docker serve: make Create idempotent to avoid "volume already exists" after restart

Docker may re-send Create requests for volumes that already exist,
especially after a plugin restart. Previously this returned
ErrVolumeExists which Docker surfaced as "volume name must be unique".

Now if a volume with the same name already exists, Create returns
success (no-op), matching the Docker volume plugin protocol's
expectation of idempotent operations.

cmd/serve/docker/docker_test.go

@@ -94,7 +94,8 @@ func TestDockerPluginLogic(t *testing.T) {
 	assert.NoError(t, drv.Create(volReq))
 	path1 := filepath.Join(testDir, "vol1")
 
-	assert.ErrorIs(t, drv.Create(volReq), docker.ErrVolumeExists)
+	// Create is idempotent - creating the same volume again should succeed
+	assert.NoError(t, drv.Create(volReq))
 
 	getReq := &docker.GetRequest{Name: "vol1"}
 	getRes, err := drv.Get(getReq)
@@ -373,8 +374,9 @@ func testMountAPI(t *testing.T, sockAddr string) {
 	}
 	cli.request("Create", createReq, &res, false)
 	assert.Equal(t, "{}", res)
-	cli.request("Create", createReq, &res, true)
-	assert.Contains(t, res, "volume already exists")
+	// Create is idempotent - creating the same volume again should succeed
+	cli.request("Create", createReq, &res, false)
+	assert.Equal(t, "{}", res)
 
 	mountReq := docker.MountRequest{Name: "vol1", ID: "id1"}
 	var mountRes docker.MountResponse

cmd/serve/docker/driver.go

@@ -182,8 +182,12 @@ func reportErr(err error) {
 	}
 }
 
-// Create volume
-// To use subpath we are limited to defining a new volume definition via alias
+// Create volume.
+//
+// If the volume already exists, the request is treated as a no-op
+// (idempotent) so that Docker can safely re-send Create requests
+// after a plugin restart without getting "volume already exists".
+// To use subpath we are limited to defining a new volume definition via alias.
 func (drv *Driver) Create(req *CreateRequest) error {
 	ctx := context.Background()
 	drv.mu.Lock()
@@ -193,7 +197,8 @@ func (drv *Driver) Create(req *CreateRequest) error {
 	fs.Debugf(nil, "Create volume %q", name)
 
 	if vol, _ := drv.getVolume(name); vol != nil {
-		return ErrVolumeExists
+		fs.Debugf(nil, "Volume %q already exists, treating Create as no-op", name)
+		return nil
 	}
 
 	vol, err := newVolume(ctx, name, req.Options, drv)

cmd/serve/docker/volume.go

@@ -20,7 +20,6 @@ import (
 // Errors
 var (
 	ErrVolumeNotFound   = errors.New("volume not found")
-	ErrVolumeExists     = errors.New("volume already exists")
 	ErrMountpointExists = errors.New("non-empty mountpoint already exists")
 )