fix(lsp): handle null id in JSON-RPC responses

Problem:
LSP spec allows response message to have a null request-id.
This may happen when for example client sends unparseable request.
https://github.com/microsoft/language-server-protocol/issues/196

Solution:
Guard the server response branches against id=vim.NIL (json null),
and handle error responses with null id by logging a warning
and dispatching on error.

Problem:
CI (ubuntu asan, ubuntu tsan, windows) reports `uv_loop_close()
hang?` from the two new null-id response tests. The leaked
handle is the server-side accepted TCP socket created inside
`server:listen` callback. The tests closed only the listener
but not the accepted socket, so libuv could not finish shutting
down the loop and each test session took ~2s extra to exit.

Solution:
Hoist the accepted socket to the outer `exec_lua` scope and
close it at teardown before closing the listener. The close
runs synchronously inside `exec_lua`, so the loop has time to
dispose the handle before the session exits.

* test(lsp): close accepted socket on read-loop exit/error

Match the precedent in the handler test ("handler can return
false as response") and the shared `_create_tcp_server` helper
in `test/functional/plugin/lsp/testutil.lua`: close the
accepted socket from inside the `create_read_loop` exit/error
callbacks. The teardown close added in the previous commit
remains as belt-and-suspenders, so the socket is disposed
whether the server goes away first or the client does.

2 files changed, 161 insertions, 1 deletions

diff --git a/runtime/lua/vim/lsp/rpc.lua b/runtime/lua/vim/lsp/rpc.lua
index 1cc5ffef44..761c0aac17 100644
--- a/runtime/lua/vim/lsp/rpc.lua
+++ b/runtime/lua/vim/lsp/rpc.lua
@@ -380,7 +380,9 @@ function Client:handle_body(body)
 
