Fix infinite autocompletion loop (#5130)

* Check for timeout while waiting for language server result
* Return empty result if language server has no completion capabilities
* Add test for OmniFunc timeout
* Add documentation for timeout option

autoload/ale/completion.vim

@@ -764,6 +764,9 @@ function! s:OnReady(linter, lsp_details) abort
     let l:id = a:lsp_details.connection_id
 
     if !ale#lsp#HasCapability(l:id, 'completion')
+        " Return at least an empty result to avoid OmniFunc timeout
+        call ale#completion#Show([])
+
         return
     endif
 
@@ -945,9 +948,18 @@ function! ale#completion#OmniFunc(findstart, base) abort
     else
         let l:result = ale#completion#GetCompletionResult()
 
+        let l:timeout = get(g:, 'ale_completion_timeout', 3)
+        let l:timeout_start = reltime()
+
         while l:result is v:null && !complete_check()
             sleep 2ms
             let l:result = ale#completion#GetCompletionResult()
+
+            if reltimefloat(reltime(l:timeout_start)) > l:timeout
+                " no-custom-checks
+                echoerr 'no result within timeout (' . l:timeout . 's)'
+                break
+            endif
         endwhile
 
         return l:result isnot v:null ? l:result : []