From 2c4cdedcaebe9412a1f2ca05b0102ff96b21d147 Mon Sep 17 00:00:00 2001
From: Joachim Bauch <bauch@struktur.de>
Date: Tue, 30 Jan 2024 16:14:28 +0100
Subject: [PATCH] Don't run monitor if empty.

---
 dns_monitor.go      | 36 +++++++++++++++++++++++++++++++++++-
 dns_monitor_test.go | 17 +++++++++++++++++
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/dns_monitor.go b/dns_monitor.go
index be19d63..48f6231 100644
--- a/dns_monitor.go
+++ b/dns_monitor.go
@@ -126,7 +126,11 @@ type DnsMonitor struct {
 	stopFunc func()
 
 	mu        sync.RWMutex
+	cond      *sync.Cond
 	hostnames map[string]*dnsMonitorEntry
+
+	// Can be overwritten from tests.
+	checkHostnames func()
 }
 
 func NewDnsMonitor(interval time.Duration) (*DnsMonitor, error) {
@@ -143,6 +147,8 @@ func NewDnsMonitor(interval time.Duration) (*DnsMonitor, error) {
 
 		hostnames: make(map[string]*dnsMonitorEntry),
 	}
+	monitor.cond = sync.NewCond(&monitor.mu)
+	monitor.checkHostnames = monitor.doCheckHostnames
 	return monitor, nil
 }
 
@@ -153,6 +159,7 @@ func (m *DnsMonitor) Start() error {
 
 func (m *DnsMonitor) Stop() {
 	m.stopFunc()
+	m.cond.Signal()
 }
 
 func (m *DnsMonitor) Add(target string, callback DnsMonitorCallback) (*DnsMonitorEntry, error) {
@@ -191,6 +198,7 @@ func (m *DnsMonitor) Add(target string, callback DnsMonitorCallback) (*DnsMonito
 	}
 	e.entry = entry
 	entry.entries[e] = true
+	m.cond.Signal()
 	return e, nil
 }
 
@@ -209,11 +217,37 @@ func (m *DnsMonitor) Remove(entry *DnsMonitorEntry) {
 
 	entry.entry = nil
 	delete(e.entries, entry)
+	if len(e.entries) == 0 {
+		delete(m.hostnames, e.hostname)
+	}
+}
+
+func (m *DnsMonitor) waitForEntries() (waited bool) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	for len(m.hostnames) == 0 && m.stopCtx.Err() == nil {
+		m.cond.Wait()
+		waited = true
+	}
+	return
 }
 
 func (m *DnsMonitor) run() {
 	ticker := time.NewTicker(m.interval)
+	defer ticker.Stop()
+
 	for {
+		if m.waitForEntries() {
+			ticker.Reset(m.interval)
+			if m.stopCtx.Err() == nil {
+				// Initial check when a new entry was added. More checks will be
+				// triggered by the Ticker.
+				m.checkHostnames()
+				continue
+			}
+		}
+
 		select {
 		case <-m.stopCtx.Done():
 			return
@@ -223,7 +257,7 @@ func (m *DnsMonitor) run() {
 	}
 }
 
-func (m *DnsMonitor) checkHostnames() {
+func (m *DnsMonitor) doCheckHostnames() {
 	m.mu.RLock()
 	defer m.mu.RUnlock()
 
diff --git a/dns_monitor_test.go b/dns_monitor_test.go
index 72cd89d..0b1466d 100644
--- a/dns_monitor_test.go
+++ b/dns_monitor_test.go
@@ -27,6 +27,7 @@ import (
 	"net"
 	"reflect"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 )
@@ -315,3 +316,19 @@ func TestDnsMonitorIP(t *testing.T) {
 	rec1.ExpectNone()
 	time.Sleep(5 * interval)
 }
+
+func TestDnsMonitorNoLookupIfEmpty(t *testing.T) {
+	interval := time.Millisecond
+	monitor := newDnsMonitorForTest(t, interval)
+
+	var checked atomic.Bool
+	monitor.checkHostnames = func() {
+		checked.Store(true)
+		monitor.doCheckHostnames()
+	}
+
+	time.Sleep(10 * interval)
+	if checked.Load() {
+		t.Error("should not have checked hostnames")
+	}
+}