Long if-then-else statement crashes language server

[SimonCockx]

A long if-then-else expression can results in a null pointer in the generated ANTRL parser, which causes the language server to crash.

Stacktrace:

java.util.concurrent.CompletionException: java.lang.NullPointerException: parseResult is null
	at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:332)
	at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:347)
	at java.base/java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:708)
	at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)
	at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2162)
	at org.eclipse.xtext.ide.server.concurrent.AbstractRequest.logAndCompleteExceptionally(AbstractRequest.java:73)
	at org.eclipse.xtext.ide.server.concurrent.ReadRequest.lambda$doRun$0(ReadRequest.java:69)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: java.lang.NullPointerException: parseResult is null
	at org.eclipse.xtext.ide.editor.contentassist.antlr.ContentAssistContextFactory.create(ContentAssistContextFactory.java:130)
	at org.eclipse.xtext.ide.server.contentassist.ContentAssistService.createProposals(ContentAssistService.java:104)
	at org.eclipse.xtext.ide.server.contentassist.ContentAssistService.createCompletionList(ContentAssistService.java:78)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.lambda$completion$23(LanguageServerImpl.java:554)
	at org.eclipse.xtext.ide.server.WorkspaceManager.doRead(WorkspaceManager.java:438)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.completion(LanguageServerImpl.java:553)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.lambda$completion$22(LanguageServerImpl.java:539)
	at org.eclipse.xtext.ide.server.concurrent.ReadRequest.lambda$doRun$0(ReadRequest.java:66)

Reproduce: create an if-then-else expression having 30+ branches.

[SimonCockx]

Related: https://www.eclipse.org/forums/index.php/m/1861451/

[SimonCockx]

Potential solutions:

• somehow restructure parser to use a loop rather than recursive descent (https://stackoverflow.com/questions/40571941/antlr-parser-stackoverflowexception)
• implement a kind of switch statement that can replace long nested if-then-elses.

[SimonCockx]

Since #671, the language server will fail more gracefully by giving back a diagnostic to the user.