Re: [PATCH] Per-inferior target_terminal state, fix PR gdb/13211, more

Christophe Lyon
2018-01-31T09:27:49Z

On 30 January 2018 at 16:37, Pedro Alves <palves@redhat.com> wrote:
> On 12/04/2017 07:44 PM, Pedro Alves wrote:
>> (This applies on top of:
>>
>> [PATCH] linux-nat: Eliminate custom target_terminal_{inferior,ours}, stop using set_sigint_trap
>> https://sourceware.org/ml/gdb-patches/2017-11/msg00368.html
>> )
>>
>> In my multi-target branch I ran into problems with GDB's terminal
>> handling that exist in master as well, with multi-inferior debugging.
>>
>> This patch adds a testcase for said problems
>> (gdb.multi/multi-term-settings.exp), fixes the problems, fixes PR
>> gdb/13211 as well (and adds a testcase for that too,
>> gdb.base/interrupt-daemon.exp).
>>
>> The basis of the problem I ran into is the following. Consider a
>> scenario where you have:
>>
>> - inferior 1 - started with "attach", process is running on some
>> other terminal.
>>
>> - inferior 2 - started with "run", process is sharing gdb's terminal.
>>
>> In this scenario, when you stop/resume both inferiors, you want GDB to
>> save/restore the terminal settings of inferior 2, the one that is
>> sharing GDB's terminal. I.e., you want inferior 2 to "own" the
>> terminal (in target_terminal::is_ours/target_terminal::is_inferior
>> sense).
>>
>> Unfortunately, that's not what you get currently. Because GDB doesn't
>> know whether an attached inferior is actually sharing GDB's terminal,
>> it tries to save/restore its settings anyway, ignoring errors. In
>> this case, this is pointless, because inferior 1 is running on a
>> different terminal, but GDB doesn't know better.
>>
>> And then, because it is only possible to have the terminal settings of
>> a single inferior be in effect at a time, or make one inferior/pgrp be
>> the terminal's foreground pgrp (aka, only one inferior can "own" the
>> terminal, ignoring fork children here), if GDB happens to try to
>> restore the terminal settings of inferior 1 first, then GDB never
>> restores the terminal settings of inferior 2.
>>
>> This patch fixes that and a few things more along the way:
>>
>> - Moves enum target_terminal::terminal_state out of the
>> target_terminal class (it's currently private) and makes it a
>> scoped enum so that it can be easily used elsewhere.
>>
>> - Replaces the inflow.c:terminal_is_ours boolean with a
>> target_terminal_state variable. This allows distinguishing is_ours
>> and is_ours_for_output states. This allows finally making
>> child_terminal_ours_1 do something with its "output_only"
>> parameter.
>>
>> - Makes each inferior have its own copy of the
>> is_ours/is_ours_for_output/is_inferior state.
>>
>> - Adds a way for GDB to tell whether the inferior is sharing GDB's
>> terminal. Works best on Linux and Solaris; the fallback works just
>> as well as currently.
>>
>> - With that, we can remove the inf->attach_flag tests from
>> child_terminal_inferior/child_terminal_ours.
>>
>> - Currently target_ops.to_ours is responsible for both saving the
>> current inferior's terminal state, and restoring gdb's state.
>> Because each inferior has its own terminal state (possibly handled
>> by different targets in a multi-target world, even), we need to
>> split the inferior-saving part from the gdb-restoring part. The
>> patch adds a new target_ops.to_save_inferior target method for
>> that.
>>
>> - Adds a new target_terminal::save_inferior() function, so that
>> sequences like:
>>
>> scoped_restore_terminal_state save_state;
>> target_terminal::ours_for_output ();
>>
>> ... restore back inferiors that were
>> target_terminal_state::is_inferior before back to is_inferior, and
>> leaves inferiors that were is_ours alone.
>>
>> - Along the way, this adds a default implementation of
>> target_pass_ctrlc to inflow.c (for inf-child.c), that handles
>> passing the Ctrl-C to a process running on GDB's terminal or to
