Re: [PATCH v5 06/15] do_target_wait_1: Clear TARGET_WNOHANG if the target isn't async.

[Simon Marchi <simon.marchi@polymtl.ca>]

> Thanks for digging the details up.  I assumed it had something to do with
> an initial misunderstanding I had which was that a target was always in
> async mode vs only being in async mode when there are non-stopped
> inferiors.  Can you please include these details in your commit reverting
> this change?  (Or did you want me to push the revert?  I'm happy for you
> to do so since you did the testing and investigation.)

I'm not sure I understand what you want me to add.  Here's what I came
up with, if you have suggestions of anything to add, let me know.

Simon

From 6adb0a6fb006b32b0ff51dcc59a0a5e90f944a82 Mon Sep 17 00:00:00 2001
From: John Baldwin <jhb@FreeBSD.org>
Date: Tue, 22 Feb 2022 11:22:14 -0800
Subject: [PATCH] Revert "do_target_wait_1: Clear TARGET_WNOHANG if the target
 isn't async."

Commit 14b3360508b1 ("do_target_wait_1: Clear
TARGET_WNOHANG if the target isn't async.") broke some multi-target
tests, such as gdb.multi/multi-target-info-inferiors.exp.  The symptom
is that execution just hangs at some point.  What happens is:

1. One remote inferior is started, and now sits stopped at a breakpoint.
   It is not "async" at this point (but it "can async").

2. We run a native inferior, the event loop gets woken up by the native
   target's fd.

3. In do_target_wait, we randomly choose an inferior to call target_wait
   on first, it happens to be the remote inferior.

4. Because the target is currently not "async", we clear
   TARGET_WNOHANG, resulting in synchronous wait.  We therefore block
   here:

  #0  0x00007fe9540dbb4d in select () from /usr/lib/libc.so.6
  #1  0x000055fc7e821da7 in gdb_select (n=15, readfds=0x7ffdb77c1fb0, writefds=0x0, exceptfds=0x7ffdb77c2050, timeout=0x7ffdb77c1f90) at /home/simark/src/binutils-gdb/gdb/posix-hdep.c:31
  #2  0x000055fc7ddef905 in interruptible_select (n=15, readfds=0x7ffdb77c1fb0, writefds=0x0, exceptfds=0x7ffdb77c2050, timeout=0x7ffdb77c1f90) at /home/simark/src/binutils-gdb/gdb/event-top.c:1134
  #3  0x000055fc7eda58e4 in ser_base_wait_for (scb=0x6250002e4100, timeout=1) at /home/simark/src/binutils-gdb/gdb/ser-base.c:240
  #4  0x000055fc7eda66ba in do_ser_base_readchar (scb=0x6250002e4100, timeout=-1) at /home/simark/src/binutils-gdb/gdb/ser-base.c:365
  #5  0x000055fc7eda6ff6 in generic_readchar (scb=0x6250002e4100, timeout=-1, do_readchar=0x55fc7eda663c <do_ser_base_readchar(serial*, int)>) at /home/simark/src/binutils-gdb/gdb/ser-base.c:444
  #6  0x000055fc7eda718a in ser_base_readchar (scb=0x6250002e4100, timeout=-1) at /home/simark/src/binutils-gdb/gdb/ser-base.c:471
  #7  0x000055fc7edb1ecd in serial_readchar (scb=0x6250002e4100, timeout=-1) at /home/simark/src/binutils-gdb/gdb/serial.c:393
  #8  0x000055fc7ec48b8f in remote_target::readchar (this=0x617000038780, timeout=-1) at /home/simark/src/binutils-gdb/gdb/remote.c:9446
  #9  0x000055fc7ec4da82 in remote_target::getpkt_or_notif_sane_1 (this=0x617000038780, buf=0x6170000387a8, forever=1, expecting_notif=1, is_notif=0x7ffdb77c24f0) at /home/simark/src/binutils-gdb/gdb/remote.c:9928
  #10 0x000055fc7ec4f045 in remote_target::getpkt_or_notif_sane (this=0x617000038780, buf=0x6170000387a8, forever=1, is_notif=0x7ffdb77c24f0) at /home/simark/src/binutils-gdb/gdb/remote.c:10037
  #11 0x000055fc7ec354d4 in remote_target::wait_ns (this=0x617000038780, ptid=..., status=0x7ffdb77c33c8, options=...) at /home/simark/src/binutils-gdb/gdb/remote.c:8147
  #12 0x000055fc7ec38aa1 in remote_target::wait (this=0x617000038780, ptid=..., status=0x7ffdb77c33c8, options=...) at /home/simark/src/binutils-gdb/gdb/remote.c:8337
  #13 0x000055fc7f1409ce in target_wait (ptid=..., status=0x7ffdb77c33c8, options=...) at /home/simark/src/binutils-gdb/gdb/target.c:2612
  #14 0x000055fc7e19da98 in do_target_wait_1 (inf=0x617000038080, ptid=..., status=0x7ffdb77c33c8, options=...) at /home/simark/src/binutils-gdb/gdb/infrun.c:3636
  #15 0x000055fc7e19e26b in operator() (__closure=0x7ffdb77c2f90, inf=0x617000038080) at /home/simark/src/binutils-gdb/gdb/infrun.c:3697
  #16 0x000055fc7e19f0c4 in do_target_wait (ecs=0x7ffdb77c33a0, options=...) at /home/simark/src/binutils-gdb/gdb/infrun.c:3716
  #17 0x000055fc7e1a31f7 in fetch_inferior_event () at /home/simark/src/binutils-gdb/gdb/infrun.c:4061

Before the aforementioned commit, we would not have cleared
TARGET_WNOHANG, the remote target's wait would have returned nothing,
and we would have consumed the native target's event.

After applying this revert, the testsuite state looks as good as before
for me on Ubuntu 20.04 amd64.

Change-Id: Ic17a1642935cabcc16c25cb6899d52e12c2f5c3f
---
 gdb/infrun.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 17ec8b27fa4e..5311822fcb84 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -3627,7 +3627,7 @@ do_target_wait_1 (inferior *inf, ptid_t ptid,
 
   /* We can't ask a non-async target to do a non-blocking wait, so this will be
      a blocking wait.  */
-  if (!target_is_async_p ())
+  if (!target_can_async_p ())
     options &= ~TARGET_WNOHANG;
 
   if (deprecated_target_wait_hook)

base-commit: 7190ae0d393d09c3c1f24add83d1e90727402749
-- 
2.35.1