Fix PR19388: Can't access $_siginfo in breakpoint (catch signal) condition

This commit merges both the registers and $_siginfo "thread
running/executing" checks into a single function.

Accessing $_siginfo from a "catch signal" breakpoint condition doesn't
work.  The condition always fails with "Selected thread is running":

 (gdb) catch signal
 Catchpoint 3 (standard signals)
 (gdb)
 condition $bpnum $_siginfo.si_signo == 5
 (gdb) continue
 Continuing.
 Error in testing breakpoint condition:
 Selected thread is running.

 Catchpoint 3 (signal SIGUSR1), 0x0000003615e35877 in __GI_raise (sig=10) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
 56        return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);
 (gdb)

When accessing the $_siginfo object, we check whether the thread is
marked running (external/public) state and refuse the access if so.
This is so "print $_siginfo" at the prompt fails nicelly when the
current thread is running.  While evaluating breakpoint conditionals,
we haven't decided yet whether the thread is going to stop, so
is_running still returns true, and we thus always error out.

Evaluating an expression that requires registers access is really
conceptually the same -- we could think of $_siginfo as a pseudo
register.  However, in that case we check whether the thread is marked
executing (internal/private state), not running (external/public
state).  Changing the $_siginfo validation to check is_executing as
well fixes the bug in question.

Note that checking is_executing is not fully correct, not even for
registers.  See PR 19389.  However, I think this is the lesser of two
evils and ends up as an improvement.  We at least now have a single
place to fix.

Tested on x86_64 GNU/Linux.

gdb/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	PR breakpoints/19388
	* frame.c (get_current_frame): Use validate_registers_access.
	* gdbthread.h (validate_registers_access): Declare.
	* infrun.c (validate_siginfo_access): Delete.
	(siginfo_value_read, siginfo_value_write): Use
	validate_registers_access.
	* thread.c (validate_registers_access): New function.

gdb/testsuite/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	PR breakpoints/19388
	* gdb.base/catch-signal-siginfo-cond.c: New file.
	* gdb.base/catch-signal-siginfo-cond.exp: New file.

1 files changed, 6 insertions, 21 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 0c61d26..f92643f 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -8684,25 +8684,6 @@ signals_info (char *signum_exp, int from_tty)
 		     "to change these tables.\n"));
 }
 
-/* Check if it makes sense to read $_siginfo from the current thread
-   at this point.  If not, throw an error.  */
-
-static void
-validate_siginfo_access (void)
-{
-  /* No current inferior, no siginfo.  */
-  if (ptid_equal (inferior_ptid, null_ptid))
-    error (_("No thread selected."));
-
-  /* Don't try to read from a dead thread.  */
-  if (is_exited (inferior_ptid))
-    error (_("The current thread has terminated"));
-
-  /* ... or from a spinning thread.  */
-  if (is_running (inferior_ptid))
-    error (_("Selected thread is running."));
-}
-
 /* The $_siginfo convenience variable is a bit special.  We don't know
    for sure the type of the value until we actually have a chance to
    fetch the data.  The type can change depending on gdbarch, so it is
@@ -8721,7 +8702,9 @@ siginfo_value_read (struct value *v)
 {
   LONGEST transferred;
 
-  validate_siginfo_access ();
+  /* If we can access registers, so can we access $_siginfo.  Likewise
+     vice versa.  */
+  validate_registers_access ();
 
   transferred =
     target_read (&current_target, TARGET_OBJECT_SIGNAL_INFO,
@@ -8742,7 +8725,9 @@ siginfo_value_write (struct value *v, struct value *fromval)
 {
   LONGEST transferred;
 
-  validate_siginfo_access ();
+  /* If we can access registers, so can we access $_siginfo.  Likewise
+     vice versa.  */
+  validate_registers_access ();
 
   transferred = target_write (&current_target,
 			      TARGET_OBJECT_SIGNAL_INFO,