feat: support dequeue event to properly match enqueue counterpart

lib/pool.js

@@ -87,10 +87,12 @@ class Pool extends EventEmitter {
       // The connection has been removed from the pool and is no longer good.
       if (this._connectionQueue.length) {
         cb = this._connectionQueue.shift();
+        this.emit('dequeue', connection);
         process.nextTick(this.getConnection.bind(this, cb));
       }
     } else if (this._connectionQueue.length) {
       cb = this._connectionQueue.shift();
+      this.emit('dequeue', connection);
       process.nextTick(cb.bind(null, null, connection));
     } else {
       this._freeConnections.push(connection);

promise.d.ts

@@ -90,6 +90,7 @@ export interface Pool extends Connection {
   on(event: 'acquire', listener: (connection: PoolConnection) => any): this;
   on(event: 'release', listener: (connection: PoolConnection) => any): this;
   on(event: 'enqueue', listener: () => any): this;
+  on(event: 'dequeue', listener: () => any): this;
 
   end(): Promise<void>;
 

promise.js

@@ -331,7 +331,7 @@ class PromisePool extends EventEmitter {
     super();
     this.pool = pool;
     this.Promise = thePromise || Promise;
-    inheritEvents(pool, this, ['acquire', 'connection', 'enqueue', 'release']);
+    inheritEvents(pool, this, ['acquire', 'connection', 'enqueue', 'release', 'dequeue']);
   }
 
   getConnection() {

test/integration/promise-wrappers/test-promise-wrappers.js

@@ -326,17 +326,22 @@ function testEventsPool() {
       assert.equal(this, pool);
       ++events;
     })
+    .once('dequeue', function() {
+      assert.equal(this, pool);
+      ++events;
+    })
     .once('release', function() {
       assert.equal(this, pool);
       ++events;
 
-      doneEventsPool = events === 4;
+      doneEventsPool = events === 5;
     });
   /* eslint-enable no-invalid-this */
 
   pool.pool.emit('acquire');
   pool.pool.emit('connection');
   pool.pool.emit('enqueue');
+  pool.pool.emit('dequeue');
   pool.pool.emit('release');
 
   for (const eventName in expectedListeners) {

typings/mysql/lib/Pool.d.ts

@@ -68,6 +68,7 @@ declare class Pool extends QueryableBase(ExecutableBase(EventEmitter)) {
   on(event: 'acquire', listener: (connection: PoolConnection) => any): this;
   on(event: 'release', listener: (connection: PoolConnection) => any): this;
   on(event: 'enqueue', listener: () => any): this;
+  on(event: 'dequeue', listener: () => any): this;
 
   unprepare(sql: string): PrepareStatementInfo;