[improve] return ErrMaxConcurrentOpsReached when too many concurren…

…t ops in transaction coordinator client (#1242)

### Motivation

Currently, the client will return an UnknownError when there are too many concurrent ops in transaction coordinator client

### Modifications

- Add new error `ErrMaxConcurrentOpsReached` 
- Return `ErrMaxConcurrentOpsReached` when too many concurrent ops in transaction coordinator client

pulsar/error.go

@@ -121,6 +121,10 @@ const (
 	// fenced. Applications are now supposed to close it and create a
 	// new producer
 	ProducerFenced
+	// MaxConcurrentOperationsReached indicates that the maximum number of concurrent operations
+	// has been reached. This means that no additional operations can be started until some
+	// of the current operations complete.
+	MaxConcurrentOperationsReached
 	// TransactionCoordinatorNotEnabled indicates that the transaction coordinator is not enabled.
 	// This error is returned when an operation that requires the transaction coordinator is attempted
 	// but the transaction coordinator feature is not enabled in the system or the transaction coordinator

pulsar/transaction_coordinator_client.go

@@ -41,6 +41,7 @@ type transactionCoordinatorClient struct {
 // where the TC located.
 const TransactionCoordinatorAssign = "persistent://pulsar/system/transaction_coordinator_assign"
 
+var ErrMaxConcurrentOpsReached = newError(MaxConcurrentOperationsReached, "Max concurrent operations reached")
 var ErrTransactionCoordinatorNotEnabled = newError(TransactionCoordinatorNotEnabled, "The broker doesn't enable "+
 	"the transaction coordinator, or the transaction coordinator has not initialized")
 
@@ -212,7 +213,7 @@ func getTCAssignTopicName(partition uint64) string {
 
 func (tc *transactionCoordinatorClient) canSendRequest() error {
 	if !tc.semaphore.Acquire(context.Background()) {
-		return newError(UnknownError, "Failed to acquire semaphore")
+		return ErrMaxConcurrentOpsReached
 	}
 	return nil
 }