.

go/zodb/internal/weak/weak.go

-// Copyright (C) 2018  Nexedi SA and Contributors.
-//                     Kirill Smelkov <kirr@nexedi.com>
+// Copyright (C) 2018-2020  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
 //
 // based on:
 // https://groups.google.com/d/msg/golang-nuts/PYWxjT2v6ps/dL71oJk1mXEJ
@@ -24,15 +24,20 @@
 // Package weak provides weak references for Go.
 package weak
 
+//go:generate gotrace gen .
+
 import (
+	"fmt"
 	"runtime"
 	"sync"
+//	"time"
 	"unsafe"
 )
 
 // iface is how Go runtime represents an interface.
 //
 // NOTE layout must be synchronized to Go runtime representation.
+// NOTE correctness depends on non-moving property of Go GC.
 type iface struct {
 	typ  uintptr // type
 	data uintptr // data
@@ -63,6 +68,8 @@ type Ref struct {
 	state weakRefState
 }
 
+//trace:event traceRelease(w *Ref, released bool)
+
 // NewRef creates new weak reference pointing to obj.
 //
 // TODO + onrelease callback?
@@ -77,6 +84,12 @@ func NewRef(obj interface{}) *Ref {
 
 	var release func(interface{})
 	release = func(obj interface{}) {
+		// assert that the object was not moved
+		iobj := *(*iface)(unsafe.Pointer(&obj))
+		if w.iface != iobj {
+			panic(fmt.Sprintf("weak: release: object moved:  w.iface=%x  obj=%x", w.iface, iobj))
+		}
+
 		// GC decided that the object is no longer reachable and
 		// scheduled us to run as finalizer. During the time till we
 		// actually run, Ref.Get might have been come to run and
@@ -87,16 +100,21 @@ func NewRef(obj interface{}) *Ref {
 		if w.state == objGot {
 			w.state = objLive
 			runtime.SetFinalizer(obj, release)
+			traceRelease(w, false)
 		} else {
 			w.state = objReleased
+			traceRelease(w, true)
 		}
 		w.mu.Unlock()
+
 	}
 
 	runtime.SetFinalizer(obj, release)
 	return w
 }
 
+//trace:event traceGotPre(w *Ref)
+
 // Get returns object pointed to by this weak reference.
 //
 // If original object is still alive - it is returned.
@@ -106,6 +124,12 @@ func (w *Ref) Get() (obj interface{}) {
 	if w.state != objReleased {
 		w.state = objGot
 
+		traceGotPre(w)
+		//time.Sleep(100*time.Nanosecond)
+		//time.Sleep(10*time.Millisecond)
+		//runtime.GC()
+		//runtime.GC()
+
 		// recreate interface{} from saved words.
 		// XXX do writes as pointers so that compiler emits write barriers to notify GC?
 		i := (*iface)(unsafe.Pointer(&obj))

go/zodb/internal/weak/weak_test.go

@@ -24,6 +24,8 @@ import (
 	"testing"
 	"time"
 	"unsafe"
+
+	"lab.nexedi.com/kirr/go123/tracing"
 )
 
 // verify that interface <-> iface works ok.
@@ -60,6 +62,22 @@ func TestWeakRef(t *testing.T) {
 	w := NewRef(p)
 	pptr := uintptr(unsafe.Pointer(p))
 
+	wrelease := make(chan bool) // events from traceRelease(w)
+	tpg := &tracing.ProbeGroup{}
+	tracing.Lock()
+	traceRelease_Attach(tpg, func(w_ *Ref, released bool) {
+		if w_ != w {
+			panic("release: w != w_")
+		}
+		wrelease <- released
+	})
+	traceGotPre_Attach(tpg, func(w *Ref) {
+		// nop for now
+		//panic("TODO GotPre")
+	})
+	tracing.Unlock()
+	defer tpg.Done()
+
 	assertEq := func(a, b interface{}) {
 		t.Helper()
 		if a != b {
@@ -68,24 +86,39 @@ func TestWeakRef(t *testing.T) {
 	}
 
