Cache wheels built from immutable Git requirements

Cache wheels that are built from Git requirements
that contain an immutable revision (i.e. a sha).

Author: sbidoul

news/6640.feature

@@ -0,0 +1,2 @@
+Cache wheels built from Git requirements that are considered immutable,
+because they point to a commit hash.

src/pip/_internal/vcs/git.py

@@ -59,6 +59,12 @@ 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

@@ -319,6 +319,20 @@ 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[CommandArgs]) -> RevOptions

src/pip/_internal/wheel.py

@@ -34,6 +34,7 @@
 )
 from pip._internal.locations import get_major_minor_version
 from pip._internal.models.link import Link
+from pip._internal.operations.prepare import _get_used_vcs_backend
 from pip._internal.utils.logging import indent_log
 from pip._internal.utils.marker_files import has_delete_marker_file
 from pip._internal.utils.misc import captured_stdout, ensure_dir, read_chunks
@@ -838,8 +839,13 @@ def should_cache(
         return False
 
     if req.link and req.link.is_vcs:
-        # VCS checkout. Build wheel just for this run.
-        return False
+        # VCS checkout. Build wheel just for this run
+        # unless it points to an immutable commit hash in which
+        # case it can be cached.
+        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
     base, ext = link.splitext()