Ensure variable.fetch_children calls callback on empty result

Author: mfussenegger

lua/dap/entity.lua

@@ -100,10 +100,16 @@ function variable.fetch_children(var, cb)
       elseif resp then
         local variables = resp.variables
         local unloaded = #variables
+        var.variables = variables
+
+        if unloaded == 0 then
+          cb(variables)
+          return
+        end
+
         local function countdown()
           unloaded = unloaded - 1
           if unloaded == 0 then
-            var.variables = variables
             cb(variables)
           end
         end

spec/entity_spec.lua

@@ -0,0 +1,45 @@
+local dap = require('dap')
+local helpers = require("spec.helpers")
+
+local config = {
+  type = 'dummy',
+  request = 'launch',
+  name = 'Launch file',
+}
+
+
+describe("variable entity", function()
+  local server
+  before_each(function()
+    server = require('spec.server').spawn()
+    dap.adapters.dummy = server.adapter
+  end)
+  after_each(function()
+    server.stop()
+    dap.close()
+    helpers.wait(function() return dap.session() == nil end, "session should become nil")
+  end)
+
+  it("fetch_children triggers callback on empty variables", function()
+    server.client.variables = function(self, request)
+      self:send_response(request, {
+        variables = {}
+      })
+    end
+    helpers.run_and_wait_until_initialized(config, server)
+    local variable = require("dap.entity").variable
+
+    local var = {
+      name = "x",
+      value = 1,
+      variablesReference = 1,
+    }
+    local variables = nil
+    variable.fetch_children(var, function (vars)
+      variables = vars
+    end)
+
+    helpers.wait(function() return variables ~= nil end, "must call callback with variables")
+    assert.are.same({}, variables)
+  end)
+end)