fix: ensure SQLite has a writable temporary directory (#876)

Co-authored-by: Elias Schneider <login@eliasschneider.com>

backend/internal/utils/file_util.go

@@ -1,12 +1,15 @@
 package utils
 
 import (
+	"crypto/rand"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
 	"mime/multipart"
 	"os"
 	"path/filepath"
+	"syscall"
 
 	"github.com/google/uuid"
 	"github.com/pocket-id/pocket-id/backend/resources"
@@ -136,3 +139,41 @@ func FileExists(path string) (bool, error) {
 	}
 	return !s.IsDir(), nil
 }
+
+// IsWritableDir checks if a directory exists and is writable
+func IsWritableDir(dir string) (bool, error) {
+	// Check if directory exists and it's actually a directory
+	info, err := os.Stat(dir)
+	if os.IsNotExist(err) {
+		return false, nil
+	} else if err != nil {
+		return false, fmt.Errorf("failed to stat '%s': %w", dir, err)
+	}
+	if !info.IsDir() {
+		return false, nil
+	}
+
+	// Generate a random suffix for the test file to avoid conflicts
+	randomBytes := make([]byte, 8)
+	_, err = io.ReadFull(rand.Reader, randomBytes)
+	if err != nil {
+		return false, fmt.Errorf("failed to generate random bytes: %w", err)
+	}
+
+	// Check if directory is writable by trying to create a temporary file
+	testFile := filepath.Join(dir, ".pocketid_test_write_"+hex.EncodeToString(randomBytes))
+	defer os.Remove(testFile)
+
+	file, err := os.Create(testFile)
+	if err != nil {
+		if os.IsPermission(err) || errors.Is(err, syscall.EROFS) {
+			return false, nil
+		}
+
+		return false, fmt.Errorf("failed to create test file: %w", err)
+	}
+
+	_ = file.Close()
+
+	return true, nil
+}