[jboss-jira] [JBoss JIRA] (JGRP-1416) NAKACK / NAKACK2: shorten scope of seqno_lock

[Bela Ban (JIRA)]

Bela Ban created JGRP-1416:
------------------------------

             Summary: NAKACK / NAKACK2: shorten scope of seqno_lock
                 Key: JGRP-1416
                 URL: https://issues.jboss.org/browse/JGRP-1416
             Project: JGroups
          Issue Type: Enhancement
            Reporter: Bela Ban
            Assignee: Bela Ban
            Priority: Minor
             Fix For: 3.0.3, 3.1


The code in NAKACK/NAKACK2.send() acquires seqno_lock like this:
{code:title=NAKACK.send()|borderStyle=solid}
 seqno_lock.lock();
        try {
            try { // incrementing seqno and adding the msg to sent_msgs needs to be atomic
                msg_id=seqno +1;
                msg.putHeader(this.id, NakAckHeader.createMessageHeader(msg_id));
                win.add(msg_id, msg);
                seqno=msg_id;
            }
            catch(Throwable t) {
                throw new RuntimeException("failure adding msg " + msg + " to the retransmit table for " + local_addr, t);
            }
        }
        finally {
            seqno_lock.unlock();
        }
{code}

This slows concurrent sender threads down if add() takes a while. Method add() can take a while if we have many concurrent adds and removes.
The reason we use seqno_lock is to prevent gaps in the sequence numbers, e.g. if we have an exception in add().

SOLUTION:
- Assign a new msg_id by incrementing seqno *atomically* (seqno_lock is now de-scoped to only increment seqno, might replace this with an AtomicLong anyway)
- In a loop: add the message (calling add()), until add() returns successfully, then break

Example:
{code}
msg_id=seqno.incrementAndGet(); // uses an AtomicLong
while(running) { // maybe bound with a counter
  try {
      msg.adddHeader(...);
      add();
      break;
  }
  catch(Throwable t) {
      // log
  }
}
{code}

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