Send only the digests we have from PackageFile (#776)

* Send only the digests we have from PackageFile

Once again, we have to deal with third-party package repositories doing
their own thing and users needing this to just work. Those repositories
combined with FIPS mean that we will need to send the information we
actually have and only that when it comes to digests.

Closes gh-775

* Update function and comments for clarity

Let's be clearer about some of the bizarre logic we use to support 
repositories other than PyPI and distributions using FIPS

Co-authored-by: Brian Rutledge <bhrutledge@gmail.com>

Co-authored-by: Brian Rutledge <bhrutledge@gmail.com>

changelog/776.bugfix.rst

@@ -0,0 +1,3 @@
+Do not include md5_digest or blake2_256_digest if FIPS mode is enabled on the
+host. This removes those fields from the metadata before sending the metadata
+to the repository.

tests/test_package.py

@@ -262,6 +262,24 @@ def test_fips_hash_manager_blake2(monkeypatch):
     assert hasher.hexdigest() == hashes
 
 
+def test_fips_metadata_excludes_md5_and_blake2(monkeypatch):
+    """Generate a valid metadata dictionary for Nexus when FIPS is enabled.
+
+    See also: https://github.com/pypa/twine/issues/775
+    """
+    replaced_blake2b = pretend.raiser(ValueError("fipsmode"))
+    replaced_md5 = pretend.raiser(ValueError("fipsmode"))
+    monkeypatch.setattr(package_file.hashlib, "md5", replaced_md5)
+    monkeypatch.setattr(package_file.hashlib, "blake2b", replaced_blake2b)
+
+    filename = "tests/fixtures/twine-1.5.0-py2.py3-none-any.whl"
+    pf = package_file.PackageFile.from_filename(filename, None)
+
+    mddict = pf.metadata_dictionary()
+    assert "md5_digest" not in mddict
+    assert "blake2_256_digest" not in mddict
+
+
 def test_pkginfo_returns_no_metadata(monkeypatch):
     """Raise an exception when pkginfo can't interpret the metadata.
 

twine/package.py

@@ -152,9 +152,7 @@ def metadata_dictionary(self) -> Dict[str, MetadataValue]:
             "download_url": meta.download_url,
             "supported_platform": meta.supported_platforms,
             "comment": self.comment,
-            "md5_digest": self.md5_digest,
             "sha256_digest": self.sha2_digest,
-            "blake2_256_digest": self.blake2_256_digest,
             # PEP 314
             "provides": meta.provides,
             "requires": meta.requires,
@@ -174,6 +172,15 @@ def metadata_dictionary(self) -> Dict[str, MetadataValue]:
         if self.gpg_signature is not None:
             data["gpg_signature"] = self.gpg_signature
 
+        # FIPS disables MD5 and Blake2, making the digest values None. Some package
+        # repositories don't allow null values, so this only sends non-null values.
+        # See also: https://github.com/pypa/twine/issues/775
+        if self.md5_digest:
+            data["md5_digest"] = self.md5_digest
+
+        if self.blake2_256_digest:
+            data["blake2_256_digest"] = self.blake2_256_digest
+
         return data
 
     def add_gpg_signature(