Add DictMetadata for adapting zip data to Distribution

pkg_resources.Distribution classes delegate to the IMetadataProvider
implementation provided at construction. This is the one we'll use for
adapting a ZipFile to pkg_resources.DistInfoDistribution.

src/pip/_internal/utils/pkg_resources.py

@@ -0,0 +1,44 @@
+from pip._vendor.pkg_resources import yield_lines
+from pip._vendor.six import ensure_str
+
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import Dict, Iterable, List
+
+
+class DictMetadata(object):
+    """IMetadataProvider that reads metadata files from a dictionary.
+    """
+    def __init__(self, metadata):
+        # type: (Dict[str, bytes]) -> None
+        self._metadata = metadata
+
+    def has_metadata(self, name):
+        # type: (str) -> bool
+        return name in self._metadata
+
+    def get_metadata(self, name):
+        # type: (str) -> str
+        try:
+            return ensure_str(self._metadata[name])
+        except UnicodeDecodeError as e:
+            # Mirrors handling done in pkg_resources.NullProvider.
+            e.reason += " in {} file".format(name)
+            raise
+
+    def get_metadata_lines(self, name):
+        # type: (str) -> Iterable[str]
+        return yield_lines(self.get_metadata(name))
+
+    def metadata_isdir(self, name):
+        # type: (str) -> bool
+        return False
+
+    def metadata_listdir(self, name):
+        # type: (str) -> List[str]
+        return []
+
+    def run_script(self, script_name, namespace):
+        # type: (str, str) -> None
+        pass

tests/unit/test_utils_pkg_resources.py

@@ -0,0 +1,57 @@
+from email.message import Message
+
+import pytest
+from pip._vendor.pkg_resources import DistInfoDistribution, Requirement
+from pip._vendor.six import ensure_binary
+
+from pip._internal.utils.packaging import get_metadata, get_requires_python
+from pip._internal.utils.pkg_resources import DictMetadata
+from tests.lib import skip_if_python2
+
+
+def test_dict_metadata_works():
+    name = "simple"
+    version = "0.1.0"
+    require_a = "a==1.0"
+    require_b = "b==1.1; extra == 'also_b'"
+    requires = [require_a, require_b, "c==1.2; extra == 'also_c'"]
+    extras = ["also_b", "also_c"]
+    requires_python = ">=3"
+
+    metadata = Message()
+    metadata["Name"] = name
+    metadata["Version"] = version
+    for require in requires:
+        metadata["Requires-Dist"] = require
+    for extra in extras:
+        metadata["Provides-Extra"] = extra
+    metadata["Requires-Python"] = requires_python
+
+    inner_metadata = DictMetadata({
+        "METADATA": ensure_binary(metadata.as_string())
+    })
+    dist = DistInfoDistribution(
+        location="<in-memory>", metadata=inner_metadata, project_name=name
+    )
+
+    assert name == dist.project_name
+    assert version == dist.version
+    assert set(extras) == set(dist.extras)
+    assert [Requirement.parse(require_a)] == dist.requires([])
+    assert [
+        Requirement.parse(require_a), Requirement.parse(require_b)
+    ] == dist.requires(["also_b"])
+    assert metadata.as_string() == get_metadata(dist).as_string()
+    assert requires_python == get_requires_python(dist)
+
+
+# Metadata is not decoded on Python 2, so no chance for error.
+@skip_if_python2
+def test_dict_metadata_throws_on_bad_unicode():
+    metadata = DictMetadata({
+        "METADATA": b"\xff"
+    })
+
+    with pytest.raises(UnicodeDecodeError) as e:
+        metadata.get_metadata("METADATA")
+    assert "METADATA" in str(e.value)