 	// perform GC + give finalizers a chance to run.
-	GC := func() {
+	GC := func(expectRelease bool) {
+		t.Helper()
 		runtime.GC()
 
 		// GC only queues finalizers, not runs them directly. Give it
 		// some time so that finalizers could have been run.
-		time.Sleep(10 * time.Millisecond) // XXX hack
+		if expectRelease {
+			select {
+			case <-wrelease:
+				// ok
+			case <-time.After(100 * time.Millisecond):
+				t.Fatal("no release event")
+			}
+		} else {
+			select {
+			case <-time.After(10 * time.Millisecond):
+				// ok
+			case <-wrelease:
+				t.Fatal("unexpected release event")
+			}
+		}
 	}
 
 	assertEq(w.state, objLive)
 	assertEq(w.Get(), p)
 	assertEq(w.state, objGot)
-	GC()
+	GC(false)
 	assertEq(w.state, objGot) // fin has not been run at all (p is live)
 	assertEq(w.Get(), p)
 	assertEq(w.state, objGot)
 
 	p = nil
-	GC()
+	GC(true)
 	assertEq(w.state, objLive) // fin ran and downgraded got -> live
 	switch p_ := w.Get().(type) {
 	default:
@@ -97,10 +130,10 @@ func TestWeakRef(t *testing.T) {
 	}
 	assertEq(w.state, objGot)
 
-	GC()
+	GC(true)
 	assertEq(w.state, objLive) // fin ran again and again downgraded got -> live
 
-	GC()
+	GC(true)
 	assertEq(w.state, objReleased) // fin ran again and released the object
 	assertEq(w.Get(), nil)
 }

go/zodb/internal/weak/ztrace.go

+// Code generated by lab.nexedi.com/kirr/go123/tracing/cmd/gotrace; DO NOT EDIT.
+
+package weak
+// code generated for tracepoints
+
+import (
+	"lab.nexedi.com/kirr/go123/tracing"
+	"unsafe"
+)
+
+// traceevent: traceGotPre(w *Ref)
+
+type _t_traceGotPre struct {
+	tracing.Probe
+	probefunc     func(w *Ref)
+}
+
+var _traceGotPre *_t_traceGotPre
+
+func traceGotPre(w *Ref) {
+	if _traceGotPre != nil {
+		_traceGotPre_run(w)
+	}
+}
+
+func _traceGotPre_run(w *Ref) {
+	for p := _traceGotPre; p != nil; p = (*_t_traceGotPre)(unsafe.Pointer(p.Next())) {
+		p.probefunc(w)
+	}
+}
+
+func traceGotPre_Attach(pg *tracing.ProbeGroup, probe func(w *Ref)) *tracing.Probe {
+	p := _t_traceGotPre{probefunc: probe}
+	tracing.AttachProbe(pg, (**tracing.Probe)(unsafe.Pointer(&_traceGotPre)), &p.Probe)
+	return &p.Probe
+}
+
+// traceevent: traceRelease(w *Ref, released bool)
+
+type _t_traceRelease struct {
+	tracing.Probe
+	probefunc     func(w *Ref, released bool)
+}
+
+var _traceRelease *_t_traceRelease
+
+func traceRelease(w *Ref, released bool) {
+	if _traceRelease != nil {
+		_traceRelease_run(w, released)
+	}
+}
+
+func _traceRelease_run(w *Ref, released bool) {
+	for p := _traceRelease; p != nil; p = (*_t_traceRelease)(unsafe.Pointer(p.Next())) {
+		p.probefunc(w, released)
+	}
+}
+
+func traceRelease_Attach(pg *tracing.ProbeGroup, probe func(w *Ref, released bool)) *tracing.Probe {
+	p := _t_traceRelease{probefunc: probe}
+	tracing.AttachProbe(pg, (**tracing.Probe)(unsafe.Pointer(&_traceRelease)), &p.Probe)
+	return &p.Probe
+}
+
+// trace export signature
+func _trace_exporthash_c65df3d81975319429a288a09095370dace6a87b() {}