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, 22 insertions, 0 deletions

diff --git a/gdb/thread.c b/gdb/thread.c
index 4c2259f..56526e4 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1098,6 +1098,28 @@ finish_thread_state_cleanup (void *arg)
   finish_thread_state (*ptid_p);
 }
 
+/* See gdbthread.h.  */
+
+void
+validate_registers_access (void)
+{
+  /* No selected thread, no registers.  */
+  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.  FIXME: This isn't actually fully
+     correct.  It'll allow an user-requested access (e.g., "print $pc"
+     at the prompt) when a thread is not executing for some internal
+     reason, but is marked running from the user's perspective.  E.g.,
+     the thread is waiting for its turn in the step-over queue.  */
+  if (is_executing (inferior_ptid))
+    error (_("Selected thread is running."));
+}
+
 int
 pc_in_thread_step_range (CORE_ADDR pc, struct thread_info *thread)
 {