Make sure metadata has Requires-Dist and Provides-Extra

egg-info distributions may not have the Requires-Dist and Provides-Extra
fields in their metadata. For consistency and to provide an
unsurprising metadata property, we emulate it by reading requires.txt.

src/pip/_internal/metadata/base.py

@@ -1,5 +1,6 @@
 import csv
 import email.message
+import functools
 import json
 import logging
 import pathlib
@@ -13,6 +14,7 @@
     Iterable,
     Iterator,
     List,
+    NamedTuple,
     Optional,
     Tuple,
     Union,
@@ -33,6 +35,7 @@
 from pip._internal.utils.compat import stdlib_pkgs  # TODO: Move definition here.
 from pip._internal.utils.egg_link import egg_link_path_from_sys_path
 from pip._internal.utils.misc import is_local, normalize_path
+from pip._internal.utils.packaging import safe_extra
 from pip._internal.utils.urls import url_to_path
 
 if TYPE_CHECKING:
@@ -91,6 +94,12 @@ def _convert_installed_files_path(
     return str(pathlib.Path(*info, *entry))
 
 
+class RequiresEntry(NamedTuple):
+    requirement: str
+    extra: str
+    marker: str
+
+
 class BaseDistribution(Protocol):
     @classmethod
     def from_directory(cls, directory: str) -> "BaseDistribution":
@@ -348,6 +357,17 @@ def read_text(self, path: InfoPath) -> str:
     def iter_entry_points(self) -> Iterable[BaseEntryPoint]:
         raise NotImplementedError()
 
+    def _metadata_impl(self) -> email.message.Message:
+        raise NotImplementedError()
+
+    @functools.lru_cache(maxsize=1)
+    def _metadata(self) -> email.message.Message:
+        # When we drop python 3.7 support, move this to the metadata property and use
+        # functools.cached_property instead of lru_cache.
+        metadata = self._metadata_impl()
+        self._add_egg_info_requires(metadata)
+        return metadata
+
     @property
     def metadata(self) -> email.message.Message:
         """Metadata of distribution parsed from e.g. METADATA or PKG-INFO.
@@ -357,7 +377,7 @@ def metadata(self) -> email.message.Message:
         :raises NoneMetadataError: If the metadata file is available, but does
             not contain valid metadata.
         """
-        raise NotImplementedError()
+        return self._metadata()
 
     @property
     def metadata_version(self) -> Optional[str]:
@@ -451,6 +471,76 @@ def iter_declared_entries(self) -> Optional[Iterator[str]]:
             or self._iter_declared_entries_from_legacy()
         )
 
+    def _iter_requires_txt_entries(self) -> Iterator[RequiresEntry]:
+        """Parse a ``requires.txt`` in an egg-info directory.
+
+        This is an INI-ish format where an egg-info stores dependencies. A
+        section name describes extra other environment markers, while each entry
+        is an arbitrary string (not a key-value pair) representing a dependency
+        as a requirement string (no markers).
+
+        There is a construct in ``importlib.metadata`` called ``Sectioned`` that
+        does mostly the same, but the format is currently considered private.
+        """
+        try:
+            content = self.read_text("requires.txt")
+        except FileNotFoundError:
+            return
+        extra = marker = ""  # Section-less entries don't have markers.
+        for line in content.splitlines():
+            line = line.strip()
+            if not line or line.startswith("#"):  # Comment; ignored.
+                continue
+            if line.startswith("[") and line.endswith("]"):  # A section header.
+                extra, _, marker = line.strip("[]").partition(":")
+                continue
+            yield RequiresEntry(requirement=line, extra=extra, marker=marker)
+
+    def _iter_egg_info_extras(self) -> Iterable[str]:
+        """Get extras from the egg-info directory."""
+        known_extras = {""}
+        for entry in self._iter_requires_txt_entries():
+            if entry.extra in known_extras:
+                continue
+            known_extras.add(entry.extra)
+            yield entry.extra
+
+    def _iter_egg_info_dependencies(self) -> Iterable[str]:
+        """Get distribution dependencies from the egg-info directory.
+
+        To ease parsing, this converts a legacy dependency entry into a PEP 508
+        requirement string. Like ``_iter_requires_txt_entries()``, there is code
+        in ``importlib.metadata`` that does mostly the same, but not do exactly
+        what we need.
+
+        Namely, ``importlib.metadata`` does not normalize the extra name before
+        putting it into the requirement string, which causes marker comparison
+        to fail because the dist-info format do normalize. This is consistent in
+        all currently available PEP 517 backends, although not standardized.
+        """
+        for entry in self._iter_requires_txt_entries():
+            if entry.extra and entry.marker:
+                marker = f'({entry.marker}) and extra == "{safe_extra(entry.extra)}"'
+            elif entry.extra:
+                marker = f'extra == "{safe_extra(entry.extra)}"'
+            elif entry.marker:
+                marker = entry.marker
+            else:
+                marker = ""
+            if marker:
+                yield f"{entry.requirement} ; {marker}"
+            else:
+                yield entry.requirement
+
+    def _add_egg_info_requires(self, metadata: email.message.Message) -> None:
+        """Add egg-info requires.txt information to the metadata."""
+        if not metadata.get_all("Requires-Dist"):
+            for dep in self._iter_egg_info_dependencies():
+                metadata["Requires-Dist"] = dep
+        if not metadata.get_all("Provides-Extra"):
+            for extra in self._iter_egg_info_extras():
+                metadata["Provides-Extra"] = extra
+
 
