UNDERTOW-1701: Fix race condition in GracefulShutdownHandler

[carterkozak]

From undertow-io/undertow#875

1. Thread T1 calls decrementRequests. Shutdown is false, so this thread will follow the 'else' path in preparation for decrementAndGet
2. Thread T2 calls shutdown. Increments from 1 to 2, sets shutdown=true, and calls decrementRequests where decrementAndGet results in 1
3. Thread T1 completes the decrementRequests invocation reducing the active request count to zero in the else path without calling shutdownComplete.

[carterkozak]

This case could be optimized to avoid CAS increment+decrement after the shutdown flag has been set by extracting the LongUnaryOperator into a loop until CAS succeeds similarly to the jboss-threads view executor pattern: https://github.com/jbossas/jboss-threads/blob/a203e13fd739902f850b55a14dc9c77e46a361a6/src/main/java/org/jboss/threads/EnhancedViewExecutor.java#L250-L280

Unclear if it's helpful to optimize this case though, I'm most interested in the hot common case in which the server is actively accepting requests.

e.g.

for (;;) {
    long snapshot = state;
    if (isShutdown(snapshot)) {
        exchange.setStatusCode(StatusCodes.SERVICE_UNAVAILABLE);
        exchange.endExchange();
        return;
    }
    if (stateUpdater.compareAndSet(this, snapshot, snapshot + 1)) {
        exchange.addExchangeCompleteListener(listener);
        next.handleRequest(exchange);
        break;
   }
}

[quarkus-bot]

Status for workflow Build

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Failing Jobs

Status	Name	Step	Failures	Logs	Raw logs
✔️	Build - JDK 11			Logs	Raw logs
✖	Build - JDK 17	Build with Maven	Failures	Logs	Raw logs

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Failures

⚙️ Build - JDK 17 #

- Failing: servlet 
! Skipped: websocket websocket/core websocket/servlet and 1 more

📦 servlet

✖ io.undertow.servlet.test.dispatcher.DispatcherForwardTestCase.testPathBasedInclude line 137 - More details - Source on GitHub

org.junit.ComparisonFailure: expected:<...etContext/dispatch /[dispatch]> but was:<...etContext/dispatch /[forward]>
	at org.junit.Assert.assertEquals(Assert.java:117)
	at org.junit.Assert.assertEquals(Assert.java:146)
	at io.undertow.servlet.test.dispatcher.DispatcherForwardTestCase.testPathBasedInclude(DispatcherForwardTestCase.java:137)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)