From 80d487111ba8fe1d0ddcd04972046070e2e4bbe9 Mon Sep 17 00:00:00 2001 From: Changkun Ou Date: Mon, 26 Jul 2021 14:23:26 +0200 Subject: [PATCH] runtime: clarify finalizer semantics for tiny objects 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 TryBot-Result: Gopher Robot Reviewed-by: Ian Lance Taylor Reviewed-by: Joedian Reid Reviewed-by: Michael Knyszek --- src/runtime/mfinal.go | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/src/runtime/mfinal.go b/src/runtime/mfinal.go index a1d08d9293b..257e9d15602 100644 --- a/src/runtime/mfinal.go +++ b/src/runtime/mfinal.go @@ -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.