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.

1 files changed, 73 insertions, 2 deletions

diff --git a/gdb/event-top.c b/gdb/event-top.c
index 2f0a758..eef1514 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -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