[PATCH] was Re: [PATCH 2/3] [delete] Not-so-harmless spurious call to `wait4`

[Dominique Quatravaux via Gdb-patches <gdb-patches@sourceware.org>]

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> Le 8 avr. 2021 à 21:26, Simon Marchi <simon.marchi@polymtl.ca> a écrit :
> 
>   Can you please explain in the commit message what harm
> that wait4 call does and why we want to remove it?


In trying to make a better case in the commit message, I found that swapping the two patches around made more sense. Here is an updated, rebased, single patch (working under the assumption that the other two will be merged first):

From 5c3756e9eb0342b1a5a23bcb54d5b8317743ce0d Mon Sep 17 00:00:00 2001
From: Dominique Quatravaux <dominique.quatravaux@epfl.ch>
Date: Thu, 8 Apr 2021 21:35:57 +0200
Subject: [PATCH] [delete] not-so-harmless spurious call to `wait4`
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

As seen in https://sourceware.org/bugzilla/show_bug.cgi?id=24069 this
code will typically wait4() a second time on the same process that was
already wait4()'d a few lines above. While this used to be
harmless/idempotent (when we assumed that the process already exited),
this now causes a deadlock in the WIFSTOPPED case.

The early (~2019) history of bug #24069 cautiously suggests to use
WNOHANG instead of outright deleting the call. However, tests on the
current version of Darwin (Big Sur) demonstrate that gdb runs just
fine without a redundant call to wait4(), as would be expected.
Notwithstanding the debatable value of conserving bug compatibility
with an OS release that is more than a decade old, there is scant
evidence of what that double-wait4() was supposed to achieve in the
first place - A cursory investigation with `git blame` pinpoints
commits bb00b29d7802 and a80b95ba67e2 from the 2008-2009 era, but
fails to answer the “why” question conclusively.
---
 gdb/darwin-nat.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/gdb/darwin-nat.c b/gdb/darwin-nat.c
index d3edcdf3a74..ac19d330ffb 100644
--- a/gdb/darwin-nat.c
+++ b/gdb/darwin-nat.c
@@ -1113,9 +1113,6 @@ darwin_nat_target::decode_message (mach_msg_header_t *hdr,
 			      res_pid, status->value.sig);
 		}
 
-	      /* Looks necessary on Leopard and harmless...  */
-	      wait4 (inf->pid, &wstatus, 0, NULL);
-
 	      return ptid_t (inf->pid);
 	    }
 	  else
-- 
2.31.1



— 
  Dominique Quatravaux
  +41 21 69 35624






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