 class BaseEnvironment:
     """An environment containing distributions to introspect."""

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

@@ -3,16 +3,7 @@
 import os
 import pathlib
 import zipfile
-from typing import (
-    Collection,
-    Dict,
-    Iterable,
-    Iterator,
-    Mapping,
-    NamedTuple,
-    Optional,
-    Sequence,
-)
+from typing import Collection, Dict, Iterable, Iterator, Mapping, Optional, Sequence
 
 from pip._vendor.packaging.requirements import Requirement
 from pip._vendor.packaging.utils import NormalizedName, canonicalize_name
@@ -92,12 +83,6 @@ def read_text(self, filename: str) -> Optional[str]:
         return text
 
 
-class RequiresEntry(NamedTuple):
-    requirement: str
-    extra: str
-    marker: str
-
-
 class Distribution(BaseDistribution):
     def __init__(
         self,
@@ -187,84 +172,17 @@ def iter_entry_points(self) -> Iterable[BaseEntryPoint]:
         # importlib.metadata's EntryPoint structure sasitfies BaseEntryPoint.
         return self._dist.entry_points
 
-    @property
-    def metadata(self) -> email.message.Message:
+    def _metadata_impl(self) -> email.message.Message:
         return self._dist.metadata
 
-    def _iter_requires_txt_entries(self) -> Iterator[RequiresEntry]:
-        """Parse a ``requires.txt`` in an egg-info directory.
-
-        This is an INI-ish format where an egg-info stores dependencies. A
-        section name describes extra other environment markers, while each entry
-        is an arbitrary string (not a key-value pair) representing a dependency
-        as a requirement string (no markers).
-
-        There is a construct in ``importlib.metadata`` called ``Sectioned`` that
-        does mostly the same, but the format is currently considered private.
-        """
-        content = self._dist.read_text("requires.txt")
-        if content is None:
-            return
-        extra = marker = ""  # Section-less entries don't have markers.
-        for line in content.splitlines():
-            line = line.strip()
-            if not line or line.startswith("#"):  # Comment; ignored.
-                continue
-            if line.startswith("[") and line.endswith("]"):  # A section header.
-                extra, _, marker = line.strip("[]").partition(":")
-                continue
-            yield RequiresEntry(requirement=line, extra=extra, marker=marker)
-
-    def _iter_egg_info_extras(self) -> Iterable[str]:
-        """Get extras from the egg-info directory."""
-        known_extras = {""}
-        for entry in self._iter_requires_txt_entries():
-            if entry.extra in known_extras:
-                continue
-            known_extras.add(entry.extra)
-            yield entry.extra
-
     def iter_provided_extras(self) -> Iterable[str]:
-        iterator = (
-            self._dist.metadata.get_all("Provides-Extra")
-            or self._iter_egg_info_extras()
+        return (
+            safe_extra(extra) for extra in self.metadata.get_all("Provides-Extra", [])
         )
-        return (safe_extra(extra) for extra in iterator)
-
-    def _iter_egg_info_dependencies(self) -> Iterable[str]:
-        """Get distribution dependencies from the egg-info directory.
-
-        To ease parsing, this converts a legacy dependency entry into a PEP 508
-        requirement string. Like ``_iter_requires_txt_entries()``, there is code
-        in ``importlib.metadata`` that does mostly the same, but not do exactly
-        what we need.
-
-        Namely, ``importlib.metadata`` does not normalize the extra name before
-        putting it into the requirement string, which causes marker comparison
-        to fail because the dist-info format do normalize. This is consistent in
-        all currently available PEP 517 backends, although not standardized.
-        """
-        for entry in self._iter_requires_txt_entries():
-            if entry.extra and entry.marker:
-                marker = f'({entry.marker}) and extra == "{safe_extra(entry.extra)}"'
-            elif entry.extra:
-                marker = f'extra == "{safe_extra(entry.extra)}"'
-            elif entry.marker:
-                marker = entry.marker
-            else:
-                marker = ""
-            if marker:
-                yield f"{entry.requirement} ; {marker}"
-            else:
-                yield entry.requirement
 
