Add compat shim to find eggs in importlib backend

.github/workflows/ci.yml

@@ -223,7 +223,7 @@ jobs:
     name: tests for importlib.metadata backend
     runs-on: ubuntu-latest
     env:
-      _PIP_METADATA_BACKEND_IMPORTLIB: '1'
+      _PIP_METADATA_BACKEND_IMPORTLIB: egg-compat
 
     needs: [pre-commit, packaging, determine-changes]
     if: >-

src/pip/_internal/metadata/importlib/_envs.py

@@ -1,11 +1,16 @@
+import functools
 import importlib.metadata
+import os
 import pathlib
 import sys
+import zipfile
+import zipimport
 from typing import Iterator, List, Optional, Sequence, Set, Tuple
 
 from pip._vendor.packaging.utils import NormalizedName, canonicalize_name
 
 from pip._internal.metadata.base import BaseDistribution, BaseEnvironment
+from pip._internal.utils.deprecation import deprecated
 
 from ._compat import BasePath, get_dist_name, get_info_location
 from ._dists import Distribution
@@ -81,6 +86,45 @@ class _DistributionFinder:
             for dist, info_location in self._find_impl(target_location):
                 yield Distribution(dist, info_location, path)
 
+    def _find_eggs_in_dir(self, location: str) -> Iterator[BaseDistribution]:
+        from pip._vendor.pkg_resources import find_distributions
+
+        from pip._internal.metadata import pkg_resources as legacy
+
+        with os.scandir(location) as it:
+            for entry in it:
+                if not entry.name.endswith(".egg"):
+                    continue
+                for dist in find_distributions(entry.path):
+                    yield legacy.Distribution(dist)
+
+    def _find_eggs_in_zip(self, location: str) -> Iterator[BaseDistribution]:
+        from pip._vendor.pkg_resources import find_eggs_in_zip
+
+        from pip._internal.metadata import pkg_resources as legacy
+
+        try:
+            importer = zipimport.zipimporter(location)
+        except zipimport.ZipImportError:
+            return
+        for dist in find_eggs_in_zip(importer, location):
+            yield legacy.Distribution(dist)
+
+    def find_eggs(self, location: str) -> Iterator[BaseDistribution]:
+        if os.path.isdir(location):
+            yield from self._find_eggs_in_dir(location)
+        if zipfile.is_zipfile(location):
+            yield from self._find_eggs_in_zip(location)
+
+
+@functools.lru_cache(maxsize=None)  # Warn a distribution exactly once.
+def _emit_egg_deprecation(location: Optional[str]) -> None:
+    deprecated(
+        reason=f"Loading egg at {location} is deprecated.",
+        replacement="to use pip for package installation.",
+        gone_in=None,
+    )
+
 
 class Environment(BaseEnvironment):
     def __init__(self, paths: Sequence[str]) -> None:
@@ -107,6 +151,15 @@ class Environment(BaseEnvironment):
             yield from finder.find(location)
             yield from finder.find_linked(location)
 
+            # Compatibility mode: Also find eggs in path. This uses the old
+            # pkg_resources backend, so it's on the way out on day one.
+            # TODO: This should only be enabled behind a flag because importing
+            # pkg_resources is slow.
+            for dist in finder.find_eggs(location):
+                # TODO: Enable deprecation message.
+                # _emit_egg_deprecation(dist.location)
+                yield dist
+
     def get_distribution(self, name: str) -> Optional[BaseDistribution]:
         matches = (
             distribution