Fix connection leak due to race condition in counting resources

Since resources were being acquired in one async context, but being
counted in another, it was possible that under high load checks of the
total number of connections could report zero when connections were
actually created.

This commit moves the call to resourceAcquired() to the same async
context where it's being created initially or reacquired from the
existing pool.

Author: voutilad

src/internal/pool.js

@@ -74,10 +74,7 @@ class Pool {
       const key = address.asKey()
 
       if (resource) {
-        resourceAcquired(key, this._activeResourceCounts)
-        if (this._log.isDebugEnabled()) {
-          this._log.debug(`${resource} acquired from the pool ${key}`)
-        }
+        // New or existing resource acquired
         return resource
       }
 
@@ -190,7 +187,11 @@ class Pool {
         }
 
         // idle resource is valid and can be acquired
-        return Promise.resolve(resource)
+        resourceAcquired(key, this._activeResourceCounts)
+        if (this._log.isDebugEnabled()) {
+          this._log.debug(`${resource} acquired from the pool ${key}`)
+        }
+        return resource
       } else {
         await this._destroy(resource)
       }
@@ -215,6 +216,11 @@ class Pool {
     try {
       // Invoke callback that creates actual connection
       resource = await this._create(address, this._release)
+
+      resourceAcquired(key, this._activeResourceCounts)
+      if (this._log.isDebugEnabled()) {
+        this._log.debug(`${resource} created for the pool ${key}`)
+      }
     } finally {
       this._pendingCreates[key] = this._pendingCreates[key] - 1
     }
@@ -307,7 +313,6 @@ class Pool {
                 this._release(address, resource)
               } else {
                 // request is still pending and can be resolved with the newly acquired resource
-                resourceAcquired(key, this._activeResourceCounts) // increment the active counter
                 pendingRequest.resolve(resource) // resolve the pending request with the acquired resource
               }
             }