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Introduce interruptible_select
We have places where we call a blocking gdb_select expecting that a
Ctrl-C will unblock it. However, if the Ctrl-C is pressed just before
gdb_select, the SIGINT handler runs before gdb_select, and thus
gdb_select won't return.
For example gdb_readline_no_editing:
QUIT;
/* Wait until at least one byte of data is available. Control-C
can interrupt gdb_select, but not fgetc. */
FD_ZERO (&readfds);
FD_SET (fd, &readfds);
if (gdb_select (fd + 1, &readfds, NULL, NULL, NULL) == -1)
and stdio_file_read:
/* For the benefit of Windows, call gdb_select before reading from
the file. Wait until at least one byte of data is available.
Control-C can interrupt gdb_select, but not read. */
{
fd_set readfds;
FD_ZERO (&readfds);
FD_SET (stdio->fd, &readfds);
if (gdb_select (stdio->fd + 1, &readfds, NULL, NULL, NULL) == -1)
return -1;
}
return read (stdio->fd, buf, length_buf);
This is a race classically fixed with either the self-pipe trick, or
by blocking SIGINT and then using pselect instead of select.
Blocking SIGINT most of the time would mean that check_quit_flag (and
thus QUIT) would need to do a syscall every time it is called, which
sounds best avoided, since QUIT is called in many loops. Thus we take
the self-pipe trick route (wrapped in a serial event).
Instead of having all places that need this manually add an extra file
descriptor to the set of gdb_select's watched file descriptors, we
introduce a wrapper, interruptible_select, that does that.
The Windows version of gdb_select actually does not suffer from this,
because mingw-hdep.c:gdb_call_async_signal_handler sets a Windows
event that gdb_select always waits on. So this patch can be seen as
generalization of that technique. We can't remove that extra event
from mingw-hdep.c until we get rid of immediate_quit though.
gdb/ChangeLog:
2016-04-12 Pedro Alves <palves@redhat.com>
* defs.h: Extend QUIT-related comments to mention
interruptible_select.
(quit_serial_event_set, quit_serial_event_clear): Declare.
* event-top.c: Include "ser-event.h" and "gdb_select.h".
(quit_serial_event): New global.
(async_init_signals): Make quit_serial_event.
(quit_serial_event_set, quit_serial_event_clear)
(quit_serial_event_fd, interruptible_select): New functions.
* extension.c (set_quit_flag): Set the quit serial event.
(check_quit_flag): Clear the quit serial event.
* gdb_select.h (interruptible_select): New declaration.
* guile/scm-ports.c (ioscm_input_waiting): Use
interruptible_select instead of gdb_select.
* top.c (gdb_readline_no_editing): Likewise.
* ui-file.c (stdio_file_read): Likewise.
This commit is contained in:
Pedro Alves
parent
5cc3ce8b5f
commit
f0881b37b6
@ -1,3 +1,21 @@
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2016-04-12 Pedro Alves <palves@redhat.com>
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* defs.h: Extend QUIT-related comments to mention
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interruptible_select.
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(quit_serial_event_set, quit_serial_event_clear): Declare.
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* event-top.c: Include "ser-event.h" and "gdb_select.h".
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(quit_serial_event): New global.
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(async_init_signals): Make quit_serial_event.
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(quit_serial_event_set, quit_serial_event_clear)
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(quit_serial_event_fd, interruptible_select): New functions.
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* extension.c (set_quit_flag): Set the quit serial event.
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(check_quit_flag): Clear the quit serial event.
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* gdb_select.h (interruptible_select): New declaration.
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* guile/scm-ports.c (ioscm_input_waiting): Use
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interruptible_select instead of gdb_select.
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* top.c (gdb_readline_no_editing): Likewise.
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* ui-file.c (stdio_file_read): Likewise.
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2016-04-12 Pedro Alves <palves@redhat.com>
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* event-loop.c: Include "ser-event.h".
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11
gdb/defs.h
11
gdb/defs.h
@ -131,6 +131,11 @@ extern char *debug_file_directory;
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take a long time, and which ought to be interruptible, checks this
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flag using the QUIT macro.
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In addition to setting a flag, the SIGINT handler also marks a
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select/poll-able file descriptor as read-ready. That is used by
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interruptible_select in order to support interrupting blocking I/O
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in a race-free manner.
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These functions use the extension_language_ops API to allow extension
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language(s) and GDB SIGINT handling to coexist seamlessly. */
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@ -159,6 +164,12 @@ extern void maybe_quit (void);
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connection. */
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#define QUIT maybe_quit ()
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/* Set the serial event associated with the quit flag. */
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extern void quit_serial_event_set (void);
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/* Clear the serial event associated with the quit flag. */
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extern void quit_serial_event_clear (void);
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/* * Languages represented in the symbol table and elsewhere.
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This should probably be in language.h, but since enum's can't
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be forward declared to satisfy opaque references before their
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@ -38,6 +38,8 @@
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#include "annotate.h"
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#include "maint.h"
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#include "buffer.h"
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#include "ser-event.h"
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#include "gdb_select.h"
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/* readline include files. */
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#include "readline/readline.h"
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@ -733,6 +735,12 @@ gdb_readline_no_editing_callback (gdb_client_data client_data)
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}
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/* The serial event associated with the QUIT flag. set_quit_flag sets
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this, and check_quit_flag clears it. Used by interruptible_select
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to be able to do interruptible I/O with no race with the SIGINT
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handler. */
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static struct serial_event *quit_serial_event;
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/* Initialization of signal handlers and tokens. There is a function
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handle_sig* for each of the signals GDB cares about. Specifically:
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SIGINT, SIGFPE, SIGQUIT, SIGTSTP, SIGHUP, SIGWINCH. These
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@ -750,6 +758,8 @@ async_init_signals (void)
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{
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initialize_async_signal_handlers ();
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quit_serial_event = make_serial_event ();
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signal (SIGINT, handle_sigint);
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sigint_token =
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create_async_signal_handler (async_request_quit, NULL);
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@ -794,8 +804,33 @@ async_init_signals (void)
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#endif
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}
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/* Tell the event loop what to do if SIGINT is received.
