client: add new nested variables to task's hcl ctx

The error messages are really bad, but it's extremely difficult to
produce good error messages without the original HCL.

client/allocrunner/taskrunner/task_runner.go

@@ -462,8 +462,29 @@ func (tr *TaskRunner) runDriver() error {
 	// TODO(nickethier): make sure this uses alloc.AllocatedResources once #4750 is rebased
 	taskConfig := tr.buildTaskConfig()
 
+	// Build hcl context variables
+	vars, errs, err := tr.envBuilder.Build().AllValues()
+	if err != nil {
+		return err
+	}
+
+	// Handle per-key errors
+	if len(errs) > 0 {
+		keys := make([]string, 0, len(errs))
+		for k, err := range errs {
+			keys = append(keys, k)
+
+			if tr.logger.IsTrace() {
+				// Verbosely log every diagnostic for debugging
+				tr.logger.Trace("error building environment variables", "key", k, "error", err)
+			}
+		}
+
+		tr.logger.Warn("some environment variables not available for rendering", "keys", strings.Join(keys, ", "))
+	}
+
 	evalCtx := &hcl.EvalContext{
-		Variables: tr.envBuilder.Build().AllValues(),
+		Variables: vars,
 		Functions: shared.GetStdlibFuncs(),
 	}
 

client/allocrunner/taskrunner/task_runner_test.go

@@ -176,9 +176,8 @@ func TestTaskRunner_TaskEnv(t *testing.T) {
 	// Use interpolation from both node attributes and meta vars
 	task.Config = map[string]interface{}{
 		"run_for":       time.Millisecond,
-		"stdout_string": `${node.region} ${NOMAD_META_foo}`,
+		"stdout_string": `${node.region} ${NOMAD_META_foo} ${NOMAD_META_common_user}`,
 	}
-	task.User = "${NOMAD_META_common_user}"
 
 	conf, cleanup := testTaskRunnerConfig(t, alloc, task.Name)
 	defer cleanup()
@@ -211,6 +210,5 @@ func TestTaskRunner_TaskEnv(t *testing.T) {
 	driverCfg, mockCfg := mockDriver.GetTaskConfig()
 	require.NotNil(driverCfg)
 	require.NotNil(mockCfg)
-	assert.Equal(t, "somebody", driverCfg.User)
-	assert.Equal(t, "global bar", mockCfg.StdoutString)
+	assert.Equal(t, "global bar somebody", mockCfg.StdoutString)
 }