Fix handling of continuation lines in Description metadata field

Fixes #1218.

changelog/1220.bugfix.rst

@@ -0,0 +1 @@
+Fix handling of multiline ``Description`` metadata fields.

tests/test_package.py

@@ -463,3 +463,58 @@ def test_setuptools_license_file(read_data, filtered, monkeypatch):
     package = package_file.PackageFile.from_filename(filename, comment=None)
     meta = package.metadata_dictionary()
     assert filtered != ("license_file" in meta)
+
+
+@pytest.mark.parametrize(
+    "description,expected",
+    [
+        # fmt: off
+        pytest.param(
+            "\n"
+            "Two\n"
+            "Lines\n",
+            "Two\nLines\n",
+            id="body",
+        ),
+        pytest.param(
+            "Description: Two\n"
+            "       |Lines\n"
+            "       |\n",
+            "Two\nLines\n",
+            id="multiline-header",
+        ),
+        pytest.param(
+            "Description: Two\n"
+            "        Lines\n"
+            "        \n",
+            "Two\nLines\n",
+            id="multiline-header-setuptools",
+        ),
+        pytest.param(
+            "Description: Two\n"
+            "        Lines\n"
+            "  Maybe Three",
+            "Two\n        Lines\n  Maybe Three",
+            id="multiline-inconsistent",
+        ),
+        pytest.param(
+            "Description: Two\n"
+            "       |Lines\n"
+            "        Maybe Three",
+            "Two\n       |Lines\n        Maybe Three",
+            id="multiline-mixed",
+        ),
+        # fmt: on
+    ],
+)
+def test_description_field_continuation(description, expected, monkeypatch):
+    """License-File metadata entries are kept when Metadata-Version is 2.4."""
+    read_data = (
+        "Metadata-Version: 2.4\n"
+        "Name: test-package\n"
+        "Version: 1.0.0\n" + description
+    )
+    monkeypatch.setattr(package_file.wheel.Wheel, "read", lambda _: read_data)
+    filename = "tests/fixtures/twine-1.5.0-py2.py3-none-any.whl"
+    package = package_file.PackageFile.from_filename(filename, comment=None)
+    assert package.metadata["description"] == expected

twine/package.py

@@ -65,6 +65,28 @@ def _safe_name(name: str) -> str:
     return re.sub("[^A-Za-z0-9.]+", "-", name)
 
 
+def _dedent(string: str) -> str:
+    """Remove line continuation suffix used for the ``Description`` metadata field.
+
+    The metadata standard prescribes that continuation lines should be
+    prefixed with 7 spaces followed by a pipe character, however, setuptools
+    used to prefix continuation lines with 8 spaces. Handle both here.
+
+    If not all lines following the first start with either of the accepted
+    prefixes, the string is returned without modifications.
+    """
+    lines = string.splitlines()
+    if (
+        False  # This is here only to force black into something sensible.
+        or all(line.startswith("       |") for line in lines[1:])
+        or all(line.startswith("        ") for line in lines[1:])
+    ):
+        for i in range(1, len(lines)):
+            lines[i] = lines[i][8:]
+        return "\n".join(lines)
+    return string
+
+
 # Map ``metadata.RawMetadata`` fields to ``PackageMetadata`` fields.  Some
 # fields are renamed to match the names expected in the upload form.
 _RAW_TO_PACKAGE_METADATA = {
@@ -232,6 +254,11 @@ def from_filename(cls, filename: str, comment: Optional[str]) -> "PackageFile":
         # than 2.4.
         if version.Version(meta.get("metadata_version", "0")) < version.Version("2.4"):
             meta.pop("license_files", None)
+
+        description = meta.get("description", None)
+        if description is not None:
+            meta["description"] = _dedent(description)
+
         try:
             metadata.Metadata.from_raw(meta)
         except metadata.ExceptionGroup as group: