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d3a071228e
Debugging with "maintenance set target-async off" on Linux has been
broken since 5b6d1e4fa4f ("Multi-target support").
The issue is easy to reproduce:
$ ./gdb -q --data-directory=data-directory -nx ./test
Reading symbols from ./test...
(gdb) maintenance set target-async off
(gdb) start
Temporary breakpoint 1 at 0x1151: file test.c, line 5.
Starting program: /home/simark/build/binutils-gdb/gdb/test
... and it hangs there.
The difference between pre-5b6d1e4fa4f and 5b6d1e4fa4f is that
fetch_inferior_event now calls target_wait with TARGET_WNOHANG for
non-async-capable targets, whereas it didn't before.
For non-async-capable targets, this is how it's expected to work when
resuming execution:
1. we call resume
2. the infrun async handler is marked in prepare_to_wait, to immediately
wake up the event loop when we get back to it
3. fetch_inferior_event calls the target's wait method without
TARGET_WNOHANG, effectively blocking until the target has something
to report
However, since we call the target's wait method with TARGET_WNOHANG,
this happens:
1. we call resume
2. the infrun async handler is marked in prepare_to_wait, to immediately
wake up the event loop when we get back to it
3. fetch_inferior_event calls the target's wait method with
TARGET_WNOHANG, the target has nothing to report yet
4. we go back to blocking on the event loop
5. SIGCHLD finally arrives, but the event loop is not woken up, because
we are not in async mode. Normally, we should have been stuck in
waitpid the SIGCHLD would have unblocked us.
We end up in this situation because these two necessary conditions are
met:
1. GDB uses the TARGET_WNOHANG option with a target that can't do async.
I don't think this makes sense. I mean, it's technically possible,
the doc for TARGET_WNOHANG is:
/* Return immediately if there's no event already queued. If this
options is not requested, target_wait blocks waiting for an
event. */
TARGET_WNOHANG = 1,
... which isn't in itself necessarily incompatible with synchronous
targets. It could be possible for a target to support non-blocking
polls, while not having a way to asynchronously wake up the event
loop, which is also necessary to support async. But as of today,
we don't expect GDB and sync targets to work this way.
2. The linux-nat target, even in the mode where it emulates a
synchronous target (with "maintenance set target-async off") respects
TARGET_WNOHANG. Other non-async targets, such as windows_nat_target,
simply don't check / support TARGET_WNOHANG, so their wait method is
always blocking.
Fix the first issue by avoiding using TARGET_WNOHANG on non-async
targets, in do_target_wait_1. Add an assert in target_wait to verify it
doesn't happen.
The new test gdb.base/maint-target-async-off.exp is a simple test that
just tries running to main and then to the end of the program, with
"maintenance set target-async off".
gdb/ChangeLog:
PR gdb/26642
* infrun.c (do_target_wait_1): Clear TARGET_WNOHANG if the
target can't do async.
* target.c (target_wait): Assert that we don't pass
TARGET_WNOHANG to a target that can't async.
gdb/testsuite/ChangeLog:
PR gdb/26642
* gdb.base/maint-target-async-off.c: New test.
* gdb.base/maint-target-async-off.exp: New test.
Change-Id: I69ad3a14598863d21338a8c4e78700a58ce7ad86
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