fix(lsp): send didClose, didOpen when languageId changes #39499

Problem:
If a buffer's filetype changes after the LSP client has already
attached (e.g. from json to jsonc via a modeline), but the client
supports both filetypes, it stays attached. It does not notify the
server of the new languageId, causing the server to incorrectly process
the file using the old languageId.

Solution:
Save the languageId used during textDocument/didOpen, and send
textDocument/didClose + textDocument/didOpen when buffer's languageId
changed.

Lsp spec:
0003fb53f1/_specifications/lsp/3.18/textDocument/didOpen.md (L5)
> If the language id of a document changes, the client
> needs to send a textDocument/didClose to the server followed by a
> textDocument/didOpen with the new language id if the server handles
> the new language id as well.

AI-assisted: Gemini 3.1 Pro

runtime/doc/lsp.txt

@@ -1723,7 +1723,8 @@ Lua module: vim.lsp.client                                        *lsp-client*
 *vim.lsp.Client*
 
     Fields: ~
-      • {attached_buffers}      (`table<integer,true>`)
+      • {attached_buffers}      (`table<integer,string>`) Each buffer's last
+                                used `languageId`.
       • {cancel_request}        (`fun(self: vim.lsp.Client, id: integer): boolean`)
                                 See |Client:cancel_request()|.
       • {capabilities}          (`lsp.ClientCapabilities`) Capabilities

runtime/doc/news.txt

@@ -88,7 +88,7 @@ EVENTS
 
 LSP
 
-• todo
+• `client.attached_buffers[buf]` now stores `languageId` string (was boolean).
 
 LUA
 

runtime/lua/vim/lsp.lua

@@ -531,9 +531,20 @@ local function lsp_enable_callback(bufnr)
       lsp.is_enabled(client.name)
       -- Check that the client is managed by vim.lsp.config before deciding to detach it!
       and lsp.config[client.name]
-      and not can_start(bufnr, lsp.config[client.name], false)
     then
-      lsp.buf_detach_client(bufnr, client.id)
+      if can_start(bufnr, lsp.config[client.name], false) then
+        -- When switch between lsp supported filetype (e.g. json to jsonc like #39498),
+        -- client should send `textDocument/didClose` + `textDocument/didOpen` with new language id
+        local new_language_id = client.get_language_id(bufnr, vim.bo[bufnr].filetype)
+        local old_language_id = client.attached_buffers[bufnr] ---@type string?
+        if old_language_id and old_language_id ~= new_language_id then
+          client:_text_document_did_close_handler(bufnr)
+          client.attached_buffers[bufnr] = new_language_id
+          client:_text_document_did_open_handler(bufnr)
+        end
+      else
+        lsp.buf_detach_client(bufnr, client.id)
+      end
     end
   end
 
@@ -1002,7 +1013,7 @@ function lsp.buf_attach_client(bufnr, client_id)
     return true
   end
 
-  client.attached_buffers[bufnr] = true
+  client.attached_buffers[bufnr] = client.get_language_id(bufnr, vim.bo[bufnr].filetype)
 
   -- This is our first time attaching this client to this buffer.
   -- Send didOpen for the client if it is initialized. If it isn't initialized

runtime/lua/vim/lsp/client.lua

@@ -167,7 +167,8 @@ local all_clients = {}
 
 --- @class vim.lsp.Client
 ---
---- @field attached_buffers table<integer,true>
+--- Each buffer's last used `languageId`.
+--- @field attached_buffers table<integer,string>
 ---
 --- Capabilities provided by the client (editor or tool), at startup.
 --- @field capabilities lsp.ClientCapabilities
@@ -1124,6 +1125,18 @@ function Client:exec_cmd(cmd, context, handler)
   self:request('workspace/executeCommand', params, handler, context.bufnr)
 end
 
+--- Default handler for the 'textDocument/didClose' LSP notification.
+---
+--- @param buf integer Number of the buffer, or 0 for current
+function Client:_text_document_did_close_handler(buf)
+  if not self:supports_method('textDocument/didClose') then
+    return
+  end
+  local uri = vim.uri_from_bufnr(buf)
+  local params = { textDocument = { uri = uri } }
+  self:notify('textDocument/didClose', params)
+end
+
 --- Default handler for the 'textDocument/didOpen' LSP notification.
 ---
 --- @param bufnr integer Number of the buffer, or 0 for current
@@ -1192,7 +1205,7 @@ function Client:on_attach(bufnr)
     end
   end)
 
-  self.attached_buffers[bufnr] = true
+  self.attached_buffers[bufnr] = self:_get_language_id(bufnr)
 end
 
 --- @private
@@ -1383,11 +1396,7 @@ function Client:_on_detach(bufnr)
 
   changetracking.reset_buf(self, bufnr)
 
-  if self:supports_method('textDocument/didClose') then
-    local uri = vim.uri_from_bufnr(bufnr)
-    local params = { textDocument = { uri = uri } }
-    self:notify('textDocument/didClose', params)
-  end
+  self:_text_document_did_close_handler(bufnr)
 
   self.attached_buffers[bufnr] = nil
 

test/functional/plugin/lsp_spec.lua

@@ -4488,6 +4488,52 @@ describe('LSP', function()
       eq({ 0, 'foo', 1, 'bar' }, count_clients())
     end)
 
+    it('sends didClose and didOpen when languageId changes', function()
+      exec_lua(create_server_definition)
+
+      local tmp1 = t.tmpname(true)
+
+      exec_lua(function()
+        _G.server = _G._create_server({
+          handlers = {
+            initialize = function(_, _, callback)
+              callback(nil, { capabilities = { textDocumentSync = { openClose = true } } })
+            end,
+          },
+        })
+        vim.lsp.config('foo', { cmd = _G.server.cmd, filetypes = { 'foo', 'bar' } })
+        vim.lsp.enable('foo')
+        vim.cmd.edit(tmp1)
+      end)
+
+      local function test_messages()
+        local opens = 0
+        local closes = 0
+        local msgs = exec_lua([[ return _G.server.messages ]])
+        local num_clients = exec_lua([[ return #vim.lsp.get_clients() ]])
+        for _, msg in ipairs(msgs) do
+          opens = opens + (msg.method == 'textDocument/didOpen' and 1 or 0)
+          closes = closes + (msg.method == 'textDocument/didClose' and 1 or 0)
+        end
+        return { opens, 'did_open', closes, 'did_close', num_clients, 'clients' }
+      end
+
+      -- No filetype on the buffer yet.
+      eq({ 0, 'did_open', 0, 'did_close', 0, 'clients' }, test_messages())
+
+      -- Set the filetype to 'foo', confirm didOpen is sent.
+      exec_lua([[vim.bo.filetype = 'foo']])
+      retry(nil, 1000, function()
+        eq({ 1, 'did_open', 0, 'did_close', 1, 'clients' }, test_messages())
+      end)
+
+      -- Set to anohter lsp-supported filetype 'bar', confirm didClose and didOpen are sent.
+      exec_lua([[vim.bo.filetype = 'bar']])
+      retry(nil, 1000, function()
+        eq({ 2, 'did_open', 1, 'did_close', 1, 'clients' }, test_messages())
+      end)
+    end)
+
     it('validates config on attach', function()
       local tmp1 = t.tmpname(true)
       exec_lua(function()