util: Add new blocked timer utility

This adds a new *BlockedTimer struct which can be used to run a function
or a printf after a short duration. It can be cancelled early. A short
blog post about the topic is available here:

https://purpleidea.com/blog/2023/05/26/blocked-select-logging-in-golang/

util/bt_test.go

@@ -0,0 +1,123 @@
+// Mgmt
+// Copyright (C) 2013-2023+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+//go:build !root
+
+package util
+
+import (
+	"testing"
+	"time"
+)
+
+func TestBlockedTimer1(t *testing.T) {
+	bt := &BlockedTimer{Seconds: 1}
+	defer bt.Cancel()
+	ch := make(chan struct{})
+	bt.Run(func() { close(ch) })
+	select {
+	case <-time.After(time.Duration(3) * time.Second):
+		t.Errorf("the timer was too slow")
+	case <-ch:
+	}
+}
+
+func TestBlockedTimer2(t *testing.T) {
+	bt := &BlockedTimer{Seconds: 3}
+	defer bt.Cancel()
+	ch := make(chan struct{})
+	bt.Run(func() { close(ch) })
+	select {
+	case <-time.After(time.Duration(1) * time.Second):
+	case <-ch:
+		t.Errorf("the timer was too fast")
+	}
+}
+
+func TestBlockedTimer3(t *testing.T) {
+	bt := &BlockedTimer{Seconds: 2}
+	defer bt.Cancel()
+	ch := make(chan struct{})
+	bt.Run(func() { close(ch) })
+	select {
+	case <-time.After(time.Duration(1) * time.Second):
+	}
+	bt.Cancel()
+	select {
+	case <-time.After(time.Duration(2) * time.Second):
+	case <-ch:
+		t.Errorf("the channel should not have closed")
+	}
+}
+
+func TestBlockedTimer1b(t *testing.T) {
+	bt := BlockedTimer{Seconds: 1}
+	defer bt.Cancel()
+	ch := make(chan struct{})
+	bt.Run(func() { close(ch) })
+	select {
+	case <-time.After(time.Duration(3) * time.Second):
+		t.Errorf("the timer was too slow")
+	case <-ch:
+	}
+}
+
+func TestBlockedTimer2b(t *testing.T) {
+	bt := BlockedTimer{Seconds: 3}
+	defer bt.Cancel()
+	ch := make(chan struct{})
+	bt.Run(func() { close(ch) })
+	select {
+	case <-time.After(time.Duration(1) * time.Second):
+	case <-ch:
+		t.Errorf("the timer was too fast")
+	}
+}
+
+func TestBlockedTimer3b(t *testing.T) {
+	bt := BlockedTimer{Seconds: 3}
+	defer bt.Cancel()
+	ch := make(chan struct{})
+	bt.Run(func() { close(ch) })
+	select {
+	case <-time.After(time.Duration(1) * time.Second):
+	}
+	bt.Cancel()
+	select {
+	case <-time.After(time.Duration(2) * time.Second):
+	case <-ch:
+		t.Errorf("the channel should not have closed")
+	}
+}
+
+// just an example to see how to use AfterFunc instead of BlockedTimer
+func TestAfterFunc1(t *testing.T) {
+	ch := make(chan struct{})
+	af := time.AfterFunc(time.Duration(2)*time.Second, func() { close(ch) })
+	defer af.Stop()
+	select {
+	case <-time.After(time.Duration(1) * time.Second):
+	}
+	af.Stop()
+	af.Stop()
+	af.Stop()
+	select {
+	case <-ch:
+		t.Errorf("the timer was too fast")
+	default:
+	}
+}