1// Copyright 2024 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5/*
6The weak package is a package for managing weak pointers.
7
8Weak pointers are pointers that explicitly do not keep a value live and
9must be queried for a regular Go pointer.
10The result of such a query may be observed as nil at any point after a
11weakly-pointed-to object becomes eligible for reclamation by the garbage
12collector.
13More specifically, weak pointers become nil as soon as the garbage collector
14identifies that the object is unreachable, before it is made reachable
15again by a finalizer.
16In terms of the C# language, these semantics are roughly equivalent to the
17the semantics of "short" weak references.
18In terms of the Java language, these semantics are roughly equivalent to the
19semantics of the WeakReference type.
20
21Using go:linkname to access this package and the functions it references
22is explicitly forbidden by the toolchain because the semantics of this
23package have not gone through the proposal process. By exposing this
24functionality, we risk locking in the existing semantics due to Hyrum's Law.
25
26If you believe you have a good use-case for weak references not already
27covered by the standard library, file a proposal issue at
28https://github.com/golang/go/issues instead of relying on this package.
29*/
30package weak
31
32import (
33	"internal/abi"
34	"runtime"
35	"unsafe"
36)
37
38// Pointer is a weak pointer to a value of type T.
39//
40// This value is comparable is guaranteed to compare equal if the pointers
41// that they were created from compare equal. This property is retained even
42// after the object referenced by the pointer used to create a weak reference
43// is reclaimed.
44//
45// If multiple weak pointers are made to different offsets within same object
46// (for example, pointers to different fields of the same struct), those pointers
47// will not compare equal.
48// If a weak pointer is created from an object that becomes reachable again due
49// to a finalizer, that weak pointer will not compare equal with weak pointers
50// created before it became unreachable.
51type Pointer[T any] struct {
52	u unsafe.Pointer
53}
54
55// Make creates a weak pointer from a strong pointer to some value of type T.
56func Make[T any](ptr *T) Pointer[T] {
57	// Explicitly force ptr to escape to the heap.
58	ptr = abi.Escape(ptr)
59
60	var u unsafe.Pointer
61	if ptr != nil {
62		u = runtime_registerWeakPointer(unsafe.Pointer(ptr))
63	}
64	runtime.KeepAlive(ptr)
65	return Pointer[T]{u}
66}
67
68// Strong creates a strong pointer from the weak pointer.
69// Returns nil if the original value for the weak pointer was reclaimed by
70// the garbage collector.
71// If a weak pointer points to an object with a finalizer, then Strong will
72// return nil as soon as the object's finalizer is queued for execution.
73func (p Pointer[T]) Strong() *T {
74	return (*T)(runtime_makeStrongFromWeak(p.u))
75}
76
77// Implemented in runtime.
78
79//go:linkname runtime_registerWeakPointer
80func runtime_registerWeakPointer(unsafe.Pointer) unsafe.Pointer
81
82//go:linkname runtime_makeStrongFromWeak
83func runtime_makeStrongFromWeak(unsafe.Pointer) unsafe.Pointer
84