fix(client): three-way merge when a file changed on both sides

backend/internal/client/sync_test.go

@@ -203,6 +203,73 @@ 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()
+
+	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()
+
+	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)
+	}
+}
+
+func TestConflictingOrBinaryEditsFallBackToLaterWins(t *testing.T) {
+	for name, versions := range map[string][3]string{
+		"same line":    {"1\n2\n3\n4\n5\n", "mac\n2\n3\n4\n5\n", "dell\n2\n3\n4\n5\n"},
+		"nul byte":     {"1\n2\n3\n4\n5\n\x00\n", "mac\n2\n3\n4\n5\n\x00\n", "1\n2\n3\n4\ndell\n\x00\n"},
+		"invalid utf8": {"1\n2\n3\n4\n5\n\xff\n", "mac\n2\n3\n4\n5\n\xff\n", "1\n2\n3\n4\ndell\n\xff\n"},
+	} {
+		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, versions[1])
+			old := time.Now().Add(-time.Hour)
+			if err := os.Chtimes(filepath.Join(mac.root, path), old, old); err != nil {
+				t.Fatal(err)
+			}
+			write(t, dell.root, path, versions[2])
+			dell.sync()
+			mac.sync()
+			dell.sync()
+
+			mac.expect(path, versions[2])
+			dell.expect(path, versions[2])
+			revisions, err := mac.client.Revisions(context.Background(), repoKey(t, mac), path)
+			if err != nil || len(revisions) != 4 {
+				t.Fatalf("want base, dell, losing mac and restored dell revisions, got %d (%v)", len(revisions), err)
+			}
+		})
+	}
+}
+
 func TestRulesFileTravelsAndHostileServerPathsAreIgnored(t *testing.T) {
 	mac, dell := setup(t)
 	write(t, mac.root, RulesFile, "shared notes\n")