     def iter_dependencies(self, extras: Collection[str] = ()) -> Iterable[Requirement]:
-        req_string_iterator = (
-            self._dist.metadata.get_all("Requires-Dist")
-            or self._iter_egg_info_dependencies()
-        )
         contexts: Sequence[Dict[str, str]] = [{"extra": safe_extra(e)} for e in extras]
-        for req_string in req_string_iterator:
+        for req_string in self.metadata.get_all("Requires-Dist", []):
             req = Requirement(req_string)
             if not req.marker:
                 yield req

src/pip/_internal/metadata/pkg_resources.py

@@ -171,8 +171,7 @@ def iter_entry_points(self) -> Iterable[BaseEntryPoint]:
                 name, _, value = str(entry_point).partition("=")
                 yield EntryPoint(name=name.strip(), value=value.strip(), group=group)
 
-    @property
-    def metadata(self) -> email.message.Message:
+    def _metadata_impl(self) -> email.message.Message:
         """
         :raises NoneMetadataError: if the distribution reports `has_metadata()`
             True but `get_metadata()` returns None.

tests/unit/metadata/test_metadata.py

@@ -1,11 +1,12 @@
 import logging
+from pathlib import Path
 from typing import cast
 from unittest import mock
 
 import pytest
 from pip._vendor.packaging.utils import NormalizedName
 
-from pip._internal.metadata import BaseDistribution
+from pip._internal.metadata import BaseDistribution, get_directory_distribution
 from pip._internal.models.direct_url import DIRECT_URL_METADATA_NAME, ArchiveInfo
 
 
@@ -40,6 +41,32 @@ class FakeDistribution(BaseDistribution):
     )
 
 
+def test_metadata_reads_egg_info_requires_txt(tmp_path: Path) -> None:
+    """Check Requires-Dist is obtained from requires.txt if absent in PKG-INFO."""
+    egg_info_path = tmp_path / "whatever.egg-info"
+    egg_info_path.mkdir()
+    dist = get_directory_distribution(str(egg_info_path))
+    assert dist.installed_with_setuptools_egg_info
+    pkg_info_path = egg_info_path / "PKG-INFO"
+    pkg_info_path.write_text("Name: whatever\n")
+    egg_info_path.joinpath("requires.txt").write_text("pkga\npkgb\n")
+    assert dist.metadata.get_all("Requires-Dist") == ["pkga", "pkgb"]
+
+
+def test_metadata_pkg_info_requires_priority(tmp_path: Path) -> None:
+    """Check Requires-Dist in PKG-INFO has priority over requires.txt."""
+    egg_info_path = tmp_path / "whatever.egg-info"
+    egg_info_path.mkdir()
+    dist = get_directory_distribution(str(egg_info_path))
+    assert dist.installed_with_setuptools_egg_info
+    pkg_info_path = egg_info_path / "PKG-INFO"
+    pkg_info_path.write_text(
+        "Name: whatever\nRequires-Dist: pkgc\nRequires-Dist: pkgd\n"
+    )
+    egg_info_path.joinpath("requires.txt").write_text("pkga\npkgb\n")
+    assert dist.metadata.get_all("Requires-Dist") == ["pkgc", "pkgd"]
+
+
 @mock.patch.object(
     BaseDistribution,
     "read_text",