fix(weixin): probe sync dir writability before choosing home

pkg/channels/weixin/state.go

@@ -40,6 +40,31 @@ type contextTokensFile struct {
 	Tokens map[string]string `json:"tokens"`
 }
 
+func syncDirForHome(home string) string {
+	return filepath.Join(home, "channels", "weixin", "sync")
+}
+
+func probeSyncDirWritable(home string) error {
+	syncDir := syncDirForHome(home)
+	if err := os.MkdirAll(syncDir, 0o700); err != nil {
+		return err
+	}
+
+	probeFile, err := os.CreateTemp(syncDir, ".write-probe-*")
+	if err != nil {
+		return err
+	}
+	probePath := probeFile.Name()
+	if err := probeFile.Close(); err != nil {
+		_ = os.Remove(probePath)
+		return err
+	}
+	if err := os.Remove(probePath); err != nil {
+		return err
+	}
+	return nil
+}
+
 func picoclawHomeDir() string {
 	if home := os.Getenv(config.EnvHome); home != "" {
 		return home
@@ -47,16 +72,15 @@ func picoclawHomeDir() string {
 
 	if userHome, err := os.UserHomeDir(); err == nil && userHome != "" {
 		home := filepath.Join(userHome, ".picoclaw")
-		syncDir := filepath.Join(home, "channels", "weixin", "sync")
-		mkErr := os.MkdirAll(syncDir, 0o700)
-		if mkErr == nil {
+		if probeErr := probeSyncDirWritable(home); probeErr == nil {
 			return home
-		}
+		} else {
 			logger.WarnCF("weixin", "Default picoclaw home is not writable; using temp directory for sync cursor", map[string]any{
 				"path":  home,
-			"error": mkErr.Error(),
+				"error": probeErr.Error(),
 			})
 		}
+	}
 
 	return filepath.Join(os.TempDir(), "picoclaw")
 }

pkg/channels/weixin/weixin_test.go

@@ -294,6 +294,44 @@ func TestBuildWeixinSyncBufPathFallsBackWhenHomeIsUnusable(t *testing.T) {
 	}
 }
 
+func TestBuildWeixinSyncBufPathFallsBackWhenSyncDirUnwritable(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("permission mode behavior differs on windows")
+	}
+	if os.Geteuid() == 0 {
+		t.Skip("root can usually bypass directory write permissions")
+	}
+
+	t.Setenv(config.EnvHome, "")
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+
+	syncDir := filepath.Join(home, ".picoclaw", "channels", "weixin", "sync")
+	if err := os.MkdirAll(syncDir, 0o700); err != nil {
+		t.Fatalf("MkdirAll(syncDir) error = %v", err)
+	}
+	if err := os.Chmod(syncDir, 0o500); err != nil {
+		t.Fatalf("Chmod(syncDir, 0500) error = %v", err)
+	}
+	t.Cleanup(func() { _ = os.Chmod(syncDir, 0o700) })
+
+	if probeFile, err := os.CreateTemp(syncDir, ".probe-*"); err == nil {
+		_ = probeFile.Close()
+		_ = os.Remove(probeFile.Name())
+		t.Skip("environment does not enforce non-writable directory permissions")
+	}
+
+	wxCfg := config.WeixinConfig{
+		BaseURL: "https://ilinkai.weixin.qq.com/",
+	}
+	wxCfg.SetToken("token-123")
+	got := buildWeixinSyncBufPath(wxCfg)
+	wantDir := filepath.Join(os.TempDir(), "picoclaw", "channels", "weixin", "sync")
+	if filepath.Dir(got) != wantDir {
+		t.Fatalf("sync path dir = %q, want %q", filepath.Dir(got), wantDir)
+	}
+}
+
 func TestSessionPauseGuard(t *testing.T) {
 	ch := &WeixinChannel{
 		typingCache: make(map[string]typingTicketCacheEntry),