Cache wheels built from immutable VCS requirements

Cache wheels that are built from VCS requirements
that contain an immutable revision (such as a git sha).

Author: sbidoul

news/6640.feature

@@ -0,0 +1 @@
+Cache wheels built from VCS requirements that are considered immutables

src/pip/_internal/vcs/git.py

@@ -48,6 +48,12 @@ class Git(VersionControl):
     def get_base_rev_args(rev):
         return [rev]
 
+    def is_immutable_rev_checkout(self, url, dest):
+        _, rev_options = self.get_url_rev_options(url)
+        if not rev_options.rev:
+            return False
+        return self.is_commit_id_equal(dest, rev_options.rev)
+
     def get_git_version(self):
         VERSION_PFX = 'git version '
         version = self.run_command(['version'], show_stdout=False)

src/pip/_internal/vcs/versioncontrol.py

@@ -268,6 +268,20 @@ def get_base_rev_args(rev):
         """
         raise NotImplementedError
 
+    def is_immutable_rev_checkout(self, url, dest):
+        # type: (str, str) -> bool
+        """
+        Return true if the commit hash checked out at dest matches
+        the revision in url.
+
+        Always return False, if the VCS does not support immutable commit
+        hashes.
+
+        This method does not check if there are local uncommitted changes
+        in dest after checkout, as pip currently has no use case for that.
+        """
+        return False
+
     @classmethod
     def make_rev_options(cls, rev=None, extra_args=None):
         # type: (Optional[str], Optional[List[str]]) -> RevOptions

src/pip/_internal/wheel.py

@@ -27,7 +27,7 @@
 from pip._vendor.six import StringIO
 
 from pip._internal import pep425tags
-from pip._internal.download import unpack_file_url
+from pip._internal.download import _get_used_vcs_backend, unpack_file_url
 from pip._internal.exceptions import (
     InstallationError,
     InvalidWheelFilename,
@@ -845,7 +845,9 @@ def should_use_ephemeral_cache(
         # VCS checkout. Build wheel just for this run
         # unless it points to an immutable commit hash in which
         # case it can be cached.
-        # TODO if req.link.is_vcs_immutable: return False
+        vcs_backend = _get_used_vcs_backend(req.link)
+        if vcs_backend.is_immutable_rev_checkout(req.link.url, req.source_dir):
+            return False
         return True
 
     link = req.link