plumbing/object: Support mergetag in merge commits

When a merge commit is created from merging a signed tag, the tag object
is embedded in the commit object. This commit adds support for this tag
data when encoding and decoding a commit object.

Signed-off-by: Aditya Sirish <aditya@saky.in>

2 files changed, 83 insertions, 0 deletions

diff --git a/plumbing/object/commit.go b/plumbing/object/commit.go
index 8a0f35c..508e93c 100644
--- a/plumbing/object/commit.go
+++ b/plumbing/object/commit.go
@@ -20,6 +20,11 @@ const (
 	beginpgp  string = "-----BEGIN PGP SIGNATURE-----"
 	endpgp    string = "-----END PGP SIGNATURE-----"
 	headerpgp string = "gpgsig"
+
+	// https://github.com/git/git/blob/bcb6cae2966cc407ca1afc77413b3ef11103c175/Documentation/gitformat-signature.txt#L153
+	// When a merge commit is created from a signed tag, the tag is embedded in
+	// the commit with the "mergetag" header.
+	headermergetag string = "mergetag"
 )
 
 // Hash represents the hash of an object
@@ -38,6 +43,9 @@ type Commit struct {
 	// Committer is the one performing the commit, might be different from
 	// Author.
 	Committer Signature
+	// MergeTag is the embedded tag object when a merge commit is created by
+	// merging a signed tag.
+	MergeTag string
 	// PGPSignature is the PGP signature of the commit.
 	PGPSignature string
 	// Message is the commit message, contains arbitrary text.
@@ -184,6 +192,7 @@ func (c *Commit) Decode(o plumbing.EncodedObject) (err error) {
 	defer sync.PutBufioReader(r)
 
 	var message bool
+	var mergetag bool
 	var pgpsig bool
 	var msgbuf bytes.Buffer
 	for {
@@ -192,6 +201,16 @@ func (c *Commit) Decode(o plumbing.EncodedObject) (err error) {
 			return err
 		}
 
+		if mergetag {
+			if len(line) > 0 && line[0] == ' ' {
+				line = bytes.TrimLeft(line, " ")
+				c.MergeTag += string(line)
+				continue
+			} else {
+				mergetag = false
+			}
+		}
+
 		if pgpsig {
 			if len(line) > 0 && line[0] == ' ' {
 				line = bytes.TrimLeft(line, " ")
@@ -225,6 +244,9 @@ func (c *Commit) Decode(o plumbing.EncodedObject) (err error) {
 				c.Author.Decode(data)
 			case "committer":
 				c.Committer.Decode(data)
+			case headermergetag:
+				c.MergeTag += string(data) + "\n"
+				mergetag = true
 			case headerpgp:
 				c.PGPSignature += string(data) + "\n"
 				pgpsig = true
@@ -286,6 +308,22 @@ func (c *Commit) encode(o plumbing.EncodedObject, includeSig bool) (err error) {
 		return err
 	}
 
+	if c.MergeTag != "" {
+		if _, err = fmt.Fprint(w, "\n"+headermergetag+" "); err != nil {
+			return err
+		}
+
+		// Split tag information lines and re-write with a left padding and
+		// newline. Use join for this so it's clear that a newline should not be
+		// added after this section. The newline will be added either as part of
+		// the PGP signature or the commit message.
+		mergetag := strings.TrimSuffix(c.MergeTag, "\n")
+		lines := strings.Split(mergetag, "\n")
+		if _, err = fmt.Fprint(w, strings.Join(lines, "\n ")); err != nil {
+			return err
+		}
+	}
+
 	if c.PGPSignature != "" && includeSig {
 		if _, err = fmt.Fprint(w, "\n"+headerpgp+" "); err != nil {
 			return err
diff --git a/plumbing/object/commit_test.go b/plumbing/object/commit_test.go
index 4b0f6b4..f1a3447 100644
--- a/plumbing/object/commit_test.go
+++ b/plumbing/object/commit_test.go
@@ -3,6 +3,7 @@ package object
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"io"
 	"strings"
 	"time"
@@ -197,6 +198,27 @@ func (s *SuiteCommit) TestPatchContext_ToNil(c *C) {
 }
 
 func (s *SuiteCommit) TestCommitEncodeDecodeIdempotent(c *C) {
+	pgpsignature := `-----BEGIN PGP SIGNATURE-----
+
+iQEcBAABAgAGBQJTZbQlAAoJEF0+sviABDDrZbQH/09PfE51KPVPlanr6q1v4/Ut
+LQxfojUWiLQdg2ESJItkcuweYg+kc3HCyFejeDIBw9dpXt00rY26p05qrpnG+85b
+hM1/PswpPLuBSr+oCIDj5GMC2r2iEKsfv2fJbNW8iWAXVLoWZRF8B0MfqX/YTMbm
+ecorc4iXzQu7tupRihslbNkfvfciMnSDeSvzCpWAHl7h8Wj6hhqePmLm9lAYqnKp
+8S5B/1SSQuEAjRZgI4IexpZoeKGVDptPHxLLS38fozsyi0QyDyzEgJxcJQVMXxVi
+RUysgqjcpT8+iQM1PblGfHR4XAhuOqN5Fx06PSaFZhqvWFezJ28/CLyX5q+oIVk=
+=EFTF
+-----END PGP SIGNATURE-----
+`
+
+	tag := fmt.Sprintf(`object f000000000000000000000000000000000000000
+type commit
+tag change
+tagger Foo <foo@example.local> 1695827841 -0400
+
+change
+%s
+`, pgpsignature)
+
 	ts, err := time.Parse(time.RFC3339, "2006-01-02T15:04:05-07:00")
 	c.Assert(err, IsNil)
 	commits := []*Commit{
@@ -219,6 +241,29 @@ func (s *SuiteCommit) TestCommitEncodeDecodeIdempotent(c *C) {
 				plumbing.NewHash("f000000000000000000000000000000000000007"),
 			},
 		},
+		{
+			Author:    Signature{Name: "Foo", Email: "foo@example.local", When: ts},
+			Committer: Signature{Name: "Bar", Email: "bar@example.local", When: ts},
+			Message:   "Testing mergetag\n\nHere, commit is not signed",
+			TreeHash:  plumbing.NewHash("f000000000000000000000000000000000000001"),
+			ParentHashes: []plumbing.Hash{
+				plumbing.NewHash("f000000000000000000000000000000000000002"),
+				plumbing.NewHash("f000000000000000000000000000000000000003"),
+			},
+			MergeTag: tag,
+		},
+		{
+			Author:    Signature{Name: "Foo", Email: "foo@example.local", When: ts},
+			Committer: Signature{Name: "Bar", Email: "bar@example.local", When: ts},
+			Message:   "Testing mergetag\n\nHere, commit is also signed",
+			TreeHash:  plumbing.NewHash("f000000000000000000000000000000000000001"),
+			ParentHashes: []plumbing.Hash{
+				plumbing.NewHash("f000000000000000000000000000000000000002"),
+				plumbing.NewHash("f000000000000000000000000000000000000003"),
+			},
+			MergeTag:     tag,
+			PGPSignature: pgpsignature,
+		},
 	}
 	for _, commit := range commits {
 		obj := &plumbing.MemoryObject{}