syncs: add Map (#6260)

Map is a concurrent safe map that is a trivial wrapper
over a Go map and a sync.RWMutex.

It is optimized for use-cases where the entries change often,
which is the opposite use-case of what sync.Map is optimized for.

The API is patterned off of sync.Map, but made generic.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

cmd/derper/depaware.txt

@@ -76,6 +76,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         tailscale.com/util/dnsname                                   from tailscale.com/hostinfo+
    W    tailscale.com/util/endian                                    from tailscale.com/net/netns
         tailscale.com/util/lineread                                  from tailscale.com/hostinfo+
+        tailscale.com/util/mak                                       from tailscale.com/syncs
         tailscale.com/util/singleflight                              from tailscale.com/net/dnscache
    L    tailscale.com/util/strs                                      from tailscale.com/hostinfo
    W 💣 tailscale.com/util/winutil                                   from tailscale.com/hostinfo+

cmd/tailscale/depaware.txt

@@ -100,7 +100,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
    W    tailscale.com/util/endian                                    from tailscale.com/net/netns
         tailscale.com/util/groupmember                               from tailscale.com/cmd/tailscale/cli
         tailscale.com/util/lineread                                  from tailscale.com/net/interfaces+
-        tailscale.com/util/mak                                       from tailscale.com/net/netcheck
+        tailscale.com/util/mak                                       from tailscale.com/net/netcheck+
         tailscale.com/util/multierr                                  from tailscale.com/control/controlhttp
         tailscale.com/util/singleflight                              from tailscale.com/net/dnscache
    L    tailscale.com/util/strs                                      from tailscale.com/hostinfo

syncs/syncs.go

@@ -7,7 +7,10 @@
 
 import (
 	"context"
+	"sync"
 	"sync/atomic"
+
+	"tailscale.com/util/mak"
 )
 
 // ClosedChan returns a channel that's already closed.
@@ -152,3 +155,66 @@ func (s Semaphore) TryAcquire() bool {
 func (s Semaphore) Release() {
 	<-s.c
 }
+
+// Map is a Go map protected by a [sync.RWMutex].
+// It is preferred over [sync.Map] for maps with entries that change
+// at a relatively high frequency.
+// This must not be shallow copied.
+type Map[K comparable, V any] struct {
+	mu sync.RWMutex
+	m  map[K]V
+}
+
+func (m *Map[K, V]) Load(key K) (value V, ok bool) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	value, ok = m.m[key]
+	return value, ok
+}
+
+func (m *Map[K, V]) Store(key K, value V) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	mak.Set(&m.m, key, value)
+}
+
+func (m *Map[K, V]) LoadOrStore(key K, value V) (actual V, loaded bool) {
+	if actual, loaded = m.Load(key); loaded {
+		return actual, loaded
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	actual, loaded = m.m[key]
+	if !loaded {
+		actual = value
+		mak.Set(&m.m, key, value)
+	}
+	return actual, loaded
+}
+
+func (m *Map[K, V]) LoadAndDelete(key K) (value V, loaded bool) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	value, loaded = m.m[key]
+	if loaded {
+		delete(m.m, key)
+	}
+	return value, loaded
+}
+
+func (m *Map[K, V]) Delete(key K) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	delete(m.m, key)
+}
+
+func (m *Map[K, V]) Range(f func(key K, value V) bool) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	for k, v := range m.m {
+		if !f(k, v) {
+			return
+		}
+	}
+}

syncs/syncs_test.go

@@ -6,7 +6,10 @@
 
 import (
 	"context"
+	"sync"
 	"testing"
+
+	"github.com/google/go-cmp/cmp"
 )
 
 func TestWaitGroupChan(t *testing.T) {
@@ -73,3 +76,66 @@ func TestSemaphore(t *testing.T) {
 	s.Release()
 	s.Release()
 }
+
+func TestMap(t *testing.T) {
+	var m Map[string, int]
+	if v, ok := m.Load("noexist"); v != 0 || ok {
+		t.Errorf(`Load("noexist") = (%v, %v), want (0, false)`, v, ok)
+	}
+	m.Store("one", 1)
+	if v, ok := m.LoadOrStore("one", -1); v != 1 || !ok {
+		t.Errorf(`LoadOrStore("one", 1) = (%v, %v), want (1, true)`, v, ok)
+	}
+	if v, ok := m.Load("one"); v != 1 || !ok {
+		t.Errorf(`Load("one") = (%v, %v), want (1, true)`, v, ok)
+	}
+	if v, ok := m.LoadOrStore("two", 2); v != 2 || ok {
+		t.Errorf(`LoadOrStore("two", 2) = (%v, %v), want (2, false)`, v, ok)
+	}
+	got := map[string]int{}
+	want := map[string]int{"one": 1, "two": 2}
+	m.Range(func(k string, v int) bool {
+		got[k] = v
+		return true
+	})
+	if d := cmp.Diff(got, want); d != "" {
+		t.Errorf("Range mismatch (-got +want):\n%s", d)
+	}
+	if v, ok := m.LoadAndDelete("two"); v != 2 || !ok {
+		t.Errorf(`LoadAndDelete("two) = (%v, %v), want (2, true)`, v, ok)
+	}
+	if v, ok := m.LoadAndDelete("two"); v != 0 || ok {
+		t.Errorf(`LoadAndDelete("two) = (%v, %v), want (0, false)`, v, ok)
+	}
+	m.Delete("one")
+	m.Delete("noexist")
+	got = map[string]int{}
+	want = map[string]int{}
+	m.Range(func(k string, v int) bool {
+		got[k] = v
+		return true
+	})
+	if d := cmp.Diff(got, want); d != "" {
+		t.Errorf("Range mismatch (-got +want):\n%s", d)
+	}
+
+	t.Run("LoadOrStore", func(t *testing.T) {
+		var m Map[string, string]
+		var wg sync.WaitGroup
+		wg.Add(2)
+		var ok1, ok2 bool
+		go func() {
+			defer wg.Done()
+			_, ok1 = m.LoadOrStore("", "")
+		}()
+		go func() {
+			defer wg.Done()
+			_, ok2 = m.LoadOrStore("", "")
+		}()
+		wg.Wait()
+
+		if ok1 == ok2 {
+			t.Errorf("exactly one LoadOrStore should load")
+		}
+	})
+}