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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 <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>
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@ -312,12 +312,21 @@ func runfinq() {
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// bufio.Writer, because the buffer would not be flushed at program exit.
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//
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// It is not guaranteed that a finalizer will run if the size of *obj is
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// zero bytes.
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// zero bytes, because it may share same address with other zero-size
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// objects in memory. See https://go.dev/ref/spec#Size_and_alignment_guarantees.
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//
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// It is not guaranteed that a finalizer will run for objects allocated
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// in initializers for package-level variables. Such objects may be
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// linker-allocated, not heap-allocated.
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//
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// Note that because finalizers may execute arbitrarily far into the future
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// after an object is no longer referenced, the runtime is allowed to perform
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// a space-saving optimization that batches objects together in a single
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// allocation slot. The finalizer for an unreferenced object in such an
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// allocation may never run if it always exists in the same batch as a
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// referenced object. Typically, this batching only happens for tiny
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// (on the order of 16 bytes or less) and pointer-free objects.
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//
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// A finalizer may run as soon as an object becomes unreachable.
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// In order to use finalizers correctly, the program must ensure that
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// the object is reachable until it is no longer required.
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