Stop tsserver from causing errors to be rendered redundantly

autoload/ale/lsp_linter.vim

@@ -55,16 +55,29 @@ function! s:HandleTSServerDiagnostics(response, error_type) abort
     endif
 
     let l:thislist = ale#lsp#response#ReadTSServerDiagnostics(a:response)
+    let l:no_changes = 0
 
     " tsserver sends syntax and semantic errors in separate messages, so we
     " have to collect the messages separately for each buffer and join them
     " back together again.
     if a:error_type is# 'syntax'
+        if len(l:thislist) is 0 && len(get(l:info, 'syntax_loclist', [])) is 0
+            let l:no_changes = 1
+        endif
+
         let l:info.syntax_loclist = l:thislist
     else
+        if len(l:thislist) is 0 && len(get(l:info, 'semantic_loclist', [])) is 0
+            let l:no_changes = 1
+        endif
+
         let l:info.semantic_loclist = l:thislist
     endif
 
+    if l:no_changes
+        return
+    endif
+
     let l:loclist = get(l:info, 'semantic_loclist', [])
     \   + get(l:info, 'syntax_loclist', [])
 
@@ -99,9 +112,10 @@ endfunction
 
 function! ale#lsp_linter#HandleLSPResponse(conn_id, response) abort
     let l:method = get(a:response, 'method', '')
-    let l:linter_name = get(s:lsp_linter_map, a:conn_id, '')
 
     if get(a:response, 'jsonrpc', '') is# '2.0' && has_key(a:response, 'error')
+        let l:linter_name = get(s:lsp_linter_map, a:conn_id, '')
+
         call s:HandleLSPErrorMessage(l:linter_name, a:response)
     elseif l:method is# 'textDocument/publishDiagnostics'
         call s:HandleLSPDiagnostics(a:conn_id, a:response)

test/test_redundant_tsserver_rendering_avoided.vader

@@ -0,0 +1,136 @@
+Before:
+  Save g:ale_buffer_info
+
+  function! CreateError(type, message) abort
+    let l:diagnostics = []
+
+    if !empty(a:message)
+      let l:diagnostics = [{
+      \ 'start': {'line': 1, 'offset': 1},
+      \ 'end': {'line': 1, 'offset':1},
+      \ 'text': a:message,
+      \ 'code': 1005,
+      \}]
+    endif
+
+    return {
+    \ 'seq': 0,
+    \ 'type': 'event',
+    \ 'event': a:type,
+    \ 'body': {'file': expand('%:p'), 'diagnostics': l:diagnostics},
+    \}
+  endfunction
+
+  function! CreateLoclist(message) abort
+    let l:list = []
+
+    if !empty(a:message)
+      let l:list = [{
+      \ 'lnum': 1,
+      \ 'col': 1,
+      \ 'end_lnum': 1,
+      \ 'end_col': 1,
+      \ 'text': a:message,
+      \}]
+    endif
+
+    return l:list
+  endfunction
+
+  call ale#test#SetDirectory('/testplugin/test')
+  call ale#test#SetFilename('filename.ts')
+
+  runtime autoload/ale/engine.vim
+
+  let g:ale_buffer_info = {bufnr(''): {'loclist': []}}
+  let g:ale_handle_loclist_called = 0
+
+  function! ale#engine#HandleLoclist(linter_name, buffer, loclist) abort
+    let g:ale_handle_loclist_called = 1
+  endfunction
+
+After:
+  Restore
+
+  delfunction CreateError
+  delfunction CreateLoclist
+
+  call ale#test#RestoreDirectory()
+
+  runtime autoload/ale/engine.vim
+
+Execute(An initial empty list of syntax errors should be ignored):
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('syntaxDiag', ''))
+
+  Assert !g:ale_handle_loclist_called
+
+Execute(An initial list of syntax errors should be handled):
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('syntaxDiag', 'x'))
+
+  Assert g:ale_handle_loclist_called
+
+Execute(Subsequent empty lists should be ignored):
+  let g:ale_buffer_info[bufnr('')].syntax_loclist = []
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('syntaxDiag', ''))
+
+  Assert !g:ale_handle_loclist_called
+
+Execute(Empty then non-empty syntax errors should be handled):
+  let g:ale_buffer_info[bufnr('')].syntax_loclist = []
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('syntaxDiag', 'x'))
+
+  Assert g:ale_handle_loclist_called
+
+Execute(Non-empty then empty syntax errors should be handled):
+  let g:ale_buffer_info[bufnr('')].syntax_loclist = CreateLoclist('x')
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('syntaxDiag', ''))
+
+  Assert g:ale_handle_loclist_called
+
+Execute(Non-empty then non-empty syntax errors should be handled):
+  let g:ale_buffer_info[bufnr('')].syntax_loclist = CreateLoclist('x')
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('syntaxDiag', 'x'))
+
+  Assert g:ale_handle_loclist_called
+
+Execute(An initial empty list of semantic errors should be ignored):
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('semanticDiag', ''))
+
+  Assert !g:ale_handle_loclist_called
+
+Execute(An initial list of semantic errors should be handled):
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('semanticDiag', 'x'))
+
+  Assert g:ale_handle_loclist_called
+
+Execute(Subsequent empty lists should be ignored):
+  let g:ale_buffer_info[bufnr('')].semantic_loclist = []
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('semanticDiag', ''))
+
+  Assert !g:ale_handle_loclist_called
+
+Execute(Empty then non-empty semantic errors should be handled):
+  let g:ale_buffer_info[bufnr('')].semantic_loclist = []
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('semanticDiag', 'x'))
+
+  Assert g:ale_handle_loclist_called
+
+Execute(Non-empty then empty semantic errors should be handled):
+  let g:ale_buffer_info[bufnr('')].semantic_loclist = CreateLoclist('x')
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('semanticDiag', ''))
+
+  Assert g:ale_handle_loclist_called
+
+Execute(Non-empty then non-empty semantic errors should be handled):
+  let g:ale_buffer_info[bufnr('')].semantic_loclist = CreateLoclist('x')
+
+  call ale#lsp_linter#HandleLSPResponse(1, CreateError('semanticDiag', 'x'))
+
+  Assert g:ale_handle_loclist_called