CompletionHandler doesn't handle OperationCanceledException gracefully

[fbricon]

From vscode-java, while trying to autocomplete while the server is still starting:

[Error - 10:55:15 AM] 29-May-2017 10:55:11 AM Problem with codeComplete for file:///Users/fbricon/Dev/souk/vertx-game-server/src/main/java/Foo.java
null
org.eclipse.core.runtime.OperationCanceledException
	at org.eclipse.jdt.internal.codeassist.CompletionEngine.checkCancel(CompletionEngine.java:1819)
	at org.eclipse.jdt.internal.codeassist.CompletionEngine.acceptType(CompletionEngine.java:1315)
	at org.eclipse.jdt.internal.core.SearchableEnvironment$4.acceptType(SearchableEnvironment.java:422)
	at org.eclipse.jdt.internal.core.search.BasicSearchEngine$5.acceptIndexMatch(BasicSearchEngine.java:1808)
	at org.eclipse.jdt.core.search.SearchPattern.acceptMatch(SearchPattern.java:319)
	at org.eclipse.jdt.core.search.SearchPattern.findIndexMatches(SearchPattern.java:2348)
	at org.eclipse.jdt.internal.core.search.matching.MatchLocator.findIndexMatches(MatchLocator.java:280)
	at org.eclipse.jdt.internal.core.search.PatternSearchJob.search(PatternSearchJob.java:109)
	at org.eclipse.jdt.internal.core.search.PatternSearchJob.execute(PatternSearchJob.java:65)
	at org.eclipse.jdt.internal.core.search.processing.JobManager.performConcurrentJob(JobManager.java:262)
	at org.eclipse.jdt.internal.core.search.BasicSearchEngine.searchAllTypeNames(BasicSearchEngine.java:1816)
	at org.eclipse.jdt.internal.core.SearchableEnvironment.findTypes(SearchableEnvironment.java:432)
	at org.eclipse.jdt.internal.codeassist.CompletionEngine.findTypesAndPackages(CompletionEngine.java:10832)
	at org.eclipse.jdt.internal.codeassist.CompletionEngine.completionOnSingleNameReference(CompletionEngine.java:3451)
	at org.eclipse.jdt.internal.codeassist.CompletionEngine.complete(CompletionEngine.java:1849)
	at org.eclipse.jdt.internal.codeassist.CompletionEngine.complete(CompletionEngine.java:2075)
	at org.eclipse.jdt.internal.core.Openable.codeComplete(Openable.java:131)
	at org.eclipse.jdt.internal.core.CompilationUnit.codeComplete(CompilationUnit.java:357)
	at org.eclipse.jdt.internal.core.CompilationUnit.codeComplete(CompilationUnit.java:345)
	at org.eclipse.jdt.ls.core.internal.handlers.CompletionHandler.computeContentAssist(CompletionHandler.java:79)
	at org.eclipse.jdt.ls.core.internal.handlers.CompletionHandler.lambda$0(CompletionHandler.java:39)
	at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:602)
	at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:577)
	at java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:443)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

I think there's no need to propagate OperationCanceledException all the way up. It could be caught and CompletionHandler.computeContentAssist could just return an empty collection.

This raises the issue of how to deal with cancelled monitors in general. Who should be responsible for dealing with them? @gorkem?