Render consul templates using task env only

When rendering a task consul template, ensure that only task environment
variables are used.

Currently, `consul-template` always falls back to host process
environment variables when key isn't a task env var[1].  Thus, we add
an empty entry for each host process env-var not found in task env-vars.

[1] https://github.com/hashicorp/consul-template/blob/bfa5d0e133688920afd1e012404f765182e3d5e0/template/funcs.go#L61-L75

client/allocrunner/taskrunner/template/template.go

@@ -509,7 +509,7 @@ func templateRunner(config *TaskTemplateManagerConfig) (
 	}
 
 	// Set Nomad's environment variables
-	runner.Env = config.EnvBuilder.Build().All()
+	runner.Env = maskProcessEnv(config.EnvBuilder.Build().All())
 
 	// Build the lookup
 	idMap := runner.TemplateConfigMapping()
@@ -525,6 +525,20 @@ func templateRunner(config *TaskTemplateManagerConfig) (
 	return runner, lookup, nil
 }
 
+// maskProcessEnv masks away any environment variable not found in task env.
+// It manipulates the parameter directly and returns it without copying.
+func maskProcessEnv(env map[string]string) map[string]string {
+	procEnvs := os.Environ()
+	for _, e := range procEnvs {
+		ekv := strings.SplitN(e, "=", 2)
+		if _, ok := env[ekv[0]]; !ok {
+			env[ekv[0]] = ""
+		}
+	}
+
+	return env
+}
+
 // parseTemplateConfigs converts the tasks templates in the config into
 // consul-templates
 func parseTemplateConfigs(config *TaskTemplateManagerConfig) (map[ctconf.TemplateConfig]*structs.Template, error) {

client/allocrunner/taskrunner/template/template_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/hashicorp/nomad/client/config"
 	"github.com/hashicorp/nomad/client/taskenv"
 	"github.com/hashicorp/nomad/helper"
+	"github.com/hashicorp/nomad/helper/uuid"
 	"github.com/hashicorp/nomad/nomad/mock"
 	"github.com/hashicorp/nomad/nomad/structs"
 	sconfig "github.com/hashicorp/nomad/nomad/structs/config"
@@ -1022,6 +1023,52 @@ func TestTaskTemplateManager_Signal_Error(t *testing.T) {
 	require.Contains(harness.mockHooks.KillEvent.DisplayMessage, "failed to send signals")
 }
 
+// TestTaskTemplateManager_FiltersProcessEnvVars asserts that we only render
+// environment variables found in task env-vars and not read the nomad host
+// process environment variables.  nomad host process environment variables
+// are to be treated the same as not found environment variables.
+func TestTaskTemplateManager_FiltersEnvVars(t *testing.T) {
+	t.Parallel()
+
+	defer os.Setenv("NOMAD_TASK_NAME", os.Getenv("NOMAD_TASK_NAME"))
+	os.Setenv("NOMAD_TASK_NAME", "should be overriden by task")
+
+	testenv := "TESTENV_" + strings.ReplaceAll(uuid.Generate(), "-", "")
+	os.Setenv(testenv, "MY_TEST_VALUE")
+	defer os.Unsetenv(testenv)
+
+	// Make a template that will render immediately
+	content := `Hello Nomad Task: {{env "NOMAD_TASK_NAME"}}
+TEST_ENV: {{ env "` + testenv + `" }}
+TEST_ENV_NOT_FOUND: {{env "` + testenv + `_NOTFOUND" }}`
+	expected := fmt.Sprintf("Hello Nomad Task: %s\nTEST_ENV: \nTEST_ENV_NOT_FOUND: ", TestTaskName)
+
+	file := "my.tmpl"
+	template := &structs.Template{
+		EmbeddedTmpl: content,
+		DestPath:     file,
+		ChangeMode:   structs.TemplateChangeModeNoop,
+	}
+
+	harness := newTestHarness(t, []*structs.Template{template}, false, false)
+	harness.start(t)
+	defer harness.stop()
+
+	// Wait for the unblock
+	select {
+	case <-harness.mockHooks.UnblockCh:
+	case <-time.After(time.Duration(5*testutil.TestMultiplier()) * time.Second):
+		require.Fail(t, "Task unblock should have been called")
+	}
+
+	// Check the file is there
+	path := filepath.Join(harness.taskDir, file)
+	raw, err := ioutil.ReadFile(path)
+	require.NoError(t, err)
+
+	require.Equal(t, expected, string(raw))
+}
+
 // TestTaskTemplateManager_Env asserts templates with the env flag set are read
 // into the task's environment.
 func TestTaskTemplateManager_Env(t *testing.T) {