fix: prevent multiple release listeners in PostgresQueryRunner (#6708)

move on-error-release code to the queryRunner connect function
so we only need to have one listener per query runner on each
connection - cutting donw the number of listeners total &
preventing a problem with too many listeners

closes #6699

src/driver/postgres/PostgresQueryRunner.ts

@@ -79,18 +79,32 @@ export class PostgresQueryRunner extends BaseQueryRunner implements QueryRunner
             return this.databaseConnectionPromise;
 
         if (this.mode === "slave" && this.driver.isReplicated)  {
-            this.databaseConnectionPromise = this.driver.obtainSlaveConnection().then(([ connection, release]: any[]) => {
+            this.databaseConnectionPromise = this.driver.obtainSlaveConnection().then(([connection, release]: any[]) => {
                 this.driver.connectedQueryRunners.push(this);
                 this.databaseConnection = connection;
-                this.releaseCallback = release;
+
+                const onErrorCallback = () => this.release();
+                this.releaseCallback = () => {
+                    this.databaseConnection.removeListener("error", onErrorCallback);
+                    release();
+                };
+                this.databaseConnection.on("error", onErrorCallback);
+
                 return this.databaseConnection;
             });
 
         } else { // master
             this.databaseConnectionPromise = this.driver.obtainMasterConnection().then(([connection, release]: any[]) => {
                 this.driver.connectedQueryRunners.push(this);
                 this.databaseConnection = connection;
-                this.releaseCallback = release;
+
+                const onErrorCallback = () => this.release();
+                this.releaseCallback = () => {
+                    this.databaseConnection.removeListener("error", onErrorCallback);
+                    release();
+                };
+                this.databaseConnection.on("error", onErrorCallback);
+
                 return this.databaseConnection;
             });
         }
@@ -102,14 +116,14 @@ export class PostgresQueryRunner extends BaseQueryRunner implements QueryRunner
      * Releases used database connection.
      * You cannot use query runner methods once its released.
      */
-    release(err?: any): Promise<void> {
+    release(): Promise<void> {
         if (this.isReleased) {
             return Promise.resolve();
         }
 
         this.isReleased = true;
         if (this.releaseCallback)
-            this.releaseCallback(err);
+            this.releaseCallback();
 
         const index = this.driver.connectedQueryRunners.indexOf(this);
         if (index !== -1) this.driver.connectedQueryRunners.splice(index);
@@ -168,14 +182,7 @@ export class PostgresQueryRunner extends BaseQueryRunner implements QueryRunner
                 this.driver.connection.logger.logQuery(query, parameters, this);
                 const queryStartTime = +new Date();
 
-                const onError = (err: any) => {
-                    this.release(err);
-                };
-                databaseConnection.once("error", onError);
-
                 databaseConnection.query(query, parameters, (err: any, result: any) => {
-                    databaseConnection.removeListener("error", onError);
-
                     // log slow queries if maxQueryExecution time is set
                     const maxQueryExecutionTime = this.driver.connection.options.maxQueryExecutionTime;
                     const queryEndTime = +new Date();

test/github-issues/6699/issue-6699.ts

@@ -0,0 +1,30 @@
+import {closeTestingConnections, createTestingConnections, reloadTestingDatabases} from "../../utils/test-utils";
+import {Connection} from "../../../src/connection/Connection";
+import {expect} from 'chai';
+
+describe("github issues > #6699 MaxListenersExceededWarning occurs on Postgres", () => {
+
+    let connections: Connection[];
+    before(async () => connections = await createTestingConnections({
+        entities: [],
+        enabledDrivers: ["postgres"]
+    }));
+    beforeEach(() => reloadTestingDatabases(connections));
+    after(() => closeTestingConnections(connections));
+
+    it("queries in a transaction do not cause an EventEmitter memory leak", () => Promise.all(connections.map(async connection => {
+        await connection.transaction(async manager => {
+            const queryPromises = [...Array(10)].map(
+                () => manager.query('SELECT pg_sleep(0.0001)')
+            );
+
+            const pgConnection = await manager.queryRunner!.connect();
+
+            expect(pgConnection.listenerCount('error')).to.equal(1);
+
+            // Wait for all of the queries to finish and drain the backlog
+            await Promise.all(queryPromises);
+        });
+    })));
+
+});