Fix regression: cannot start with LD_PRELOAD=libSegFault.so (PR gdb/18653#c7)

At https://sourceware.org/bugzilla/show_bug.cgi?id=18653#c7, Andrew
reports that the fix for PR gdb/18653 made GDB useless if you preload
libSegFault.so, because GDB internal-errors on startup:

 $ LD_PRELOAD=libSegFault.so gdb
 src/gdb/common/signals-state-save-restore.c:64: internal-error: unexpected signal handler
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Aborted (core dumped)
 $

The internal error comes from the code saving the signal dispositions
inherited from gdb's parent:

 (top-gdb) bt
 #0  0x000000000056b001 in internal_error(char const*, int, char const*, ...) (file=0xaf5f38 "src/gdb/common/signals-state-save-restore.c", line=64, fmt=0xaf5f18 "unexpected signal handler") at src/gdb/common/errors.c:54
 #1  0x00000000005752c9 in save_original_signals_state() () at src/gdb/common/signals-state-save-restore.c:64
 #2  0x00000000007425de in captured_main_1(captured_main_args*) (context=0x7fffffffd860)
     at src/gdb/main.c:509
 #3  0x0000000000743622 in captured_main(void*) (data=0x7fffffffd860) at src/gdb/main.c:1145
 During symbol reading, cannot get low and high bounds for subprogram DIE at 24065.
 #4  0x00000000007436f9 in gdb_main(captured_main_args*) (args=0x7fffffffd860) at src/gdb/main.c:1171
 #5  0x0000000000413acd in main(int, char**) (argc=1, argv=0x7fffffffd968) at src/gdb/gdb.c:32

This commit downgrades the internal error to a warning.  You'll get
instead:

~~~
 $ LD_PRELOAD=libSegFault.so gdb
 warning: Found custom handler for signal 11 (Segmentation fault) preinstalled.
 Some signal dispositions inherited from the environment (SIG_DFL/SIG_IGN)
 won't be propagated to spawned programs.
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 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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 (gdb)
~~~

This also moves the location where save_original_signals_state is
called a bit further below (to after option processing), so that "-q"
disables the warning:

~~~
 $ LD_PRELOAD=libSegFault.so gdb -q
 (gdb)
~~~

New testcase included.

gdb/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* common/signals-state-save-restore.c
	(save_original_signals_state): New parameter 'quiet'.  Warn if we
	find a custom handler preinstalled, instead of internal erroring.
	But only warn if !quiet.
	* common/signals-state-save-restore.h
	(save_original_signals_state): New parameter 'quiet'.
	* main.c (captured_main_1): Move save_original_signals_state call
	after option handling, and pass QUIET.

gdb/gdbserver/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* server.c (captured_main): Pass quiet=false to
	save_original_signals_state.

gdb/testsuite/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	PR gdb/18653
	* gdb.base/libsegfault.exp: New.

2 files changed, 31 insertions, 6 deletions

diff --git a/gdb/common/signals-state-save-restore.c b/gdb/common/signals-state-save-restore.c
index 7c7afd3..eb281dd 100644
--- a/gdb/common/signals-state-save-restore.c
+++ b/gdb/common/signals-state-save-restore.c
@@ -35,7 +35,7 @@ static sigset_t original_signal_mask;
 /* See signals-state-save-restore.h.   */
 
 void
-save_original_signals_state (void)
+save_original_signals_state (bool quiet)
 {
 #ifdef HAVE_SIGACTION
   int i;
@@ -45,6 +45,8 @@ save_original_signals_state (void)
   if (res == -1)
     perror_with_name (("sigprocmask"));
 
+  bool found_preinstalled = false;
+
   for (i = 1; i < NSIG; i++)
     {
       struct sigaction *oldact = &original_signal_actions[i];
@@ -59,9 +61,31 @@ save_original_signals_state (void)
 	perror_with_name (("sigaction"));
 
       /* If we find a custom signal handler already installed, then
-	 this function was called too late.  */
-      if (oldact->sa_handler != SIG_DFL && oldact->sa_handler != SIG_IGN)
-	internal_error (__FILE__, __LINE__, _("unexpected signal handler"));
+	 this function was called too late.  This is a warning instead
+	 of an internal error because this can also happen if you
+	 LD_PRELOAD a library that installs a signal handler early via
+	 __attribute__((constructor)), like libSegFault.so.  */
+      if (!quiet
+	  && oldact->sa_handler != SIG_DFL
+	  && oldact->sa_handler != SIG_IGN)
+	{
+	  found_preinstalled = true;
+
+	  /* Use raw fprintf here because we're being called in early
+	     startup, because GDB's filtered streams are are
+	     created.  */
+	  fprintf (stderr,
+		   _("warning: Found custom handler for signal "
+		     "%d (%s) preinstalled.\n"), i,
+		   strsignal (i));
+	}
+    }
+
+  if (found_preinstalled)
+    {
+      fprintf (stderr, _("\
+Some signal dispositions inherited from the environment (SIG_DFL/SIG_IGN)\n\
+won't be propagated to spawned programs.\n"));
     }
 #endif
 }
diff --git a/gdb/common/signals-state-save-restore.h b/gdb/common/signals-state-save-restore.h
index 71ec08e..276ddc4 100644
--- a/gdb/common/signals-state-save-restore.h
+++ b/gdb/common/signals-state-save-restore.h
@@ -28,9 +28,10 @@
    back to what was originally inherited from gdb/gdbserver's parent,
    just before execing the target program to debug.  */
 
-/* Save the signal state of all signals.  */
+/* Save the signal state of all signals.  If !QUIET, warn if we detect
+   a custom signal handler preinstalled.  */
 
-extern void save_original_signals_state (void);
+extern void save_original_signals_state (bool quiet);
 
 /* Restore the signal state of all signals.  */