add TransientTransactionError label to poolClearedError

Author: zhouselena

x/mongo/driver/topology/pool.go

@@ -50,8 +50,9 @@ func (pe PoolError) Error() string { return string(pe) }
 // poolClearedError is an error returned when the connection pool is cleared or currently paused. It
 // is a retryable error.
 type poolClearedError struct {
-	err     error
-	address address.Address
+	err         error
+	address     address.Address
+	errorLabels []string
 }
 
 func (pce poolClearedError) Error() string {
@@ -503,7 +504,7 @@ func (p *pool) checkOut(ctx context.Context) (conn *connection, err error) {
 		}
 		return nil, ErrPoolClosed
 	case poolPaused:
-		err := poolClearedError{err: p.lastClearErr, address: p.address}
+		err := poolClearedError{err: p.lastClearErr, address: p.address, errorLabels: []string{"TransientTransactionError"}}
 		p.stateMu.RUnlock()
 
 		duration := time.Since(start)
@@ -1049,7 +1050,7 @@ func (p *pool) clearImpl(err error, serviceID *bson.ObjectID, interruptAllConnec
 	}
 
 	if serviceID == nil {
-		pcErr := poolClearedError{err: err, address: p.address}
+		pcErr := poolClearedError{err: err, address: p.address, errorLabels: []string{"TransientTransactionError"}}
 
 		// Clear the idle connections wait queue.
 		p.idleMu.Lock()

x/mongo/driver/topology/pool_test.go

@@ -1584,3 +1584,29 @@ func TestPool_PoolMonitor(t *testing.T) {
 			"expected ConnectionCheckOutFailed Duration to be set")
 	})
 }
+
+func TestPool_Error(t *testing.T) {
+
+	t.Parallel()
+
+	t.Run("should have TransientTransactionError", func(t *testing.T) {
+		t.Parallel()
+
+		p := newPool(poolConfig{})
+		assert.Equalf(t, poolPaused, p.getState(), "expected new pool to be paused")
+
+		// Since new pool is paused, checkout should throw poolClearedError.
+		_, err := p.checkOut(context.Background())
+
+		var pce poolClearedError
+		if errors.As(err, &pce) {
+			expectedLabel := "TransientTransactionError"
+			assert.Contains(t, pce.errorLabels, expectedLabel, `expected error to include the "TransientTransactionError" label`)
+		} else {
+			t.Errorf("expected poolClearedError, got %v", err)
+		}
+
+		p.close(context.Background())
+
+	})
+}