   if type(decoded) ~= 'table' then
     self:on_error(M.client_errors.INVALID_SERVER_MESSAGE, decoded)
-  elseif type(decoded.method) == 'string' and decoded.id then
+  elseif type(decoded.method) == 'string' and decoded.id and decoded.id ~= vim.NIL then
+    local id_type = type(decoded.id)
+    assert(id_type == 'number' or id_type == 'string', 'Request id must be a number or a string')
     local err --- @type lsp.ResponseError?
     -- Schedule here so that the users functions don't trigger an error and
     -- we can still use the result.
@@ -426,6 +428,7 @@ function Client:handle_body(body)
   -- - If 'result' is nil, then 'error' must be present (and not vim.NIL).
   elseif
     decoded.id
+    and decoded.id ~= vim.NIL
     and (
       (decoded.error == nil and decoded.result ~= nil)
       or (decoded.result == nil and decoded.error ~= nil and decoded.error ~= vim.NIL)
@@ -477,6 +480,9 @@ function Client:handle_body(body)
       self:on_error(M.client_errors.NO_RESULT_CALLBACK_FOUND, decoded)
       log.error('No callback found for server response id ' .. result_id)
     end
+  elseif decoded.id == vim.NIL then
+    log.warn('Server sent response with null id', decoded.error)
+    self:on_error(M.client_errors.INVALID_SERVER_MESSAGE, decoded)
   elseif type(decoded.method) == 'string' then
     -- Notification
     self:try_call(
diff --git a/test/functional/plugin/lsp_spec.lua b/test/functional/plugin/lsp_spec.lua
index 84eb63beea..cd6629b378 100644
--- a/test/functional/plugin/lsp_spec.lua
+++ b/test/functional/plugin/lsp_spec.lua
@@ -3040,6 +3040,160 @@ describe('LSP', function()
       }
       eq(expected, result)
     end)
+
+    it('does not crash on error response with null id (JSON-RPC 2.0 parse error)', function()
+      local result = exec_lua(function()
+        local server = assert(vim.uv.new_tcp())
+        local accepted
+        local messages = {}
+        server:bind('127.0.0.1', 0)
+        server:listen(127, function(err)
+          assert(not err, err)
+          accepted = assert(vim.uv.new_tcp())
+          server:accept(accepted)
+          accepted:read_start(require('vim.lsp.rpc').create_read_loop(function(body)
+            local payload = vim.json.decode(body)
+            if payload.method then
+              table.insert(messages, payload.method)
+              if payload.method == 'initialize' then
+                -- Send a valid initialize response first
+                local msg = vim.json.encode({
+                  id = payload.id,
+                  jsonrpc = '2.0',
+                  result = {
+                    capabilities = {},
+                  },
+                })
+                accepted:write(
+                  table.concat({ 'Content-Length: ', tostring(#msg), '\r\n\r\n', msg })
+                )
+              elseif payload.method == 'initialized' then
+                -- Then send an error response with null id (parse error per JSON-RPC 2.0 §5)
+                local msg =
+                  '{"jsonrpc":"2.0","error":{"code":-32700,"message":"Parse error"},"id":null}'
+                accepted:write(
+                  table.concat({ 'Content-Length: ', tostring(#msg), '\r\n\r\n', msg })
+                )
+              end
+            end
+          end, function()
+            if accepted and not accepted:is_closing() then
+              accepted:close()
+            end
+          end, function()
+            if accepted and not accepted:is_closing() then
+              accepted:close()
+            end
+          end))
+        end)
+        local port = server:getsockname().port
+        local on_error_called = false
+        local client_id = assert(vim.lsp.start({
+          name = 'null-id-test',
+          cmd = vim.lsp.rpc.connect('127.0.0.1', port),
+          on_error = function(_code, _err)
+            on_error_called = true
+          end,
+        }))
+        vim.lsp.get_client_by_id(client_id)
+        -- Wait for the initialized notification to be sent and the null-id error to be received
+        vim.wait(1000, function()
+          return #messages >= 2 and on_error_called
+        end)
+        if accepted and not accepted:is_closing() then
+          accepted:close()
+        end
+        server:close()
+        server:shutdown()
+        return {
+          messages = messages,
+          on_error_called = on_error_called,
+        }
+      end)
+      -- The key assertion: Neovim should not crash, and the error handler should be called
+      eq(true, result.on_error_called)
+      eq(true, #result.messages >= 2)
+    end)
+
+    it('does not misclassify server request with null id as notification', function()
+      local result = exec_lua(function()
+        local server = assert(vim.uv.new_tcp())
+        local accepted
+        local messages = {}
+        server:bind('127.0.0.1', 0)
+        server:listen(127, function(err)
+          assert(not err, err)
+          accepted = assert(vim.uv.new_tcp())
+          server:accept(accepted)
+          accepted:read_start(require('vim.lsp.rpc').create_read_loop(function(body)
+            local payload = vim.json.decode(body)
+            if payload.method then
+              table.insert(messages, payload.method)
+              if payload.method == 'initialize' then
+                local msg = vim.json.encode({
+                  id = payload.id,
+                  jsonrpc = '2.0',
+                  result = {
+                    capabilities = {},
+                  },
+                })
+                accepted:write(
+                  table.concat({ 'Content-Length: ', tostring(#msg), '\r\n\r\n', msg })
+                )
+              elseif payload.method == 'initialized' then
+                -- Send a server request with null id (invalid per JSON-RPC 2.0)
+                local msg =
+                  '{"jsonrpc":"2.0","method":"workspace/configuration","params":{"items":[]},"id":null}'
+                accepted:write(
+                  table.concat({ 'Content-Length: ', tostring(#msg), '\r\n\r\n', msg })
+                )
+              end
+            end
+          end, function()
+            if accepted and not accepted:is_closing() then
+              accepted:close()
+            end
+          end, function()
+            if accepted and not accepted:is_closing() then
+              accepted:close()
+            end
+          end))
+        end)
+        local port = server:getsockname().port
+        local on_error_called = false
+        local notification_received = false
+        local client_id = assert(vim.lsp.start({
+          name = 'null-id-request-test',
+          cmd = vim.lsp.rpc.connect('127.0.0.1', port),
+          on_error = function(_code, _err)
+            on_error_called = true
+          end,
+          handlers = {
+            ['workspace/configuration'] = function()
+              notification_received = true
+              return {}
+            end,
+          },
+        }))
+        vim.lsp.get_client_by_id(client_id)
+        vim.wait(1000, function()
+          return #messages >= 2 and (on_error_called or notification_received)
+        end)
+        if accepted and not accepted:is_closing() then
+          accepted:close()
+        end
+        server:close()
+        server:shutdown()
+        return {
+          messages = messages,
+          on_error_called = on_error_called,
+          notification_received = notification_received,
+        }
+      end)
+      -- Should be dispatched as an error, NOT silently handled as a notification
+      eq(true, result.on_error_called)
+      eq(false, result.notification_received)
+    end)
   end)
 
   describe('#dynamic vim.lsp._dynamic', function()