git: Extract function to get object hash by format name

Extract a function that allows the caller to retrieve an object hash by
the format name, e.g. "sha1" or "sha256". This functionality will be
required by the `stats` package in a subsequent commit.

2 files changed, 49 insertions, 9 deletions

diff --git a/internal/git/object_id.go b/internal/git/object_id.go
index 2826215d3..d03832283 100644
--- a/internal/git/object_id.go
+++ b/internal/git/object_id.go
@@ -21,6 +21,7 @@ var (
 		Hash:         sha1.New,
 		EmptyTreeOID: ObjectID("4b825dc642cb6eb9a060e54bf8d69288fbee4904"),
 		ZeroOID:      ObjectID("0000000000000000000000000000000000000000"),
+		Format:       "sha1",
 	}
 
 	// ObjectHashSHA256 is the implementation of an object ID via SHA256.
@@ -29,6 +30,7 @@ var (
 		Hash:         sha256.New,
 		EmptyTreeOID: ObjectID("6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321"),
 		ZeroOID:      ObjectID("0000000000000000000000000000000000000000000000000000000000000000"),
+		Format:       "sha256",
 	}
 
 	// ErrInvalidObjectID is returned in case an object ID's string
@@ -45,6 +47,20 @@ type ObjectHash struct {
 	EmptyTreeOID ObjectID
 	// ZeroOID is the special value that Git uses to signal a ref or object does not exist
 	ZeroOID ObjectID
+	// Format is the name of the object hash.
+	Format string
+}
+
+// ObjectHashByFormat looks up the ObjectHash by its format name.
+func ObjectHashByFormat(format string) (ObjectHash, error) {
+	switch format {
+	case ObjectHashSHA1.Format:
+		return ObjectHashSHA1, nil
+	case ObjectHashSHA256.Format:
+		return ObjectHashSHA256, nil
+	default:
+		return ObjectHash{}, fmt.Errorf("unknown object format: %q", format)
+	}
 }
 
 // DetectObjectHash detects the object-hash used by the given repository.
@@ -60,15 +76,7 @@ func DetectObjectHash(ctx context.Context, repoExecutor RepositoryExecutor) (Obj
 		return ObjectHash{}, fmt.Errorf("reading object format: %w, stderr: %q", err, stderr.String())
 	}
 
-	objectFormat := text.ChompBytes(stdout.Bytes())
-	switch objectFormat {
-	case "sha1":
-		return ObjectHashSHA1, nil
-	case "sha256":
-		return ObjectHashSHA256, nil
-	default:
-		return ObjectHash{}, fmt.Errorf("unknown object format: %q", objectFormat)
-	}
+	return ObjectHashByFormat(text.ChompBytes(stdout.Bytes()))
 }
 
 // EncodedLen returns the length of the hex-encoded string of a full object ID.
diff --git a/internal/git/object_id_test.go b/internal/git/object_id_test.go
index 69b900cc9..9711e7e2e 100644
--- a/internal/git/object_id_test.go
+++ b/internal/git/object_id_test.go
@@ -20,6 +20,38 @@ import (
 	"gitlab.com/gitlab-org/gitaly/v15/proto/go/gitalypb"
 )
 
+func TestObjectHashByFormat(t *testing.T) {
+	for _, tc := range []struct {
+		format             string
+		expectedErr        error
+		expectedObjectHash git.ObjectHash
+	}{
+		{
+			format:             "sha1",
+			expectedObjectHash: git.ObjectHashSHA1,
+		},
+		{
+			format:             "sha256",
+			expectedObjectHash: git.ObjectHashSHA256,
+		},
+		{
+			format:      "invalid",
+			expectedErr: fmt.Errorf("unknown object format: %q", "invalid"),
+		},
+	} {
+		t.Run(tc.format, func(t *testing.T) {
+			objectHash, err := git.ObjectHashByFormat(tc.format)
+			require.Equal(t, tc.expectedErr, err)
+
+			// Function pointers cannot be compared, so we need to unset them.
+			objectHash.Hash = nil
+			tc.expectedObjectHash.Hash = nil
+
+			require.Equal(t, tc.expectedObjectHash, objectHash)
+		})
+	}
+}
+
 func TestDetectObjectHash(t *testing.T) {
 	cfg := testcfg.Build(t)
 	ctx := testhelper.Context(t)