git_patch_from_diff may return a NULL patch without an error

Author: jorio

pygit2/_pygit2.pyi

@@ -432,8 +432,8 @@ class Diff:
 deffrom_c(diff, repo) ->Diff: ...
 @staticmethod
 defparse_diff(git_diff: str|bytes) ->Diff: ...
-def__getitem__(self, index: int) ->Patch: ... # Diff_getitem
-def__iter__(self) ->Iterator[Patch]: ... # -> DiffIter
+def__getitem__(self, index: int) ->Patch|None: ... # Diff_getitem
+def__iter__(self) ->Iterator[Patch|None]: ... # -> DiffIter
 def__len__(self) ->int: ...
 
 classDiffDelta:

src/diff.c

@@ -533,6 +533,11 @@ diff_get_patch_byindex(git_diff *diff, size_t idx)
 if (err<0)
 returnError_set(err);
 
+/* libgit2 may decide not to create a patch if the file is
+ "unchanged or binary", but this isn't an error case */
+if (patch==NULL)
+Py_RETURN_NONE;
+
 return (PyObject*) wrap_patch(patch, NULL, NULL);
 }
 

test/test_diff.py

@@ -28,6 +28,7 @@
 importtextwrap
 fromcollections.abcimportIterator
 fromitertoolsimportchain
+frompathlibimportPath
 
 importpytest
 
@@ -459,3 +460,33 @@ def test_diff_blobs(emptyrepo: Repository) -> None:
 assertdiff_one_context_line.text==PATCH_BLOBS_ONE_CONTEXT_LINE
 diff_all_together=repo.diff(blob1, blob2, context_lines=1, interhunk_lines=1)
 assertdiff_all_together.text==PATCH_BLOBS_DEFAULT
+
+
+deftest_diff_unchanged_file_no_patch(testrepo) ->None:
+repo=testrepo
+
+# Convert hello.txt line endings to CRLF
+path=Path(repo.workdir) /'hello.txt'
+data=path.read_bytes()
+data=data.replace(b'\n', b'\r\n')
+path.write_bytes(data)
+
+# Enable CRLF filter
+repo.config['core.autocrlf'] ='input'
+
+diff=repo.diff()
+assertlen(diff) ==1
+
+# Get patch #0 in the same diff several times.
+# git_patch_from_diff eventually decides that the file is "unchanged"
+# it returns a NULL patch in this case.
+# https://libgit2.org/docs/reference/main/patch/git_patch_from_diff
+foriinrange(10): # loop typically exits in the third iteration
+patch=diff[0]
+ifpatchisNone: # libgit2 decides the file is unchanged
+break
+assertpatch.delta.new_file.path==path.name
+assertpatch.text==''# no content change (just line endings)
+else:
+# Didn't find the edge case that this test is supposed to exercise.
+assertFalse, 'libgit2 rebuilt a new patch every time'