Second improvement of TSAN flakiness on channel tests.

The flakiness rate is higher when cleanup handlers of multiple dispatch IO are invoked at the same time on the different queues. Consequently, let's use a single serial queue to close socket for EDOSocketChannel.

PiperOrigin-RevId: 326268100

Channel/Sources/EDOSocket.m

@@ -67,12 +67,28 @@ static void edo_RunHandlerWithErrorInQueueWithBlock(int code, dispatch_queue_t q
 
 #pragma mark - EDOSocket implementation
 
+@interface EDOSocket ()
+
+/** The global serial queue for handling dispatch I/O's cleanup handler and closing socket. */
+@property(class, atomic, readonly) dispatch_queue_t closeSocketQueue;
+
+@end
+
 @implementation EDOSocket {
   dispatch_fd_t _socket;
 }
 
 @dynamic valid;
 
++ (dispatch_queue_t)closeSocketQueue {
+  static dispatch_queue_t closeSocketQueue;
+  static dispatch_once_t once_token;
+  dispatch_once(&once_token, ^{
+    closeSocketQueue = dispatch_queue_create("com.google.edo.CloseSocket", DISPATCH_QUEUE_SERIAL);
+  });
+  return closeSocketQueue;
+}
+
 + (nullable instancetype)socketWithTCPPort:(UInt16)port
                                      queue:(dispatch_queue_t _Nullable)queue
                                      error:(NSError *_Nullable *_Nullable)error {
@@ -125,7 +141,7 @@ - (nullable dispatch_io_t)releaseAsDispatchIO {
     return NULL;
   }
 
-  dispatch_queue_t queue = dispatch_queue_create("com.google.edo.SocketIO", DISPATCH_QUEUE_SERIAL);
+  dispatch_queue_t queue = [self class].closeSocketQueue;
   dispatch_io_t channel = dispatch_io_create(DISPATCH_IO_STREAM, socket, queue, ^(int error) {
     if (error) {
       NSLog(@"Error (%d) when closing dispatch channel.", error);