Refactor pg-pool to avoid potential memory leak

Reduce the closure scope captured by the "release once" lambda function
in _acquireClient so that it does not retain the pendingItem promise.

Author: Johan Levin

packages/pg-pool/index.js

@@ -252,16 +252,7 @@ class Pool extends EventEmitter {
 
     this.emit('acquire', client)
 
-    let released = false
-
-    client.release = (err) => {
-      if (released) {
-        throwOnDoubleRelease()
-      }
-
-      released = true
-      this._release(client, idleListener, err)
-    }
+    client.release = this._releaseOnce(client, idleListener)
 
     client.removeListener('error', idleListener)
 
@@ -287,6 +278,20 @@ class Pool extends EventEmitter {
     }
   }
 
+  // returns a function that wraps _release and throws if called more than once
+  _releaseOnce(client, idleListener) {
+    let released = false
+
+    return (err) => {
+      if (released) {
+        throwOnDoubleRelease()
+      }
+
+      released = true
+      this._release(client, idleListener, err)
+    }
+  }
+
   // release a client back to the poll, include an error
   // to remove it from the pool
   _release(client, idleListener, err) {