.

wcfs/weak.go

+// XXX copytight = ?
+// based on:
+// https://groups.google.com/d/msg/golang-nuts/PYWxjT2v6ps/dL71oJk1mXEJ
+// https://play.golang.org/p/f9HY6-z8Pp
+
+package main
+// weak reference
+
+import (
+	"runtime"
+	"sync"
+	"unsafe"
+)
+
+// iface is how Go runtime represents an interface.
+//
+// NOTE layout must be synchronized to Go runtime representation.
+type iface struct {
+	typ  uintptr // type
+	data uintptr // data
+}
+
+type weakRefState int32
+
+const (
+	objGot      weakRefState = +1 // WeakRef.Get returned !nil
+	objLive     weakRefState =  0 // object is alive, Get did not run yet in this GC cycle
+	objReleased weakRefState = -1 // the finalizer marked object as released
+)
+
+
+// WeakRef is a weak reference.
+//
+// Create one with NewWeakRef and retrieve referenced object with Get.
+type WeakRef struct {
+	iface
+
+	// XXX try to do without mutex and only with atomics
+	mu    sync.Mutex
+	state weakRefState
+}
+
+// XXX
+func NewWeakRef(obj interface{}) *WeakRef {
+	// original code uses ^ for pointers not to cause GC thinking that original
+	// object is still referenced from WeakRef.data. However starting with ~ Go1.4
+	// the GC is precise, so we don't need that trick this days.
+	i := (*[2]uintptr)(unsafe.Pointer(&obj))
+	w := &WeakRef{
+		iface: *(*iface)(unsafe.Pointer(&obj)),
+		state: objLive,
+	}
+
+	var release func(interface{})
+	release = func(_ interface{}) {
+		// GC decided that the object is no longer reachable and
+		// scheduled us to run as finalizer. During the time till we
+		// actually run, WeakRef.Get might have been come to run and
+		// "rematerializing" the object for use. Check if we do not
+		// race with any Get in progress, and reschedule us to retry at
+		// next GC if we do.
+		w.mu.Lock()
+		if w.state == objGot {
+			w.state = objLive
+			runtime.SetFinalizer(obj, release)
+		} else {
+			w.state = objReleased
+		}
+		w.mu.Unlock()
+	}
+
+	runtime.SetFinalizer(obj, release)
+	return w
+}
+
+// Get returns object pointed to by this weak reference.
+//
+// If original object is still alive - it is returned.
+// If not - nil is returned.
+func (w *WeakRef) Get() (obj interface{}) {
+	w.mu.Lock()
+	if w.state != objReleased {
+		w.state = objGot
+
+		// recreate interface{} from saved words.
+		// XXX do writes as pointers so that compiler emits write barriers to notify GC?
+		i := (*iface)(unsafe.Pointer(&obj))
+		*i = w.iface
+	}
+	w.mu.Unlock()
+	return obj
+}

wcfs/weak_test.go

+// Copyright (C) 2018  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+package main
+
+// sizeof(interface{}) == 2*sizeof(uintptr)
+// v -> weakref -> get -> == v
+// GC -> get -> alive -> GC get alive -> GC get alive -> GC GC get ø