binutils-gdb/gdb/ser-event.h

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Introduce a serial interface for select'able events This patch adds a new "event" struct serial type, that is an abstraction specifically for waking up blocking waits/selects, implemented on top of a pipe on POSIX, and on top of a native Windows event (CreateEvent, etc.) on Windows. This will be used to plug signal handler / mainline code races. For example, GDB can indefinitely delay handling a quit request if the user presses Ctrl-C between the last QUIT call and the next (blocking) gdb_select call in the event loop: QUIT; <<< press ctrl-c here and end up blocked in gdb_select indefinitely. gdb_select (...); // whoops, SIGINT was already handled, no EINTR. A global alone (either the quit flag, or the "ready" flag of the async signal handlers in the event loop) is not sufficient. To plug races such as these on POSIX systems, we have to register some waitable file descriptor in the set of files gdb_select waits on, and write to it from the signal handler. This is classically a pipe, and the pattern called the self-pipe trick. On Linux, it could be a more efficient eventfd instead, but I'm sticking with a pipe for simplifity, as we need it for portability anyway. (Alternatively, we could use pselect/ppoll, and block signals until the pselect. The latter is not a design I think GDB could use, because we want the QUIT macro to be super cheap, as it is used in loops. Plus, Windows.) This is a "struct serial" because Windows's gdb_select relies on that. Windows's gdb_select, our "select" replacement, knows how to wait on all kinds of handles (regular files, pipes, sockets, console, etc.) unlike the native Windows "select" function, which can only wait on sockets. Each file descriptor for a "serial" type that is not normally waitable with WaitForMultipleObjects must have a corresponding struct serial instance. gdb_select then internally looks up the struct serial instance that wraps each file descriptor, and asks it for the corresponding Windows waitable handle. We could use serial_pipe() to create a "struct serial"-wrapped pipe that is usable everywhere, including Windows. That's what currently python/python.c uses for cross-thread posting of events. However, serial_write and serial_readchar are not designed to be async-signal-safe on POSIX hosts. It's easier to bypass those when setting/clearing the event source. And writing and a serial pipe is a bit heavy weight on Windows. gdb_select requires an extra thread to wait on the pipe and several Windows events, when a single manual-reset Windows event, with no extra thread is sufficient. The intended usage is simply: - Call make_serial_event to create a serial event object. - From the signal handler call serial_event_set to set the event. - From mainline code, have select/poll wait for serial_event_fd(), in addition to whatever other files you're about to wait for. gdb/ChangeLog: 2016-04-12 Pedro Alves <palves@redhat.com> * Makefile.in (SFILES): Add ser-event.c. (HFILES_NO_SRCDIR): Add ser-event.h. (COMMON_OBS): Add ser-event.o. * ser-event.c, ser-event.h: New files. * serial.c (new_serial): New function, factored out from (serial_fdopen_ops): ... this. (serial_open_ops_1): New function, factored out from (serial_open): ... this. (serial_open_ops): New function. * serial.h (struct serial): Forware declare. (serial_open_ops): New declaration.
2016-04-12 23:49:30 +08:00
/* Serial interface for a selectable event.
Copyright (C) 2016-2017 Free Software Foundation, Inc.
Introduce a serial interface for select'able events This patch adds a new "event" struct serial type, that is an abstraction specifically for waking up blocking waits/selects, implemented on top of a pipe on POSIX, and on top of a native Windows event (CreateEvent, etc.) on Windows. This will be used to plug signal handler / mainline code races. For example, GDB can indefinitely delay handling a quit request if the user presses Ctrl-C between the last QUIT call and the next (blocking) gdb_select call in the event loop: QUIT; <<< press ctrl-c here and end up blocked in gdb_select indefinitely. gdb_select (...); // whoops, SIGINT was already handled, no EINTR. A global alone (either the quit flag, or the "ready" flag of the async signal handlers in the event loop) is not sufficient. To plug races such as these on POSIX systems, we have to register some waitable file descriptor in the set of files gdb_select waits on, and write to it from the signal handler. This is classically a pipe, and the pattern called the self-pipe trick. On Linux, it could be a more efficient eventfd instead, but I'm sticking with a pipe for simplifity, as we need it for portability anyway. (Alternatively, we could use pselect/ppoll, and block signals until the pselect. The latter is not a design I think GDB could use, because we want the QUIT macro to be super cheap, as it is used in loops. Plus, Windows.) This is a "struct serial" because Windows's gdb_select relies on that. Windows's gdb_select, our "select" replacement, knows how to wait on all kinds of handles (regular files, pipes, sockets, console, etc.) unlike the native Windows "select" function, which can only wait on sockets. Each file descriptor for a "serial" type that is not normally waitable with WaitForMultipleObjects must have a corresponding struct serial instance. gdb_select then internally looks up the struct serial instance that wraps each file descriptor, and asks it for the corresponding Windows waitable handle. We could use serial_pipe() to create a "struct serial"-wrapped pipe that is usable everywhere, including Windows. That's what currently python/python.c uses for cross-thread posting of events. However, serial_write and serial_readchar are not designed to be async-signal-safe on POSIX hosts. It's easier to bypass those when setting/clearing the event source. And writing and a serial pipe is a bit heavy weight on Windows. gdb_select requires an extra thread to wait on the pipe and several Windows events, when a single manual-reset Windows event, with no extra thread is sufficient. The intended usage is simply: - Call make_serial_event to create a serial event object. - From the signal handler call serial_event_set to set the event. - From mainline code, have select/poll wait for serial_event_fd(), in addition to whatever other files you're about to wait for. gdb/ChangeLog: 2016-04-12 Pedro Alves <palves@redhat.com> * Makefile.in (SFILES): Add ser-event.c. (HFILES_NO_SRCDIR): Add ser-event.h. (COMMON_OBS): Add ser-event.o. * ser-event.c, ser-event.h: New files. * serial.c (new_serial): New function, factored out from (serial_fdopen_ops): ... this. (serial_open_ops_1): New function, factored out from (serial_open): ... this. (serial_open_ops): New function. * serial.h (struct serial): Forware declare. (serial_open_ops): New declaration.
2016-04-12 23:49:30 +08:00
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef SER_EVENT_H
#define SER_EVENT_H
/* This is used to be able to signal the event loop (or any other
select/poll) of events, in a race-free manner.
For example, a signal handler can defer non-async-signal-safe work
to the event loop, by having the signal handler set a struct
serial_event object, and having the event loop wait for that same
object to the readable. Once readable, the event loop breaks out
of select/poll and calls a registered callback that does the
deferred work. */
struct serial_event;
/* Make a new serial_event object. */
struct serial_event *make_serial_event (void);
/* Return the FD that can be used by select/poll to wait for the
event. The only valid operation on this object is to wait until it
is readable. */
extern int serial_event_fd (struct serial_event *event);
/* Set the event. This signals the file descriptor returned by
serial_event_fd as readable. */
extern void serial_event_set (struct serial_event *event);
/* Clear the event. The file descriptor returned by serial_event_fd
is not longer readable after this, until a new serial_event_set
call is made. */
extern void serial_event_clear (struct serial_event *event);
#endif