[release-branch.go1.15] sync: delete dirty keys inside Map.LoadAndDelete

Updates #40999
Fixes #41011

Change-Id: Ie32427e5cb5ed512b976b554850f50be156ce9f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/250197
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 94953d3e59)
Reviewed-on: https://go-review.googlesource.com/c/go/+/250297
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/sync/map.go

@@ -274,6 +274,7 @@ func (m *Map) LoadAndDelete(key interface{}) (value interface{}, loaded bool) {
 		e, ok = read.m[key]
 		if !ok && read.amended {
 			e, ok = m.dirty[key]
+			delete(m.dirty, key)
 			// Regardless of whether the entry was present, record a miss: this key
 			// will take the slow path until the dirty map is promoted to the read
 			// map.

src/sync/map_test.go

@@ -9,6 +9,7 @@ import (
 	"reflect"
 	"runtime"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"testing/quick"
 )
@@ -171,3 +172,26 @@ func TestConcurrentRange(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue40999(t *testing.T) {
+	var m sync.Map
+
+	// Since the miss-counting in missLocked (via Delete)
+	// compares the miss count with len(m.dirty),
+	// add an initial entry to bias len(m.dirty) above the miss count.
+	m.Store(nil, struct{}{})
+
+	var finalized uint32
+
+	// Set finalizers that count for collected keys. A non-zero count
+	// indicates that keys have not been leaked.
+	for atomic.LoadUint32(&finalized) == 0 {
+		p := new(int)
+		runtime.SetFinalizer(p, func(*int) {
+			atomic.AddUint32(&finalized, 1)
+		})
+		m.Store(p, struct{}{})
+		m.Delete(p)
+		runtime.GC()
+	}
+}