rebase: bump google.golang.org/grpc from 1.55.0 to 1.56.0

Bumps [google.golang.org/grpc](https://github.com/grpc/grpc-go) from 1.55.0 to 1.56.0.
- [Release notes](https://github.com/grpc/grpc-go/releases)
- [Commits](https://github.com/grpc/grpc-go/compare/v1.55.0...v1.56.0)

---
updated-dependencies:
- dependency-name: google.golang.org/grpc
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

vendor/google.golang.org/grpc/internal/grpcsync/callback_serializer.go

@@ -20,6 +20,7 @@ package grpcsync
 
 import (
 	"context"
+	"sync"
 
 	"google.golang.org/grpc/internal/buffer"
 )
@@ -31,15 +32,26 @@ import (
 //
 // This type is safe for concurrent access.
 type CallbackSerializer struct {
+	// Done is closed once the serializer is shut down completely, i.e all
+	// scheduled callbacks are executed and the serializer has deallocated all
+	// its resources.
+	Done chan struct{}
+
 	callbacks *buffer.Unbounded
+	closedMu  sync.Mutex
+	closed    bool
 }
 
 // NewCallbackSerializer returns a new CallbackSerializer instance. The provided
 // context will be passed to the scheduled callbacks. Users should cancel the
 // provided context to shutdown the CallbackSerializer. It is guaranteed that no
-// callbacks will be executed once this context is canceled.
+// callbacks will be added once this context is canceled, and any pending un-run
+// callbacks will be executed before the serializer is shut down.
 func NewCallbackSerializer(ctx context.Context) *CallbackSerializer {
-	t := &CallbackSerializer{callbacks: buffer.NewUnbounded()}
+	t := &CallbackSerializer{
+		Done:      make(chan struct{}),
+		callbacks: buffer.NewUnbounded(),
+	}
 	go t.run(ctx)
 	return t
 }
@@ -48,18 +60,60 @@ func NewCallbackSerializer(ctx context.Context) *CallbackSerializer {
 //
 // Callbacks are expected to honor the context when performing any blocking
 // operations, and should return early when the context is canceled.
-func (t *CallbackSerializer) Schedule(f func(ctx context.Context)) {
+//
+// Return value indicates if the callback was successfully added to the list of
+// callbacks to be executed by the serializer. It is not possible to add
+// callbacks once the context passed to NewCallbackSerializer is cancelled.
+func (t *CallbackSerializer) Schedule(f func(ctx context.Context)) bool {
+	t.closedMu.Lock()
+	defer t.closedMu.Unlock()
+
+	if t.closed {
+		return false
+	}
 	t.callbacks.Put(f)
+	return true
 }
 
 func (t *CallbackSerializer) run(ctx context.Context) {
+	var backlog []func(context.Context)
+
+	defer close(t.Done)
 	for ctx.Err() == nil {
 		select {
 		case <-ctx.Done():
-			return
-		case callback := <-t.callbacks.Get():
+			// Do nothing here. Next iteration of the for loop will not happen,
+			// since ctx.Err() would be non-nil.
+		case callback, ok := <-t.callbacks.Get():
+			if !ok {
+				return
+			}
 			t.callbacks.Load()
 			callback.(func(ctx context.Context))(ctx)
 		}
 	}
+
+	// Fetch pending callbacks if any, and execute them before returning from
+	// this method and closing t.Done.
+	t.closedMu.Lock()
+	t.closed = true
+	backlog = t.fetchPendingCallbacks()
+	t.callbacks.Close()
+	t.closedMu.Unlock()
+	for _, b := range backlog {
+		b(ctx)
+	}
+}
+
+func (t *CallbackSerializer) fetchPendingCallbacks() []func(context.Context) {
+	var backlog []func(context.Context)
+	for {
+		select {
+		case b := <-t.callbacks.Get():
+			backlog = append(backlog, b.(func(context.Context)))
+			t.callbacks.Load()
+		default:
+			return backlog
+		}
+	}
 }