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runtime: clarify finalizer semantics for tiny objects

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This change clarifies that a finalizer is not guaranteed to run,
not only for zero bytes objects but also tiny objects (< 16bytes).

Fixes #46827

Change-Id: I193e77f6f024c79110604f86bcb1a28b16cf98ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/337391
Run-TryBot: Changkun Ou <mail@changkun.de>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Changkun Ou 2021-07-26 14:23:26 +02:00 committed by Michael Knyszek
parent 1fcd4e9099
commit 80d487111b
1 changed files with 10 additions and 1 deletions
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11
src/runtime/mfinal.go
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@ -312,12 +312,21 @@ func runfinq() {
// bufio.Writer, because the buffer would not be flushed at program exit.
//
// It is not guaranteed that a finalizer will run if the size of *obj is
// zero bytes.
// zero bytes, because it may share same address with other zero-size
// objects in memory. See https://go.dev/ref/spec#Size_and_alignment_guarantees.
//
// It is not guaranteed that a finalizer will run for objects allocated
// in initializers for package-level variables. Such objects may be
// linker-allocated, not heap-allocated.
//
// Note that because finalizers may execute arbitrarily far into the future
// after an object is no longer referenced, the runtime is allowed to perform
// a space-saving optimization that batches objects together in a single
// allocation slot. The finalizer for an unreferenced object in such an
// allocation may never run if it always exists in the same batch as a
// referenced object. Typically, this batching only happens for tiny
// (on the order of 16 bytes or less) and pointer-free objects.
//
// A finalizer may run as soon as an object becomes unreachable.
// In order to use finalizers correctly, the program must ensure that
// the object is reachable until it is no longer required.