Send JSON-RPC error responses for unknown request methods

autoload/ale/codefix.vim

@@ -244,7 +244,7 @@ function! ale#codefix#HandleLSPResponse(conn_id, response) abort
         let l:changes_map = ale#code_action#GetChanges(l:params.edit)
 
         if empty(l:changes_map)
-            return
+            return v:true
         endif
 
         let l:changes = ale#code_action#BuildChangesList(l:changes_map)
@@ -256,6 +256,8 @@ function! ale#codefix#HandleLSPResponse(conn_id, response) abort
         \   },
         \   {}
         \)
+
+        return v:true
     elseif has_key(a:response, 'id')
     \&& has_key(s:codefix_map, a:response.id)
         let l:data = remove(s:codefix_map, a:response.id)

autoload/ale/lsp.vim

@@ -395,10 +395,21 @@ function! ale#lsp#HandleMessage(conn_id, message) abort
     " responses.
     if l:conn.initialized
         for l:response in l:response_list
+            let l:handled = 0
+
             " Call all of the registered handlers with the response.
             for l:Callback in l:conn.callback_list
-                call ale#util#GetFunction(l:Callback)(a:conn_id, l:response)
+                let l:result = ale#util#GetFunction(l:Callback)(a:conn_id, l:response)
+
+                if l:result is v:true
+                    let l:handled = 1
+                endif
             endfor
+
+            " If this was a request that no handler processed, send Method Not Found error
+            if !l:handled && has_key(l:response, 'method') && has_key(l:response, 'id')
+                call s:SendMethodNotFoundResponse(a:conn_id, l:response.id, l:response.method)
+            endif
         endfor
     endif
 endfunction
@@ -676,6 +687,29 @@ function! ale#lsp#StopAll() abort
     endfor
 endfunction
 
+function! s:SendMethodNotFoundResponse(conn_id, request_id, method) abort
+    let l:conn = get(s:connections, a:conn_id, {})
+
+    if empty(l:conn)
+        return
+    endif
+
+    let l:error_response = {
+    \   'jsonrpc': '2.0',
+    \   'id': a:request_id,
+    \   'error': {
+    \       'code': -32601,
+    \       'message': 'Method not found',
+    \       'data': 'Unknown method: ' . a:method
+    \   }
+    \}
+
+    let l:body = json_encode(l:error_response)
+    let l:data = 'Content-Length: ' . strlen(l:body) . "\r\n\r\n" . l:body
+
+    call s:SendMessageData(l:conn, l:data)
+endfunction
+
 function! s:SendMessageData(conn, data) abort
     if has_key(a:conn, 'job_id')
         call ale#job#SendRaw(a:conn.job_id, a:data)

autoload/ale/lsp_linter.vim

@@ -234,6 +234,8 @@ function! ale#lsp_linter#HandleLSPResponse(conn_id, response) abort
         let l:diagnostics = a:response.params.diagnostics
 
         call ale#lsp_linter#HandleLSPDiagnostics(a:conn_id, l:uri, l:diagnostics)
+
+        return v:true
     elseif has_key(s:diagnostic_uri_map, get(a:response, 'id'))
         let l:uri = remove(s:diagnostic_uri_map, a:response.id)
         let l:diagnostics = a:response.result.kind is# 'unchanged'
@@ -247,16 +249,24 @@ function! ale#lsp_linter#HandleLSPResponse(conn_id, response) abort
         \   g:ale_lsp_show_message_format,
         \   a:response.params
         \)
+
+        return v:true
     elseif get(a:response, 'type', '') is# 'event'
     \&& get(a:response, 'event', '') is# 'semanticDiag'
         call s:HandleTSServerDiagnostics(a:response, 'semantic')
+
+        return v:true
     elseif get(a:response, 'type', '') is# 'event'
     \&& get(a:response, 'event', '') is# 'syntaxDiag'
         call s:HandleTSServerDiagnostics(a:response, 'syntax')
+
+        return v:true
     elseif get(a:response, 'type', '') is# 'event'
     \&& get(a:response, 'event', '') is# 'suggestionDiag'
     \&& get(g:, 'ale_lsp_suggestions')
         call s:HandleTSServerDiagnostics(a:response, 'suggestion')
+
+        return v:true
     endif
 endfunction