From 331c1f5b6f41d70fbffcfd22baf8ee0226194484 Mon Sep 17 00:00:00 2001
From: h <h@kotikot.com>
Date: Fri, 2 Oct 2026 23:32:44 +0000
Subject: [PATCH] fix(client): merge concurrent appends at end of file

---
 backend/internal/client/sync.go      | 17 ++++++-
 backend/internal/client/sync_test.go | 71 ++++++++++++++++------------
 2 files changed, 58 insertions(+), 30 deletions(-)

diff --git a/backend/internal/client/sync.go b/backend/internal/client/sync.go
index 22d863b..e7460bd 100644
--- a/backend/internal/client/sync.go
+++ b/backend/internal/client/sync.go
@@ -254,13 +254,28 @@ func (c *Client) merge(ctx context.Context, local, base, remote protocol.File) (
 	cmd := exec.CommandContext(ctx, "git", append([]string{"merge-file", "-p"}, names...)...)
 	cmd.Dir = dir
 	merged, err := cmd.Output()
-	if err != nil || len(merged) > protocol.MaxBlobSize {
+	clean := err == nil
+	if !clean {
+		merged, clean = appended(versions[0], versions[1], versions[2])
+	}
+	if !clean || len(merged) > protocol.MaxBlobSize {
 		return protocol.File{}, false
 	}
 	hash, err := c.cacheBlob(merged)
 	return protocol.File{Hash: hash, Time: max(local.Time, remote.Time)}, err == nil
 }
 
+func appended(local, base, remote []byte) ([]byte, bool) {
+	if !bytes.HasPrefix(local, base) || !bytes.HasPrefix(remote, base) {
+		return nil, false
+	}
+	tail, glue := local[len(base):], []byte{}
+	if !bytes.HasSuffix(remote, []byte("\n")) && !bytes.HasPrefix(tail, []byte("\n")) {
+		glue = []byte("\n")
+	}
+	return slices.Concat(remote, glue, tail), true
+}
+
 func isText(data []byte) bool {
 	return len(data) <= protocol.MaxBlobSize && utf8.Valid(data) && bytes.IndexByte(data, 0) == -1
 }
diff --git a/backend/internal/client/sync_test.go b/backend/internal/client/sync_test.go
index 00d5b6a..7a8c5d2 100644
--- a/backend/internal/client/sync_test.go
+++ b/backend/internal/client/sync_test.go
@@ -203,37 +203,50 @@ func TestLaterEditWinsAndLoserStaysInHistory(t *testing.T) {
 	}
 }
 
-func TestConcurrentEditsOfDifferentLinesMerge(t *testing.T) {
-	mac, dell := setup(t)
-	path := "docs/agents/plan.md"
-	write(t, mac.root, path, "# plan\n\n## todo\n- a\n\n## notes\n- x\n")
-	mac.sync()
-	dell.sync()
+func TestConcurrentEditsMerge(t *testing.T) {
+	for name, versions := range map[string][4]string{
+		"different lines": {
+			"# plan\n\n## todo\n- a\n\n## notes\n- x\n",
+			"# plan\n\n## todo\n- a\n- from mac\n\n## notes\n- x\n",
+			"# plan\n\n## todo\n- a\n\n## notes\n- x\n- from dell\n",
+			"# plan\n\n## todo\n- a\n- from mac\n\n## notes\n- x\n- from dell\n",
+		},
+		"appends":                    {"# log\n- a\n", "# log\n- a\n- from mac\n", "# log\n- a\n- from dell\n", "# log\n- a\n- from dell\n- from mac\n"},
+		"appends without newline":    {"# log\n- a", "# log\n- a\n- from mac", "# log\n- a\n- from dell", "# log\n- a\n- from dell\n- from mac"},
+		"appends to unfinished line": {"# log\n- a", "# log\n- a b", "# log\n- a\n- from dell", "# log\n- a\n- from dell\n b"},
+	} {
+		t.Run(name, func(t *testing.T) {
+			mac, dell := setup(t)
+			path := "docs/agents/plan.md"
+			write(t, mac.root, path, versions[0])
+			mac.sync()
+			dell.sync()
 
-	write(t, mac.root, path, "# plan\n\n## todo\n- a\n- from mac\n\n## notes\n- x\n")
-	write(t, dell.root, path, "# plan\n\n## todo\n- a\n\n## notes\n- x\n- from dell\n")
-	dell.sync()
-	mac.sync()
-	dell.sync()
+			write(t, mac.root, path, versions[1])
+			write(t, dell.root, path, versions[2])
+			dell.sync()
+			mac.sync()
+			dell.sync()
 
-	merged := "# plan\n\n## todo\n- a\n- from mac\n\n## notes\n- x\n- from dell\n"
-	mac.expect(path, merged)
-	dell.expect(path, merged)
-	state, err := mac.client.State(context.Background(), repoKey(t, mac), "")
-	if err != nil {
-		t.Fatal(err)
-	}
-	if got, _ := mac.client.Blob(context.Background(), state.Spaces[protocol.SharedSpace][path].Hash); string(got) != merged {
-		t.Fatalf("server has %q, want %q", got, merged)
-	}
-	for _, m := range []*machine{mac, dell, mac} {
-		if res := m.sync(); res.Pushed != 0 || res.Pulled != 0 {
-			t.Fatalf("merged file keeps syncing: %+v", res)
-		}
-	}
-	revisions, err := mac.client.Revisions(context.Background(), repoKey(t, mac), path)
-	if err != nil || len(revisions) != 3 {
-		t.Fatalf("want base, dell and one merged revision, got %d (%v)", len(revisions), err)
+			mac.expect(path, versions[3])
+			dell.expect(path, versions[3])
+			state, err := mac.client.State(context.Background(), repoKey(t, mac), "")
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got, _ := mac.client.Blob(context.Background(), state.Spaces[protocol.SharedSpace][path].Hash); string(got) != versions[3] {
+				t.Fatalf("server has %q, want %q", got, versions[3])
+			}
+			for _, m := range []*machine{mac, dell, mac} {
+				if res := m.sync(); res.Pushed != 0 || res.Pulled != 0 {
+					t.Fatalf("merged file keeps syncing: %+v", res)
+				}
+			}
+			revisions, err := mac.client.Revisions(context.Background(), repoKey(t, mac), path)
+			if err != nil || len(revisions) != 3 {
+				t.Fatalf("want base, dell and one merged revision, got %d (%v)", len(revisions), err)
+			}
+		})
 	}
 }