Re: [PATCH] Fix fail in gdb.base/interrupt-noterm.exp

[Pedro Alves <palves at redhat dot com>]

On 01/22/2016 05:35 PM, Pedro Alves wrote:
> On 01/22/2016 05:14 PM, Yao Qi wrote:
>> Pedro Alves <palves@redhat.com> writes:
>>
>>> Can you expand the rationale some more?
>>>
>>> E.g., why is this not a gdbserver bug?  Instintively I'd say it is.
>>
>> The interaction between GDB and GDBserver is like this,
>>
>>   1. GDB sends vCont;c and doesn't wait for the stop reply because
>>   "continue &" is background command,
>>   2. GDBserver receives vCont;c, enables the async i/o (by
>>   enable_async_io) and resumes the inferior.
>>   3. GDB sends interrupt packet,
>>
>> #1 happens before #2 and #3, but the order of #2 and #3 is not
>> determined.  If #2 happens before #3, it is fine, otherwise, the
>> GDBserver doesn't know the interrupt from GDB.
> 
> If 1. is followed by 3., then the \\003 is always read by gdb
> after the vCont;c.  We call enable_async_io before reaching
> mywait.  Since we're in all-stop, that means we'll block
> inside mywait -> waitpid, all the while \\003 is already available to
> read in the socket.  Since we're blocked in waitpid, we won't see
> the \\003 until after the next time the program happens to stop.
> 
> Agree?
> 
> It still seems to me like a gdbserver bug.
> 
> I think that after calling enable_async_io, we need to check whether
> input is already pending from GDB, and if so, process it immediately -- we
> know the only input coming from GDB at this point is a \\003.  IOW, I think
> we need to call input_interrupt after calling enable_async_io.  input_interrupt
> already uses select before reading, so it handles the case of there
> being no input available without blocking.
> 
> However, we need to be careful, because a SIGIO can race with calling
> input_interrupt from mainline code...

Might be simpler to always have the SIGIO handler installed (install
it early), and change enable_async_io/disable_async_io to use
sigprocmask to block/unblock the signal.  That way, if input comes
before the signal is unblocked, the handler is called immediately
when enable_async_io is called.

Thanks,
Pedro Alves