sync: don't keep func alive after OnceFunc panics

This moves the f = nil assignment to the defer statement,
so that in case the functions panics, the f func is not
referenced anymore.

Change-Id: I3e53b90a10f21741e26602270822c8a75679f163
GitHub-Last-Rev: bda01100c6
GitHub-Pull-Request: golang/go#68636
Reviewed-on: https://go-review.googlesource.com/c/go/+/601240
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/sync/oncefunc.go

@@ -21,6 +21,7 @@ func OnceFunc(f func()) func() {
 	return func() {
 		d.once.Do(func() {
 			defer func() {
+				d.f = nil // Do not keep f alive after invoking it.
 				d.p = recover()
 				if !d.valid {
 					// Re-panic immediately so on the first
@@ -30,7 +31,6 @@ func OnceFunc(f func()) func() {
 				}
 			}()
 			d.f()
-			d.f = nil      // Do not keep f alive after invoking it.
 			d.valid = true // Set only if f does not panic.
 		})
 		if !d.valid {
@@ -57,13 +57,13 @@ func OnceValue[T any](f func() T) func() T {
 	return func() T {
 		d.once.Do(func() {
 			defer func() {
+				d.f = nil
 				d.p = recover()
 				if !d.valid {
 					panic(d.p)
 				}
 			}()
 			d.result = d.f()
-			d.f = nil
 			d.valid = true
 		})
 		if !d.valid {
@@ -92,13 +92,13 @@ func OnceValues[T1, T2 any](f func() (T1, T2)) func() (T1, T2) {
 	return func() (T1, T2) {
 		d.once.Do(func() {
 			defer func() {
+				d.f = nil
 				d.p = recover()
 				if !d.valid {
 					panic(d.p)
 				}
 			}()
 			d.r1, d.r2 = d.f()
-			d.f = nil
 			d.valid = true
 		})
 		if !d.valid {

src/sync/oncefunc_test.go

@@ -219,6 +219,17 @@ func TestOnceXGC(t *testing.T) {
 			f := sync.OnceValues(func() (any, any) { buf[0] = 1; return nil, nil })
 			return func() { f() }
 		},
+		"OnceFunc panic": func(buf []byte) func() {
+			return sync.OnceFunc(func() { buf[0] = 1; panic("test panic") })
+		},
+		"OnceValue panic": func(buf []byte) func() {
+			f := sync.OnceValue(func() any { buf[0] = 1; panic("test panic") })
+			return func() { f() }
+		},
+		"OnceValues panic": func(buf []byte) func() {
+			f := sync.OnceValues(func() (any, any) { buf[0] = 1; panic("test panic") })
+			return func() { f() }
+		},
 	}
 	for n, fn := range fns {
 		t.Run(n, func(t *testing.T) {
@@ -230,14 +241,20 @@ func TestOnceXGC(t *testing.T) {
 			if gc.Load() != false {
 				t.Fatal("wrapped function garbage collected too early")
 			}
-			f()
+			func() {
+				defer func() { recover() }()
+				f()
+			}()
 			gcwaitfin()
 			if gc.Load() != true {
 				// Even if f is still alive, the function passed to Once(Func|Value|Values)
 				// is not kept alive after the first call to f.
 				t.Fatal("wrapped function should be garbage collected, but still live")
 			}
-			f()
+			func() {
+				defer func() { recover() }()
+				f()
+			}()
 		})
 	}
 }