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See event-signal.c. */
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/* See defs.h. */
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void
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quit_serial_event_set (void)
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{
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serial_event_set (quit_serial_event);
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}
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/* See defs.h. */
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void
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quit_serial_event_clear (void)
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{
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serial_event_clear (quit_serial_event);
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}
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/* Return the selectable file descriptor of the serial event
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associated with the quit flag. */
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static int
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quit_serial_event_fd (void)
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{
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return serial_event_fd (quit_serial_event);
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}
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/* Handle a SIGINT. */
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void
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handle_sigint (int sig)
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{
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@ -819,6 +854,42 @@ handle_sigint (int sig)
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gdb_call_async_signal_handler (sigint_token, immediate_quit);
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}
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/* See gdb_select.h. */
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int
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interruptible_select (int n,
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fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
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struct timeval *timeout)
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{
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fd_set my_readfds;
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int fd;
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int res;
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if (readfds == NULL)
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{
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readfds = &my_readfds;
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FD_ZERO (&my_readfds);
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}
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fd = quit_serial_event_fd ();
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FD_SET (fd, readfds);
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if (n <= fd)
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n = fd + 1;
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do
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{
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res = gdb_select (n, readfds, writefds, exceptfds, timeout);
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}
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while (res == -1 && errno == EINTR);
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if (res == 1 && FD_ISSET (fd, readfds))
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{
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errno = EINTR;
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return -1;
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}
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return res;
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}
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/* Handle GDB exit upon receiving SIGTERM if target_can_async_p (). */
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static void
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@ -828,7 +828,16 @@ set_quit_flag (void)
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&& active_ext_lang->ops->set_quit_flag != NULL)
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active_ext_lang->ops->set_quit_flag (active_ext_lang);
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else
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quit_flag = 1;
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{
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quit_flag = 1;
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/* Now wake up the event loop, or any interruptible_select. Do
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this after setting the flag, because signals on Windows
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actually run on a separate thread, and thus otherwise the
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main code could be woken up and find quit_flag still
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clear. */
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quit_serial_event_set ();
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}
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}
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/* Return true if the quit flag has been set, false otherwise.
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@ -852,6 +861,10 @@ check_quit_flag (void)
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/* This is written in a particular way to avoid races. */
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if (quit_flag)
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{
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/* No longer need to wake up the event loop or any
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interruptible_select. The caller handles the quit
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request. */
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quit_serial_event_clear ();
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quit_flag = 0;
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result = 1;
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}
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@ -33,4 +33,19 @@
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extern int gdb_select (int n, fd_set *readfds, fd_set *writefds,
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fd_set *exceptfds, struct timeval *timeout);
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/* Convenience wrapper around gdb_select that returns -1/EINTR if
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set_quit_flag is set, either on entry or from a signal handler or
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from a different thread while select is blocked. The quit flag is
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not cleared on exit -- the caller is responsible to check it with
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check_quit_flag or QUIT.
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Note this does NOT return -1/EINTR if any signal handler other than
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SIGINT runs, nor if the current SIGINT handler does not call
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set_quit_flag. */
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extern int interruptible_select (int n,
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fd_set *readfds,
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fd_set *writefds,
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fd_set *exceptfds,
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struct timeval *timeout);
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#endif /* !defined(GDB_SELECT_H) */
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@ -201,7 +201,9 @@ ioscm_input_waiting (SCM port)
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FD_ZERO (&input_fds);
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FD_SET (fdes, &input_fds);
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num_found = gdb_select (num_fds, &input_fds, NULL, NULL, &timeout);
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num_found = interruptible_select (num_fds,
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&input_fds, NULL, NULL,
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&timeout);
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if (num_found < 0)
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{
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/* Guile doesn't export SIGINT hooks like Python does.
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@ -613,10 +613,10 @@ gdb_readline_no_editing (const char *prompt)
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QUIT;
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/* Wait until at least one byte of data is available. Control-C
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can interrupt gdb_select, but not fgetc. */
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can interrupt interruptible_select, but not fgetc. */
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FD_ZERO (&readfds);
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FD_SET (fd, &readfds);
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if (gdb_select (fd + 1, &readfds, NULL, NULL, NULL) == -1)
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if (interruptible_select (fd + 1, &readfds, NULL, NULL, NULL) == -1)
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{
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if (errno == EINTR)
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{
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@ -567,14 +567,14 @@ stdio_file_read (struct ui_file *file, char *buf, long length_buf)
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internal_error (__FILE__, __LINE__,
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_("stdio_file_read: bad magic number"));
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/* For the benefit of Windows, call gdb_select before reading from
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the file. Wait until at least one byte of data is available.
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Control-C can interrupt gdb_select, but not read. */
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/* Wait until at least one byte of data is available, or we get
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interrupted with Control-C. */
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{
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fd_set readfds;
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FD_ZERO (&readfds);
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FD_SET (stdio->fd, &readfds);
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if (gdb_select (stdio->fd + 1, &readfds, NULL, NULL, NULL) == -1)
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if (interruptible_select (stdio->fd + 1, &readfds, NULL, NULL, NULL) == -1)
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return -1;
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}
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