Cache wheels built from immutable Git requirements

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

docs/html/reference/pip_install.rst

@@ -573,6 +573,9 @@ and use any packages found there. This is disabled via the same
 of that is not part of the pip API. As of 7.0, pip makes a subdirectory for
 each sdist that wheels are built from and places the resulting wheels inside.
 
+As of version 20.0, pip also caches wheels when building from an immutable Git
+reference (i.e. a commit hash).
+
 Pip attempts to choose the best wheels from those built in preference to
 building a new wheel. Note that this means when a package has both optional
 C extensions and builds ``py`` tagged wheels when the C extension can't be built

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

@@ -12,7 +12,7 @@ from pip._vendor.six.moves.urllib import parse as urllib_parse
 from pip._vendor.six.moves.urllib import request as urllib_request
 
 from pip._internal.exceptions import BadCommand
-from pip._internal.utils.misc import display_path
+from pip._internal.utils.misc import display_path, hide_url
 from pip._internal.utils.subprocess import make_command
 from pip._internal.utils.temp_dir import TempDirectory
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
@@ -59,6 +59,13 @@ class Git(VersionControl):
     def get_base_rev_args(rev):
         return [rev]
 
+    def is_immutable_rev_checkout(self, url, dest):
+        # type: (str, str) -> bool
+        _, rev_options = self.get_url_rev_options(hide_url(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

@@ -329,6 +329,20 @@ class VersionControl(object):
         """
         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

@@ -52,6 +52,7 @@ from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 from pip._internal.utils.ui import open_spinner
 from pip._internal.utils.unpacking import unpack_file
 from pip._internal.utils.urls import path_to_url
+from pip._internal.vcs import vcs
 
 if MYPY_CHECK_RUNNING:
     from typing import (
@@ -838,7 +839,15 @@ def should_cache(
         return False
 
     if req.link and req.link.is_vcs:
-        # VCS checkout. Build wheel just for this run.
+        # VCS checkout. Build wheel just for this run
+        # unless it points to an immutable commit hash in which
+        # case it can be cached.
+        assert not req.editable
+        assert req.source_dir
+        vcs_backend = vcs.get_backend_for_scheme(req.link.scheme)
+        assert vcs_backend
+        if vcs_backend.is_immutable_rev_checkout(req.link.url, req.source_dir):
+            return True
         return False
 
     link = req.link

tests/functional/test_vcs_git.py

@@ -221,3 +221,20 @@ def test_is_commit_id_equal(script):
     assert not Git.is_commit_id_equal(version_pkg_path, 'abc123')
     # Also check passing a None value.
     assert not Git.is_commit_id_equal(version_pkg_path, None)
+
+
+def test_is_immutable_rev_checkout(script):
+    version_pkg_path = _create_test_package(script)
+    commit = script.run(
+        'git', 'rev-parse', 'HEAD',
+        cwd=version_pkg_path
+    ).stdout.strip()
+    assert Git().is_immutable_rev_checkout(
+        "git+https://g.c/o/r@" + commit, version_pkg_path
+    )
+    assert not Git().is_immutable_rev_checkout(
+        "git+https://g.c/o/r", version_pkg_path
+    )
+    assert not Git().is_immutable_rev_checkout(
+        "git+https://g.c/o/r@master", version_pkg_path
+    )