Completion "hangs" with no response

[vrubezhny]

In some cases Completion hardly "hangs" without a response from server logged, like:

[2023-07-20T15:04:27.107621992+02:00] LSP4E_TO_LANGUAGE_SERVER org.eclipse.wildwebdeveloper.xml:
{"jsonrpc":"2.0","id":"31","method":"textDocument/completion","params":{"textDocument":{"uri":"file:///home/jeremy/projects/eclipse/source/tycho/demo/osgi-repository/repository/pom.xml"},"position":{"line":36,"character":5}}}

For reference issue see: eclipse-lemminx/lemminx-maven#444

I believe LSP4E has problems in sending a request to the server, - a problem which I'd like to be investigated - so the response cannot be generated, but even if a LS is causing the problem, completion request is never finishes on LSP4E due to "infinite waiting on the completion future by calling .get():
https://github.com/eclipse/lsp4e/blob/55ce45f20dba1147cfa8383412f0105bfe96fc2e/org.eclipse.lsp4e/src/org/eclipse/lsp4e/operations/completion/LSContentAssistProcessor.java#L114C11-L114C11

this.completionLanguageServersFuture = LanguageServers.forDocument(document)
	.withFilter(capabilities -> capabilities.getCompletionProvider() != null)
	.collectAll((w, ls) -> ls.getTextDocumentService().completion(param)
		.thenAccept(completion -> proposals.addAll(toProposals(document, offset, completion, w))));
this.completionLanguageServersFuture.get();

While it's very difficult to catch the reason of sending request/receiving response failures, the using CompletableFuture.get() is to be strongly avoided.

[angelozerr]

Using this.completionLanguageServersFuture.get(); means that you trust the language server, but if language server return a result whish never finish, the Thread this.completionLanguageServersFuture.get() is never free.

If user opens several completions, a Thread is taken again and again from the pool and after several completion, there is no Thread in the pool and language server becomes unusable not only with completion but with other features like highlighting, validation etc

I suspect that this issue with strange error comes from this problem with Thread which are not available in the pool eclipse-wildwebdeveloper/wildwebdeveloper#1251

On my side when I edit a pom.xml file after some operations like completion, validation, I loose my all LSP features and I have strange error like eclipse-wildwebdeveloper/wildwebdeveloper#1251 and when I try to higlight a tag name, it doesn't work, so I suspect that there is no Thread in the pool available to process LSP operations.