls', [])[1:]]
         attrs = {fetcher.url_attr: url, 'mirrors': mirrors}
     else:
         url = getattr(pkg, fetcher.url_attr)

lib/spack/spack/package.py

@@ -763,7 +763,7 @@ def url_for_version(self, version):
         # If no specific URL, use the default, class-level URL
         url = getattr(self, 'url', None)
         urls = getattr(self, 'urls', [None])
-        default_url = url or urls.pop(0)
+        default_url = url or urls[0]
 
         # if no exact match AND no class-level default, use the nearest URL
         if not default_url:

lib/spack/spack/test/url_fetch.py

@@ -32,7 +32,7 @@ def pkg_factory():
     def factory(url, urls):
 
         def fn(v):
-            main_url = url or urls.pop(0)
+            main_url = url or urls[0]
             return spack.url.substitute_version(main_url, v)
 
         return Pkg(