fix: bind writeback operations to agents directory

internal/agents/writeback_test.go

@@ -115,6 +115,46 @@ func TestWriteRejectsAgentsDirectoryReplacementBeforeBackup(t *testing.T) {
 	assertFileContent(t, filepath.Join(externalDir, "backend.toml"), `name = "外部"`+"\n")
 }
 
+func TestWriteBindsBackupToAgentsDirectoryAfterFinalVerify(t *testing.T) {
+	store, target := writebackFixture(t, `name = "旧名称"`+"\n")
+	validation, err := store.ValidateDraft("backend", `name = "新名称"`+"\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	root := filepath.Dir(filepath.Dir(target))
+	agentsDir := filepath.Join(root, "agents")
+	realAgentsDir := filepath.Join(root, "agents-real")
+	externalDir := filepath.Join(root, "external")
+	if err := os.MkdirAll(externalDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(externalDir, "backend.toml"), []byte(`name = "外部"`+"\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	writebackTestHookAfterVerifyBeforeBackup = func() {
+		if err := os.Rename(agentsDir, realAgentsDir); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.Symlink(externalDir, agentsDir); err != nil {
+			t.Fatal(err)
+		}
+	}
+	defer func() { writebackTestHookAfterVerifyBeforeBackup = nil }()
+
+	_, err = store.WriteDraft("backend", `name = "新名称"`+"\n", validation.CurrentHash)
+	if !errors.Is(err, ErrWriteConflict) && !errors.Is(err, codexhome.ErrForbiddenPath) {
+		t.Fatalf("expected post-verify directory replacement to be rejected, got %v", err)
+	}
+	assertFileContent(t, filepath.Join(realAgentsDir, "backend.toml"), `name = "旧名称"`+"\n")
+	assertFileContent(t, filepath.Join(externalDir, "backend.toml"), `name = "外部"`+"\n")
+	if backups, err := filepath.Glob(filepath.Join(externalDir, "*.bak-*")); err != nil {
+		t.Fatal(err)
+	} else if len(backups) != 0 {
+		t.Fatalf("backup was created through replaced symlink directory: %#v", backups)
+	}
+}
+
 func TestWriteRejectsTargetChangeAfterBackup(t *testing.T) {
 	store, target := writebackFixture(t, `name = "旧名称"`+"\n")
 	validation, err := store.ValidateDraft("backend", `name = "新名称"`+"\n")