Add API for custom LSP requests

Implement a function `ale#lsp_linter#SendRequest` that allows to send
custom LSP requests to an enabled LSP linter.

Resolves #2474

autoload/ale/lsp.vim

@@ -5,6 +5,9 @@
 let s:connections = get(s:, 'connections', {})
 let g:ale_lsp_next_message_id = 1
 
+" A Dictionary to track one-shot callbacks for custom LSP requests
+let s:custom_callbacks = get(s:, 'custom_callbacks', {})
+
 " Given an id, which can be an executable or address, and a project path,
 " create a new connection if needed. Return a unique ID for the connection.
 function! ale#lsp#Register(executable_or_address, project, init_options) abort
@@ -296,10 +299,19 @@ function! ale#lsp#HandleMessage(conn_id, message) abort
     " responses.
     if l:conn.initialized
         for l:response in l:response_list
-            " 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)
-            endfor
+            if has_key(l:response, 'id') && has_key(s:custom_callbacks, l:response.id)
+                " Response to a custom request, call the registered one-shot handler.
+                try
+                    call s:custom_callbacks[l:response.id](l:response)
+                finally
+                    call remove(s:custom_callbacks, l:response.id)
+                endtry
+            else
+                " 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)
+                endfor
+            endif
         endfor
     endif
 endfunction
@@ -525,6 +537,18 @@ function! ale#lsp#Send(conn_id, message) abort
     return l:id == 0 ? -1 : l:id
 endfunction
 
+" Send a custom request to an LSP server.
+" The given callback is called on response.
+function! ale#lsp#SendCustomRequest(conn_id, message, Callback) abort
+    let l:id = ale#lsp#Send(a:conn_id, a:message)
+
+    if l:id > 0
+        let s:custom_callbacks[l:id] = a:Callback
+    endif
+
+    return l:id
+endfunction
+
 " Notify LSP servers or tsserver if a document is opened, if needed.
 " If a document is opened, 1 will be returned, otherwise 0 will be returned.
 function! ale#lsp#OpenDocument(conn_id, buffer, language_id) abort

autoload/ale/lsp_linter.vim

@@ -413,3 +413,29 @@ endfunction
 function! ale#lsp_linter#SetLSPLinterMap(replacement_map) abort
     let s:lsp_linter_map = a:replacement_map
 endfunction
+
+" Send a custom request to an LSP linter.
+function! ale#lsp_linter#SendRequest(buffer, linter_name, method, parameters, Callback) abort
+    let l:filetype = ale#linter#ResolveFiletype(getbufvar(a:buffer, '&filetype'))
+    let l:linter_list = ale#linter#GetAll(l:filetype)
+    let l:linter_list = filter(l:linter_list, {_, v -> v.name is# a:linter_name})
+
+    if len(l:linter_list) < 1
+        throw 'Linter "' . a:linter_name . '" not found!'
+    endif
+
+    let l:linter = l:linter_list[0]
+    let l:executable_or_address = ''
+
+    if l:linter.lsp is# 'socket'
+        let l:executable_or_address = ale#linter#GetAddress(a:buffer, l:linter)
+    else
+        let l:executable_or_address = ale#linter#GetExecutable(a:buffer, l:linter)
+    endif
+
+    let l:root = ale#util#GetFunction(l:linter.project_root)(a:buffer)
+    let l:conn_id = l:executable_or_address . ':' . l:root
+    let l:message = [0, a:method, a:parameters]
+
+    return ale#lsp#SendCustomRequest(l:conn_id, l:message, a:Callback) == 0
+endfunction