>> some other process otherwise.
>>
>> - Similarly, adds a new target default implementation of
>> target_interrupt, for the "interrupt" command. The current
>> implementation of this hook in inf-ptrace.c kills the whole process
>> group, but that's incorrect/undesirable because we may not be
>> attached to all processes in the process group. And also, it's
>> incorrect because inferior_process_group() doesn't really return
>> the inferior's real process group id if the inferior is not a
>> process group leader... This is the cause of PR gdb/13211 [1],
>> which this patch fixes. While at it, that target method's "ptid"
>> parameter is eliminated, because it's not really used.
>>
>> - A new test is included that exercises and fixes PR gdb/13211, and
>> also fixes a GDB issue reported on stackoverflow that I ran into
>> while working on this [2]. The problem is similar to PR gdb/13211,
>> except that it also triggers with Ctrl-C. When debugging a daemon
>> (i.e., a process that disconnects from the controlling terminal and
>> is not a process group leader, then Ctrl-C doesn't work, you just
>> can't interrupt the inferior at all, resulting in a hung debug
>> session. The problem is that since the inferior is no longer
>> associated with gdb's session / controlling terminal, then trying
>> to put the inferior in the foreground fails. And so Ctrl-C never
>> reaches the inferior directly. pass_signal is only used when the
>> inferior is attached, but that is not the case here. This is fixed
>> by the new child_pass_ctrlc. Without the fix, the new
>> interrupt-daemon.exp testcase fails with timeout waiting for a
>> SIGINT that never arrives.
>>
>> [1] PR gdb/13211 - Async / Process group and interrupt not working
>> https://sourceware.org/bugzilla/show_bug.cgi?id=13211
>>
>> [2] GDB not reacting Ctrl-C when after fork() and setsid()
>> https://stackoverflow.com/questions/46101292/gdb-not-reacting-ctrl-c-when-after-fork-and-setsid
>>
>> Note this patch does _not_ fix:
>>
>> - PR gdb/14559 - The 'interrupt' command does not work if sigwait is in use
>> https://sourceware.org/bugzilla/show_bug.cgi?id=14559
>>
>> - PR gdb/9425 - When using "sigwait" GDB doesn't trap SIGINT. Ctrl+C terminates program when should break gdb.
>> https://sourceware.org/bugzilla/show_bug.cgi?id=9425
>>
>> The only way to fix that that I know of (without changing the kernel)
>> is to make GDB put inferiors in a separate session (create a
>> pseudo-tty master/slave pair, make the inferior run with the slave as
>> its terminal, and have gdb pump output/input on the master end). I
>> have a follow up series that builds on top of this one that does that,
>> but that's too invasive for 8.1, I think. While this one fixes a
>> couple bugs already, and I think having it in 8.1 would simplify
>> backports for (a future) 8.1.1.
>
> I chickened out of putting this in 8.1, but I pushed it to
> master now, along with the prerequisite patch.
>

Hi,

This caused GDB to fail to build for arm-none-eabi targets
../../../gdb/remote-sim.c: In function 'void init_gdbsim_ops()':
../../../gdb/remote-sim.c:1342:27: error: invalid conversion from
'void (*)(target_ops*, ptid_t)' to 'void (*)(target_ops*)'
[-fpermissive]
gdbsim_ops.to_interrupt = gdbsim_interrupt;
^
make[2]: *** [remote-sim.o] Error 1
make[2]: *** Waiting for unfinished jobs....
../../../gdb/cli/cli-cmds.c: In function 'void complete_command(const
char*, int)':
../../../gdb/cli/cli-cmds.c:304:48: warning: 'word' may be used
uninitialized in this function [-Wmaybe-uninitialized]
get_max_completions_reached_message ());
^
../../../gdb/cli/cli-cmds.c:277:71: warning: 'tracker' may be used
uninitialized in this function [-Wmaybe-uninitialized]
= tracker->build_completion_result (word, word - arg, strlen (arg));

(building in an Ubuntu Trusty container)

Can you fix it ?

Thanks,

Christophe

> Thanks,
> Pedro Alves