fix(lsp): always return boolean in lsp.buf_client_attach (#24077)

Co-authored-by: Mathias Fussenegger <f.mathias@zignar.net>

runtime/doc/lsp.txt

@@ -678,6 +678,10 @@ buf_attach_client({bufnr}, {client_id})          *vim.lsp.buf_attach_client()*
       • {bufnr}      (integer) Buffer handle, or 0 for current
       • {client_id}  (integer) Client id
 
+    Return: ~
+        (boolean) success `true` if client was attached successfully; `false`
+        otherwise
+
 buf_detach_client({bufnr}, {client_id})          *vim.lsp.buf_detach_client()*
     Detaches client from the specified buffer. Note: While the server is
     notified that the text document (buffer) was closed, it is still able to

runtime/lua/vim/lsp.lua

@@ -1799,6 +1799,7 @@ end
 ---
 ---@param bufnr (integer) Buffer handle, or 0 for current
 ---@param client_id (integer) Client id
+---@return boolean success `true` if client was attached successfully; `false` otherwise
 function lsp.buf_attach_client(bufnr, client_id)
   validate({
     bufnr = { bufnr, 'n', true },
@@ -1887,7 +1888,7 @@ function lsp.buf_attach_client(bufnr, client_id)
   end
 
   if buffer_client_ids[client_id] then
-    return
+    return true
   end
   -- This is our first time attaching this client to this buffer.
   buffer_client_ids[client_id] = true

test/functional/plugin/lsp_spec.lua

@@ -1070,7 +1070,7 @@ describe('LSP', function()
           eq(full_kind, client.server_capabilities().textDocumentSync.change)
           eq(true, client.server_capabilities().textDocumentSync.openClose)
           exec_lua [[
-            assert(not lsp.buf_attach_client(BUFFER, TEST_RPC_CLIENT_ID), "Shouldn't attach twice")
+            assert(lsp.buf_attach_client(BUFFER, TEST_RPC_CLIENT_ID), "Already attached, returns true")
           ]]
         end;
         on_exit = function(code, signal)