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 : []

doc/ale.txt

@@ -1143,6 +1143,20 @@ g:ale_completion_max_suggestions
   Adjust this option as needed, depending on the complexity of your codebase
   and your available processing power.
 
+                                               *ale-options.completion_timeout*
+                                                     *g:ale_completion_timeout*
+completion_timeout
+g:ale_completion_timeout
+  Type: |Number|
+  Default: `3`
+
+  The maximum time in seconds ale#completion#OmniFunc() will wait for a
+  completion result from the language server.
+
+  When the timeout is exceeded ale#completion#OmniFunc() will print an error
+  message and return an empty result. Setting a higher value allows longer
+  response times, but it may also keep Vim unresponsible for a longer time.
+
                                                     *ale-options.cursor_detail*
                                                           *g:ale_cursor_detail*
 cursor_detail

test/completion/test_omnifunc_completion.vader

@@ -58,3 +58,10 @@ Execute(The omnifunc function should parse and return async responses):
 
     AssertEqual ['foo'], ale#completion#OmniFunc(0, '')
   endif
+
+Execute (The omnifunc function should timeout with an error, if no result is returned):
+  " WARNING: This will lock the whole test-suite, if the timeout does not work!
+  let g:ale_completion_timeout = 0.2
+
+  AssertThrows call ale#completion#OmniFunc(0, '')
+  AssertEqual 'Vim(echoerr):no result within timeout (0.2s)', g:vader_exception