GODRIVER-3399: PoolClearedError should have TransientTransactionError label appended to it

x/mongo/driver/topology/pool.go

@@ -503,7 +503,19 @@ func (p *pool) checkOut(ctx context.Context) (conn *connection, err error) {
 		}
 		return nil, ErrPoolClosed
 	case poolPaused:
-		err := poolClearedError{err: p.lastClearErr, address: p.address}
+		// Wrap poolCleared in a driver.Error so we can add the
+		// "TransientTransactionError" label. This will add
+		// "TransientTransactionError" to all poolClearedError instances, not
+		// just those that happened during transactions. While that behavior is
+		// different than other places we add "TransientTransactionError", it is
+		// consistent with the Transactions specification and simplifies the
+		// code.
+		pcErr := poolClearedError{err: p.lastClearErr, address: p.address}
+		err := driver.Error{
+			Message: pcErr.Error(),
+			Labels:  []string{driver.TransientTransactionError},
+			Wrapped: pcErr,
+		}
 		p.stateMu.RUnlock()
 
 		duration := time.Since(start)

x/mongo/driver/topology/pool_test.go

@@ -21,6 +21,7 @@ import (
 	"go.mongodb.org/mongo-driver/v2/internal/eventtest"
 	"go.mongodb.org/mongo-driver/v2/internal/require"
 	"go.mongodb.org/mongo-driver/v2/mongo/address"
+	"go.mongodb.org/mongo-driver/v2/x/mongo/driver"
 	"go.mongodb.org/mongo-driver/v2/x/mongo/driver/operation"
 )
 
@@ -1584,3 +1585,26 @@ 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 le driver.Error
+		if errors.As(err, &le) {
+			assert.ErrorIs(t, poolClearedError{}, le.Unwrap(), "expect error to be PoolClearedError")
+			assert.True(t, le.HasErrorLabel(driver.TransientTransactionError), `expected error to include the "TransientTransactionError" label`)
+		} else {
+			t.Errorf("expected labeled error, got %v", err)
+		}
+
+		p.close(context.Background())
+	})
+}