smb: refresh Kerberos credentials when ccache file changes

This change enhances the SMB backend in Rclone to automatically refresh
Kerberos credentials when the associated ccache file is updated.

Previously, credentials were only loaded once per path and cached
indefinitely, which caused issues when service tickets expired or the
cache was renewed on the server.

backend/smb/kerberos_test.go

@@ -4,7 +4,11 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+	"time"
 
+	"github.com/jcmturner/gokrb5/v8/client"
+	"github.com/jcmturner/gokrb5/v8/config"
+	"github.com/jcmturner/gokrb5/v8/credentials"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -77,3 +81,62 @@ func TestResolveCcachePath(t *testing.T) {
 		})
 	}
 }
+
+func TestKerberosFactory_GetClient_ReloadOnCcacheChange(t *testing.T) {
+	// Create temp ccache file
+	tmpFile, err := os.CreateTemp("", "krb5cc_test")
+	assert.NoError(t, err)
+	defer func() {
+		if err := os.Remove(tmpFile.Name()); err != nil {
+			t.Logf("Failed to remove temp file %s: %v", tmpFile.Name(), err)
+		}
+	}()
+
+	unixPath := filepath.ToSlash(tmpFile.Name())
+	ccachePath := "FILE:" + unixPath
+
+	initialContent := []byte("CCACHE_VERSION 4\n")
+	_, err = tmpFile.Write(initialContent)
+	assert.NoError(t, err)
+	assert.NoError(t, tmpFile.Close())
+
+	// Setup mocks
+	loadCallCount := 0
+	mockLoadCCache := func(path string) (*credentials.CCache, error) {
+		loadCallCount++
+		return &credentials.CCache{}, nil
+	}
+
+	mockNewClient := func(cc *credentials.CCache, cfg *config.Config, opts ...func(*client.Settings)) (*client.Client, error) {
+		return &client.Client{}, nil
+	}
+
+	mockLoadConfig := func() (*config.Config, error) {
+		return &config.Config{}, nil
+	}
+	factory := &KerberosFactory{
+		loadCCache: mockLoadCCache,
+		newClient:  mockNewClient,
+		loadConfig: mockLoadConfig,
+	}
+
+	// First call — triggers loading
+	_, err = factory.GetClient(ccachePath)
+	assert.NoError(t, err)
+	assert.Equal(t, 1, loadCallCount, "expected 1 load call")
+
+	// Second call — should reuse cache, no additional load
+	_, err = factory.GetClient(ccachePath)
+	assert.NoError(t, err)
+	assert.Equal(t, 1, loadCallCount, "expected cached reuse, no new load")
+
+	// Simulate file update
+	time.Sleep(1 * time.Second) // ensure mtime changes
+	err = os.WriteFile(tmpFile.Name(), []byte("CCACHE_VERSION 4\n#updated"), 0600)
+	assert.NoError(t, err)
+
+	// Third call — should detect change, reload
+	_, err = factory.GetClient(ccachePath)
+	assert.NoError(t, err)
+	assert.Equal(t, 2, loadCallCount, "expected reload on changed ccache")
+}