pkg/vcs: be more strict in BaseForDiff

Do not tolerate unknown blob hashes - it means that we are unable to
find the correct base commit given the repository.
Explicitly ignore newly added files - we definitely won't find their
hashes.
Explicitly skip malformed patches that won't have any blob hashes -
otherwise we could end up with too many candidates and waste too much
time.

Author: a-nogikh

pkg/vcs/git.go

@@ -776,17 +776,22 @@ func (git Git) BaseForDiff(diff []byte, tracer debugtracer.DebugTracer) ([]*Base
 	}
 	var fileNames []string
 	nameToHash := map[string]string{}
+	ignoreFiles := map[string]struct{}{}
 	for _, file := range ParseGitDiff(diff) {
 		if strings.Trim(file.LeftHash, "0") == "" {
-			// Newly created file are not of any help here.
+			// Newly created files are not of any help here.
+			ignoreFiles[file.Name] = struct{}{}
+			continue
+		}
+		if _, ignore := ignoreFiles[file.Name]; ignore {
 			continue
 		}
 		if ok, err := git.verifyHash(file.LeftHash); err != nil {
 			return nil, fmt.Errorf("hash verification failed: %w", err)
 		} else if !ok {
-			// The object is not known in this repository.
-			// Ignore it, or otherwise the command will fail.
-			continue
+			// The object is not known in this repository, so we won't find the exact base commit.
+			tracer.Log("unknown object %s, stopping base commit search", file.LeftHash)
+			return nil, nil
 		}
 		if _, ok := nameToHash[file.Name]; !ok {
 			// If the diff is actually a concatenation of several diffs, we only
@@ -797,6 +802,9 @@ func (git Git) BaseForDiff(diff []byte, tracer debugtracer.DebugTracer) ([]*Base
 		args = append(args, "--find-object="+file.LeftHash)
 	}
 	tracer.Log("extracted %d left blob hashes", len(nameToHash))
+	if len(nameToHash) == 0 {
+		return nil, nil
+	}
 	output, err := git.Run(args...)
 	if err != nil {
 		return nil, err

pkg/vcs/git_test.go

@@ -551,8 +551,8 @@ func TestBaseForDiff(t *testing.T) {
 		assert.Equal(t, commit4.Hash, base[0].Hash)
 		assert.Equal(t, commit2.Hash, base[1].Hash)
 	})
-	t.Run("ignore unknown objects", func(t *testing.T) {
-		// It's okay if the diff contains unknown hashes.
+	t.Run("unknown objects", func(t *testing.T) {
+		// It's not okay if the diff contains unknown hashes.
 		diff2 := `
 diff --git a/b.txt b/b.txt
 deleted file mode 100644
@@ -561,9 +561,36 @@ index f70f10e..0000000
 +++ /dev/null
 @@ -1 +0,0 @@
 -A`
+		twoDiffs := append(append([]byte{}, diff...), diff2...)
+		base, err := repo.repo.BaseForDiff(twoDiffs, &debugtracer.TestTracer{T: t})
+		require.NoError(t, err)
+		require.Nil(t, base)
+	})
+	t.Run("ignore new files", func(t *testing.T) {
+		diff2 := `
+diff --git a/a.txt b/a.txt
+new file mode 100644
+index 0000000..fa49b07
+--- /dev/null
++++ b/a.txt
+@@ -0,0 +1 @@
++new file
+diff --git a/a.txt b/a.txt
+index fa49b07..01c887f 100644
+--- a/a.txt
++++ b/a.txt
+@@ -1 +1 @@
+-new file
++edit file
+`
 		twoDiffs := append(append([]byte{}, diff...), diff2...)
 		base, err := repo.repo.BaseForDiff(twoDiffs, &debugtracer.TestTracer{T: t})
 		require.NoError(t, err)
 		require.Len(t, base, 2)
 	})
+	t.Run("empty patch", func(t *testing.T) {
+		base, err := repo.repo.BaseForDiff([]byte{}, &debugtracer.TestTracer{T: t})
+		require.NoError(t, err)
+		require.Nil(t, base)
+	})
 }