git/odb: support git/odb.Commit.ExtraHeaders, byte-for-byte round trip

git/odb/commit.go

@@ -40,6 +40,19 @@ func (s *Signature) String() string {
 	return fmt.Sprintf("%s <%s> %d %s", s.Name, s.Email, at, zone)
 }
 
+// ExtraHeader encapsulates a key-value pairing of header key to header value.
+// It is stored as a struct{string, string} in memory as opposed to a
+// map[string]string to maintain ordering in a byte-for-byte encode/decode round
+// trip.
+type ExtraHeader struct {
+	// K is the header key, or the first run of bytes up until a ' ' (\x20)
+	// character.
+	K string
+	// V is the header value, or the remaining run of bytes in the line,
+	// stripping off the above "K" field as a prefix.
+	V string
+}
+
 // Commit encapsulates a Git commit entry.
 type Commit struct {
 	// Author is the Author this commit, or the original writer of the
@@ -59,6 +72,9 @@ type Commit struct {
 	ParentIDs [][]byte
 	// TreeID is the root Tree associated with this commit.
 	TreeID []byte
+	// ExtraHeaders stores headers not listed above, for instance
+	// "encoding", "gpgsig", or "mergetag" (among others).
+	ExtraHeaders []*ExtraHeader
 	// Message is the commit message, including any signing information
 	// associated with this commit.
 	Message string
@@ -77,10 +93,16 @@ func (c *Commit) Type() ObjectType { return CommitObjectType }
 // If any error was encountered along the way, that will be returned, along with
 // the number of bytes read up to that point.
 func (c *Commit) Decode(from io.Reader, size int64) (n int, err error) {
+	var finishedHeaders bool
+	var messageParts []string
+
 	s := bufio.NewScanner(from)
 	for s.Scan() {
 		text := s.Text()
+		n = n + len(text+"\n")
+
 		if len(s.Text()) == 0 {
+			finishedHeaders = true
 			continue
 		}
 
@@ -105,17 +127,20 @@ func (c *Commit) Decode(from io.Reader, size int64) (n int, err error) {
 			case "committer":
 				c.Committer = strings.Join(fields[1:], " ")
 			default:
-				if len(c.Message) == 0 {
-					c.Message = s.Text()
+				if finishedHeaders {
+					messageParts = append(messageParts, s.Text())
 				} else {
-					c.Message = strings.Join([]string{c.Message, s.Text()}, "\n")
+					c.ExtraHeaders = append(c.ExtraHeaders, &ExtraHeader{
+						K: fields[0],
+						V: strings.Join(fields[1:], " "),
+					})
 				}
 			}
 		}
-
-		n = n + len(text+"\n")
 	}
 
+	c.Message = strings.Join(messageParts, "\n")
+
 	if err = s.Err(); err != nil {
 		return n, err
 	}
@@ -141,11 +166,26 @@ func (c *Commit) Encode(to io.Writer) (n int, err error) {
 		n = n + n1
 	}
 
-	n2, err := fmt.Fprintf(to, "author %s\ncommitter %s\n\n%s\n",
-		c.Author, c.Committer, c.Message)
+	n2, err := fmt.Fprintf(to, "author %s\ncommitter %s\n", c.Author, c.Committer)
 	if err != nil {
 		return n, err
 	}
 
-	return n + n2, err
+	n = n + n2
+
+	for _, hdr := range c.ExtraHeaders {
+		n3, err := fmt.Fprintf(to, "%s %s\n", hdr.K, hdr.V)
+		if err != nil {
+			return n, err
+		}
+
+		n = n + n3
+	}
+
+	n4, err := fmt.Fprintf(to, "\n%s\n", c.Message)
+	if err != nil {
+		return n, err
+	}
+
+	return n + n4, err
 }

git/odb/commit_test.go

@@ -25,7 +25,10 @@ func TestCommitEncoding(t *testing.T) {
 		ParentIDs: [][]byte{
 			[]byte("aaaaaaaaaaaaaaaaaaaa"), []byte("bbbbbbbbbbbbbbbbbbbb"),
 		},
-		TreeID:  []byte("cccccccccccccccccccc"),
+		TreeID: []byte("cccccccccccccccccccc"),
+		ExtraHeaders: []*ExtraHeader{
+			{"foo", "bar"},
+		},
 		Message: "initial commit",
 	}
 
@@ -39,6 +42,7 @@ func TestCommitEncoding(t *testing.T) {
 	assertLine(t, buf, "parent 6262626262626262626262626262626262626262")
 	assertLine(t, buf, "author %s", author.String())
 	assertLine(t, buf, "committer %s", committer.String())
+	assertLine(t, buf, "foo bar")
 	assertLine(t, buf, "")
 	assertLine(t, buf, "initial commit")
 
@@ -58,8 +62,9 @@ func TestCommitDecoding(t *testing.T) {
 	fmt.Fprintf(from, "committer %s\n", committer)
 	fmt.Fprintf(from, "parent %s\n", hex.EncodeToString(p1))
 	fmt.Fprintf(from, "parent %s\n", hex.EncodeToString(p2))
+	fmt.Fprintf(from, "foo bar\n")
 	fmt.Fprintf(from, "tree %s\n", hex.EncodeToString(treeId))
-	fmt.Fprintf(from, "initial commit\n")
+	fmt.Fprintf(from, "\ninitial commit\n")
 
 	flen := from.Len()
 
@@ -72,6 +77,9 @@ func TestCommitDecoding(t *testing.T) {
 	assert.Equal(t, author.String(), commit.Author)
 	assert.Equal(t, committer.String(), commit.Committer)
 	assert.Equal(t, [][]byte{p1, p2}, commit.ParentIDs)
+	assert.Equal(t, 1, len(commit.ExtraHeaders))
+	assert.Equal(t, "foo", commit.ExtraHeaders[0].K)
+	assert.Equal(t, "bar", commit.ExtraHeaders[0].V)
 	assert.Equal(t, "initial commit", commit.Message)
 }