fix(lsp): support false result in handlers (#20252)

Closes https://github.com/neovim/neovim/issues/20111

runtime/lua/vim/lsp/rpc.lua

@@ -405,8 +405,7 @@ function Client:handle_body(body)
           { status = status, result = result, err = err }
         )
       if status then
-        if not (result or err) then
+        if result == nil and err == nil then
-          -- TODO this can be a problem if `null` is sent for result. needs vim.NIL
           error(
             string.format(
               'method %q: either a result or an error must be sent to the server in response',

test/functional/plugin/lsp_spec.lua

@@ -3292,4 +3292,75 @@ describe('LSP', function()
       eq(result.method, "initialize")
     end)
   end)
+
+  describe('handlers', function()
+    it('handler can return false as response', function()
+      local result = exec_lua [[
+        local uv = vim.loop
+        local server = uv.new_tcp()
+        local messages = {}
+        local responses = {}
+        server:bind('127.0.0.1', 0)
+        server:listen(127, function(err)
+          assert(not err, err)
+          local socket = uv.new_tcp()
+          server:accept(socket)
+          socket: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 = {}
+                  },
+                })
+                socket:write(table.concat({'Content-Length: ', tostring(#msg), '\r\n\r\n', msg}))
+              elseif payload.method == 'initialized' then
+                local msg = vim.json.encode({
+                  id = 10,
+                  jsonrpc = '2.0',
+                  method = 'dummy',
+                  params = {},
+                })
+                socket:write(table.concat({'Content-Length: ', tostring(#msg), '\r\n\r\n', msg}))
+              end
+            else
+              table.insert(responses, payload)
+              socket:close()
+            end
+          end))
+        end)
+        local port = server:getsockname().port
+        local handler_called = false
+        vim.lsp.handlers['dummy'] = function(err, result)
+          handler_called = true
+          return false
+        end
+        local client_id = vim.lsp.start({ name = 'dummy', cmd = vim.lsp.rpc.connect('127.0.0.1', port) })
+        local client = vim.lsp.get_client_by_id(client_id)
+        vim.wait(1000, function() return #messages == 2 and handler_called and #responses == 1 end)
+        server:close()
+        server:shutdown()
+        return {
+          messages = messages,
+          handler_called = handler_called,
+          responses = responses }
+      ]]
+      local expected = {
+        messages = { 'initialize', 'initialized' },
+        handler_called = true,
+        responses = {
+          {
+            id = 10,
+            jsonrpc = '2.0',
+            result = false
+          }
+        }
+      }
+      eq(expected, result)
+    end)
+  end)
 end)