fix: do the maxMessages and maxBytes checks _before_ adding

src/message-queues.ts

@@ -202,7 +202,18 @@ export abstract class MessageQueue {
     }
 
     const {maxMessages, maxMilliseconds} = this._options;
+    const size = Buffer.byteLength(message.ackId, 'utf8');
 
+    // If we will go over maxMessages or MAX_BATCH_BYTES by adding this
+    // message, flush first. (maxMilliseconds is handled by timers.)
+    if (
+      this._requests.length + 1 >= maxMessages! ||
+      this.bytes + size >= MAX_BATCH_BYTES
+    ) {
+      this.flush();
+    }
+
+    // Add the message to the current batch.
     const responsePromise = defer<void>();
     this._requests.push({
       message: {
@@ -215,14 +226,10 @@ export abstract class MessageQueue {
     });
     this.numPendingRequests++;
     this.numInFlightRequests++;
-    this.bytes += Buffer.byteLength(message.ackId, 'utf8');
+    this.bytes += size;
 
-    if (
-      this._requests.length >= maxMessages! ||
-      this.bytes >= MAX_BATCH_BYTES
-    ) {
-      this.flush();
-    } else if (!this._timer) {
+    // Ensure that we are counting toward maxMilliseconds by timer.
+    if (!this._timer) {
       this._timer = setTimeout(() => this.flush(), maxMilliseconds!);
     }