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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 2016-04-12 16:49:30 +01:00
parent 5cc3ce8b5f
commit f0881b37b6
8 changed files with 140 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
gdb/ChangeLog
View File

@ -1,3 +1,21 @@
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.
2016-04-12 Pedro Alves <palves@redhat.com>
* event-loop.c: Include "ser-event.h".

11
gdb/defs.h
View File

@ -131,6 +131,11 @@ extern char *debug_file_directory;
take a long time, and which ought to be interruptible, checks this
flag using the QUIT macro.
In addition to setting a flag, the SIGINT handler also marks a
select/poll-able file descriptor as read-ready. That is used by
interruptible_select in order to support interrupting blocking I/O
in a race-free manner.
These functions use the extension_language_ops API to allow extension
language(s) and GDB SIGINT handling to coexist seamlessly. */
@ -159,6 +164,12 @@ extern void maybe_quit (void);
connection. */
#define QUIT maybe_quit ()
/* Set the serial event associated with the quit flag. */
extern void quit_serial_event_set (void);
/* Clear the serial event associated with the quit flag. */
extern void quit_serial_event_clear (void);
/* * Languages represented in the symbol table and elsewhere.
This should probably be in language.h, but since enum's can't
be forward declared to satisfy opaque references before their

75
gdb/event-top.c
View File

@ -38,6 +38,8 @@
#include "annotate.h"
#include "maint.h"
#include "buffer.h"
#include "ser-event.h"
#include "gdb_select.h"
/* readline include files. */
#include "readline/readline.h"
@ -733,6 +735,12 @@ gdb_readline_no_editing_callback (gdb_client_data client_data)
}
/* The serial event associated with the QUIT flag. set_quit_flag sets
this, and check_quit_flag clears it. Used by interruptible_select
to be able to do interruptible I/O with no race with the SIGINT
handler. */
static struct serial_event *quit_serial_event;
/* Initialization of signal handlers and tokens. There is a function
handle_sig* for each of the signals GDB cares about. Specifically:
SIGINT, SIGFPE, SIGQUIT, SIGTSTP, SIGHUP, SIGWINCH. These
@ -750,6 +758,8 @@ async_init_signals (void)
{
initialize_async_signal_handlers ();
quit_serial_event = make_serial_event ();
signal (SIGINT, handle_sigint);
sigint_token =
create_async_signal_handler (async_request_quit, NULL);
@ -794,8 +804,33 @@ async_init_signals (void)
#endif
}
/* Tell the event loop what to do if SIGINT is received.
See event-signal.c. */
/* See defs.h. */
void
quit_serial_event_set (void)
{
serial_event_set (quit_serial_event);
}
/* See defs.h. */
void
quit_serial_event_clear (void)
{
serial_event_clear (quit_serial_event);
}
/* Return the selectable file descriptor of the serial event
associated with the quit flag. */
static int
quit_serial_event_fd (void)
{
return serial_event_fd (quit_serial_event);
}
/* Handle a SIGINT. */
void
handle_sigint (int sig)
{
@ -819,6 +854,42 @@ handle_sigint (int sig)
gdb_call_async_signal_handler (sigint_token, immediate_quit);
}
/* See gdb_select.h. */
int
interruptible_select (int n,
fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
struct timeval *timeout)
{
fd_set my_readfds;
int fd;
int res;
if (readfds == NULL)
{
readfds = &my_readfds;
FD_ZERO (&my_readfds);
}
fd = quit_serial_event_fd ();
FD_SET (fd, readfds);
if (n <= fd)
n = fd + 1;
do
{
res = gdb_select (n, readfds, writefds, exceptfds, timeout);
}
while (res == -1 && errno == EINTR);
if (res == 1 && FD_ISSET (fd, readfds))
{
errno = EINTR;
return -1;
}
return res;
}
/* Handle GDB exit upon receiving SIGTERM if target_can_async_p (). */
static void

15
gdb/extension.c
View File

@ -828,7 +828,16 @@ set_quit_flag (void)
&& active_ext_lang->ops->set_quit_flag != NULL)
active_ext_lang->ops->set_quit_flag (active_ext_lang);
else
quit_flag = 1;
{
quit_flag = 1;
/* Now wake up the event loop, or any interruptible_select. Do
this after setting the flag, because signals on Windows
actually run on a separate thread, and thus otherwise the
main code could be woken up and find quit_flag still
clear. */
quit_serial_event_set ();
}
}
/* Return true if the quit flag has been set, false otherwise.
@ -852,6 +861,10 @@ check_quit_flag (void)
/* This is written in a particular way to avoid races. */
if (quit_flag)
{
/* No longer need to wake up the event loop or any
interruptible_select. The caller handles the quit
request. */
quit_serial_event_clear ();
quit_flag = 0;
result = 1;
}

15
gdb/gdb_select.h
View File

@ -33,4 +33,19 @@
extern int gdb_select (int n, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
/* Convenience wrapper around gdb_select that returns -1/EINTR if
set_quit_flag is set, either on entry or from a signal handler or
from a different thread while select is blocked. The quit flag is
not cleared on exit -- the caller is responsible to check it with
check_quit_flag or QUIT.
Note this does NOT return -1/EINTR if any signal handler other than
SIGINT runs, nor if the current SIGINT handler does not call
set_quit_flag. */
extern int interruptible_select (int n,
fd_set *readfds,
fd_set *writefds,
fd_set *exceptfds,
struct timeval *timeout);
#endif /* !defined(GDB_SELECT_H) */

4
gdb/guile/scm-ports.c
View File

@ -201,7 +201,9 @@ ioscm_input_waiting (SCM port)
FD_ZERO (&input_fds);
FD_SET (fdes, &input_fds);
num_found = gdb_select (num_fds, &input_fds, NULL, NULL, &timeout);
num_found = interruptible_select (num_fds,
&input_fds, NULL, NULL,
&timeout);
if (num_found < 0)
{
/* Guile doesn't export SIGINT hooks like Python does.

4
gdb/top.c
View File

@ -613,10 +613,10 @@ gdb_readline_no_editing (const char *prompt)
QUIT;
/* Wait until at least one byte of data is available. Control-C
can interrupt gdb_select, but not fgetc. */
can interrupt interruptible_select, but not fgetc. */
FD_ZERO (&readfds);
FD_SET (fd, &readfds);
if (gdb_select (fd + 1, &readfds, NULL, NULL, NULL) == -1)
if (interruptible_select (fd + 1, &readfds, NULL, NULL, NULL) == -1)
{
if (errno == EINTR)
{

8
gdb/ui-file.c
View File

@ -567,14 +567,14 @@ stdio_file_read (struct ui_file *file, char *buf, long length_buf)
internal_error (__FILE__, __LINE__,
_("stdio_file_read: bad magic number"));
/* 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. */
/* Wait until at least one byte of data is available, or we get
interrupted with Control-C. */
{
fd_set readfds;
FD_ZERO (&readfds);
FD_SET (stdio->fd, &readfds);
if (gdb_select (stdio->fd + 1, &readfds, NULL, NULL, NULL) == -1)
if (interruptible_select (stdio->fd + 1, &readfds, NULL, NULL, NULL) == -1)
return -1;
}