Sporadic ArithmeticException when sending WebSocket upgrade requests on 9.4.x

[marcphilipp]

Jetty version

9.4.35.v20201120

Java version

1.8 (let me know if you need to know the exact update)

OS type/version

Linux 4.4.0-1052-aws

Description

We're sporadically seeing exceptions like the following when sending WebSocketUpgradeRequests:

Caused by: java.lang.ArithmeticException: / by zero
	at org.eclipse.jetty.util.Pool.acquire(Pool.java:278)
	at org.eclipse.jetty.client.AbstractConnectionPool.activate(AbstractConnectionPool.java:284)
	at org.eclipse.jetty.client.AbstractConnectionPool.acquire(AbstractConnectionPool.java:209)
	at org.eclipse.jetty.client.HttpDestination.process(HttpDestination.java:329)
	at org.eclipse.jetty.client.HttpDestination.send(HttpDestination.java:316)
	at org.eclipse.jetty.client.HttpDestination.send(HttpDestination.java:309)
	at org.eclipse.jetty.client.HttpDestination.send(HttpDestination.java:286)
	at org.eclipse.jetty.client.HttpDestination.send(HttpDestination.java:263)
	at org.eclipse.jetty.client.HttpClient.send(HttpClient.java:600)
	at org.eclipse.jetty.client.HttpRequest.sendAsync(HttpRequest.java:778)
	at org.eclipse.jetty.client.HttpRequest.send(HttpRequest.java:765)
	at org.eclipse.jetty.websocket.client.WebSocketUpgradeRequest.send(WebSocketUpgradeRequest.java:565)
	at org.eclipse.jetty.websocket.client.WebSocketUpgradeRequest.sendAsync(WebSocketUpgradeRequest.java:570)
	at com.gradle.enterprise.testdistribution.common.client.websocket.h.a(SourceFile:70)
	at com.gradle.enterprise.testdistribution.common.client.websocket.h.a(SourceFile:53)
	at com.gradle.enterprise.testdistribution.common.client.websocket.o.b(SourceFile:47)
	at net.jodah.failsafe.Functions.lambda$toCtxSupplier$15(Functions.java:276)
	at net.jodah.failsafe.Functions.lambda$getPromiseOfStage$6(Functions.java:154)
	at net.jodah.failsafe.Functions.lambda$getPromiseAsync$4(Functions.java:100)
	... 8 more

We've limited the maxRequestsQueuedPerDestination on the HttpClient to 4 in case that's relevant.