fix: ensure graceful shutdown on fuse error

If the FUSE mount has not succeeded due to misconfiguration, the Proxy
should still shutdown without panicing.

Related to GoogleCloudPlatform/cloud-sql-proxy#2013

internal/proxy/proxy_other.go

@@ -181,6 +181,9 @@ func (c *Client) fuseMounts() []*socketMount {
 func (c *Client) unmountFUSE() error {
 	c.fuseServerMu.Lock()
 	defer c.fuseServerMu.Unlock()
+    if c.fuseServer == nil {
+        return nil
+    }
 	return c.fuseServer.Unmount()
 }
 

internal/proxy/proxy_other_test.go

@@ -18,8 +18,11 @@
 package proxy_test
 
 import (
+	"context"
 	"os"
 	"testing"
+
+	"github.com/GoogleCloudPlatform/alloydb-auth-proxy/internal/proxy"
 )
 
 func verifySocketPermissions(t *testing.T, addr string) {
@@ -31,3 +34,19 @@ func verifySocketPermissions(t *testing.T, addr string) {
 		t.Fatalf("file mode: want = %v, got = %v", 0777|os.ModeSocket, fm)
 	}
 }
+
+func TestFuseClosesGracefully(t *testing.T) {
+	c, err := proxy.NewClient(
+		context.Background(), nil, testLogger,
+		&proxy.Config{
+			FUSEDir:     t.TempDir(),
+			FUSETempDir: t.TempDir(),
+			Token:       "mytoken",
+		})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Close(); err != nil {
+		t.Fatal(err)
+	}
+}