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runtime: warn that KeepAlive is not an unsafe.Pointer workaround

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Even experienced users occasionally mistake that runtime.KeepAlive can
be used as a workaround for following the unsafe.Pointer safety rules,
but it cannot. Add an explicit warning to this effect to dissuade
users from trying to use it as such.

Fixes #47562.

Change-Id: I842e33a3e1c080933c6b1bd1b6318448adbf495c
Reviewed-on: https://go-review.googlesource.com/c/go/+/340269
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Matthew Dempsky 2021-08-05 13:05:23 -07:00
parent 6e738868a7
commit fd45e267c2
1 changed files with 4 additions and 0 deletions
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4
src/runtime/mfinal.go
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@ -466,6 +466,10 @@ okarg:
// Without the KeepAlive call, the finalizer could run at the start of
// syscall.Read, closing the file descriptor before syscall.Read makes
// the actual system call.
//
// Note: KeepAlive should only be used to prevent finalizers from
// running prematurely. In particular, when used with unsafe.Pointer,
// the rules for valid uses of unsafe.Pointer still apply.
func KeepAlive(x interface{}) {
// Introduce a use of x that the compiler can't eliminate.
// This makes sure x is alive on entry. We need x to be alive