Return error from GetDefaultStateDir when home directory unavailable

When os.UserConfigDir() fails, DetermineStateDir falls back to
os.UserHomeDir(). Previously the error from UserHomeDir was discarded,
which could result in a dangerous root-relative path (/.config/...) if
both calls fail.

Now DetermineStateDir returns (string, error) and propagates failures
from both UserConfigDir and UserHomeDir.

Closes #14

internal/secret/helpers.go

@@ -28,16 +28,17 @@ func generateRandomString(length int, charset string) (string, error) {
 	return string(result), nil
 }
 
-// DetermineStateDir determines the state directory based on environment variables and OS
-func DetermineStateDir(customConfigDir string) string {
+// DetermineStateDir determines the state directory based on environment variables and OS.
+// It returns an error if no usable directory can be determined.
+func DetermineStateDir(customConfigDir string) (string, error) {
 	// Check for environment variable first
 	if envStateDir := os.Getenv(EnvStateDir); envStateDir != "" {
-		return envStateDir
+		return envStateDir, nil
 	}
 
 	// Use custom config dir if provided
 	if customConfigDir != "" {
-		return filepath.Join(customConfigDir, AppID)
+		return filepath.Join(customConfigDir, AppID), nil
 	}
 
 	// Use os.UserConfigDir() which handles platform-specific directories:
@@ -47,10 +48,13 @@ func DetermineStateDir(customConfigDir string) string {
 	configDir, err := os.UserConfigDir()
 	if err != nil {
 		// Fallback to a reasonable default if we can't determine user config dir
-		homeDir, _ := os.UserHomeDir()
+		homeDir, homeErr := os.UserHomeDir()
+		if homeErr != nil {
+			return "", fmt.Errorf("unable to determine state directory: config dir: %w, home dir: %w", err, homeErr)
+		}
 
-		return filepath.Join(homeDir, ".config", AppID)
+		return filepath.Join(homeDir, ".config", AppID), nil
 	}
 
-	return filepath.Join(configDir, AppID)
+	return filepath.Join(configDir, AppID), nil
 }

internal/secret/helpers_test.go

@@ -0,0 +1,50 @@
+package secret
+
+import (
+	"testing"
+)
+
+func TestDetermineStateDir_ErrorsWhenHomeDirUnavailable(t *testing.T) {
+	// Clear all env vars that could provide a home/config directory.
+	// On Darwin, os.UserHomeDir may still succeed via the password
+	// database, so we also test via an explicit empty-customConfigDir
+	// path to exercise the fallback branch.
+	t.Setenv(EnvStateDir, "")
+	t.Setenv("HOME", "")
+	t.Setenv("XDG_CONFIG_HOME", "")
+
+	result, err := DetermineStateDir("")
+	// On systems where both lookups fail, we must get an error.
+	// On systems where the OS provides a fallback (e.g. macOS pw db),
+	// result should still be valid (non-empty, not root-relative).
+	if err != nil {
+		// Good — the error case is handled.
+		return
+	}
+	if result == "/.config/"+AppID || result == "" {
+		t.Errorf("DetermineStateDir returned dangerous/empty path %q without error", result)
+	}
+}
+
+func TestDetermineStateDir_UsesEnvVar(t *testing.T) {
+	t.Setenv(EnvStateDir, "/custom/state")
+	result, err := DetermineStateDir("")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result != "/custom/state" {
+		t.Errorf("expected /custom/state, got %q", result)
+	}
+}
+
+func TestDetermineStateDir_UsesCustomConfigDir(t *testing.T) {
+	t.Setenv(EnvStateDir, "")
+	result, err := DetermineStateDir("/my/config")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	expected := "/my/config/" + AppID
+	if result != expected {
+		t.Errorf("expected %q, got %q", expected, result)
+	}
+}