GODRIVER-2867 Unpin connections when ending a session. (#1330)

Author: matthewdale

mongo/integration/load_balancer_prose_test.go

@@ -15,6 +15,7 @@ import (
 
 	"go.mongodb.org/mongo-driver/bson"
 	"go.mongodb.org/mongo-driver/internal/assert"
+	"go.mongodb.org/mongo-driver/internal/require"
 	"go.mongodb.org/mongo-driver/mongo"
 	"go.mongodb.org/mongo-driver/mongo/integration/mtest"
 	"go.mongodb.org/mongo-driver/mongo/options"
@@ -99,5 +100,33 @@ func TestLoadBalancerSupport(t *testing.T) {
 			_, err := mt.Coll.InsertOne(ctx, bson.M{"x": 1})
 			assertErrorHasInfo(mt, err, 0, 1, 0)
 		})
+
+		// GODRIVER-2867: Test that connections are unpinned from transactions
+		// when the transaction session is ended. Create a Client with
+		// maxPoolSize=1 and expect that it can start and commit 5 transactions
+		// with that 1 connection.
+		mt.RunOpts("transaction connections are unpinned", maxPoolSizeMtOpts, func(mt *mtest.T) {
+			{
+				ctx, cancel := context.WithTimeout(context.Background(), 1*time.Minute)
+				defer cancel()
+
+				for i := 0; i < 5; i++ {
+					sess, err := mt.Client.StartSession()
+					require.NoError(mt, err, "StartSession error")
+
+					err = sess.StartTransaction()
+					require.NoError(mt, err, "StartTransaction error")
+
+					ctx := mongo.NewSessionContext(ctx, sess)
+					_, err = mt.Coll.InsertOne(ctx, bson.M{"x": 1})
+					assert.NoError(mt, err, "InsertOne error")
+
+					err = sess.CommitTransaction(ctx)
+					assert.NoError(mt, err, "CommitTransaction error")
+
+					sess.EndSession(ctx)
+				}
+			}
+		})
 	})
 }

x/mongo/driver/session/client_session.go

@@ -331,9 +331,10 @@ func (c *Client) ClearPinnedResources() error {
 	return nil
 }
 
-// UnpinConnection gracefully unpins the connection associated with the session if there is one. This is done via
-// the pinned connection's UnpinFromTransaction function.
-func (c *Client) UnpinConnection() error {
+// unpinConnection gracefully unpins the connection associated with the session
+// if there is one. This is done via the pinned connection's
+// UnpinFromTransaction function.
+func (c *Client) unpinConnection() error {
 	if c == nil || c.PinnedConnection == nil {
 		return nil
 	}
@@ -353,6 +354,12 @@ func (c *Client) EndSession() {
 		return
 	}
 	c.Terminated = true
+
+	// Ignore the error when unpinning the connection because we can't do
+	// anything about it if it doesn't work. Typically the only errors that can
+	// happen here indicate that something went wrong with the connection state,
+	// like it wasn't marked as pinned or attempted to return to the wrong pool.
+	_ = c.unpinConnection()
 	c.pool.ReturnSession(c.Server)
 }