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Diffstat (limited to 'src/internal/weak/pointer.go')
| -rw-r--r-- | src/internal/weak/pointer.go | 83 |
1 files changed, 83 insertions, 0 deletions
diff --git a/src/internal/weak/pointer.go b/src/internal/weak/pointer.go new file mode 100644 index 0000000000..44d26738bc --- /dev/null +++ b/src/internal/weak/pointer.go @@ -0,0 +1,83 @@ +// Copyright 2024 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +/* +The weak package is a package for managing weak pointers. + +Weak pointers are pointers that explicitly do not keep a value live and +must be queried for a regular Go pointer. +The result of such a query may be observed as nil at any point after a +weakly-pointed-to object becomes eligible for reclamation by the garbage +collector. +More specifically, weak pointers become nil as soon as the garbage collector +identifies that the object is unreachable, before it is made reachable +again by a finalizer. +In terms of the C# language, these semantics are roughly equivalent to the +the semantics of "short" weak references. +In terms of the Java language, these semantics are roughly equivalent to the +semantics of the WeakReference type. + +Using go:linkname to access this package and the functions it references +is explicitly forbidden by the toolchain because the semantics of this +package have not gone through the proposal process. By exposing this +functionality, we risk locking in the existing semantics due to Hyrum's Law. + +If you believe you have a good use-case for weak references not already +covered by the standard library, file a proposal issue at +https://github.com/golang/go/issues instead of relying on this package. +*/ +package weak + +import ( + "internal/abi" + "runtime" + "unsafe" +) + +// Pointer is a weak pointer to a value of type T. +// +// This value is comparable is guaranteed to compare equal if the pointers +// that they were created from compare equal. This property is retained even +// after the object referenced by the pointer used to create a weak reference +// is reclaimed. +// +// If multiple weak pointers are made to different offsets within same object +// (for example, pointers to different fields of the same struct), those pointers +// will not compare equal. +// If a weak pointer is created from an object that becomes reachable again due +// to a finalizer, that weak pointer will not compare equal with weak pointers +// created before it became unreachable. +type Pointer[T any] struct { + u unsafe.Pointer +} + +// Make creates a weak pointer from a strong pointer to some value of type T. +func Make[T any](ptr *T) Pointer[T] { + // Explicitly force ptr to escape to the heap. + ptr = abi.Escape(ptr) + + var u unsafe.Pointer + if ptr != nil { + u = runtime_registerWeakPointer(unsafe.Pointer(ptr)) + } + runtime.KeepAlive(ptr) + return Pointer[T]{u} +} + +// Strong creates a strong pointer from the weak pointer. +// Returns nil if the original value for the weak pointer was reclaimed by +// the garbage collector. +// If a weak pointer points to an object with a finalizer, thhen Strong will +// return nil as soon as the object's finalizer is queued for execution. +func (p Pointer[T]) Strong() *T { + return (*T)(runtime_makeStrongFromWeak(unsafe.Pointer(p.u))) +} + +// Implemented in runtime. + +//go:linkname runtime_registerWeakPointer +func runtime_registerWeakPointer(unsafe.Pointer) unsafe.Pointer + +//go:linkname runtime_makeStrongFromWeak +func runtime_makeStrongFromWeak(unsafe.Pointer) unsafe.Pointer |
