Avoid crash when calling warning too early

I noticed that if you pass the name of an existing file (not a
directory) as the argument to --data-directory, gdb will crash:

    $ ./gdb -nx  --data-directory  ./gdb
    ../../binutils-gdb/gdb/target.c:590:56: runtime error: member call on null pointer of type 'struct target_ops'

This was later reported as PR gdb/23838.

This happens because warning ends up calling
target_supports_terminal_ours, which calls current_top_target, which
returns nullptr this early.

This fixes the problem by handling this case specially in
target_supports_terminal_ours.  I also changed
target_supports_terminal_ours to return bool.

gdb/ChangeLog
2018-11-08  Tom Tromey  <tom@tromey.com>

	PR gdb/23555:
	PR gdb/23838:
	* target.h (target_supports_terminal_ours): Return bool.
	* target.c (target_supports_terminal_ours): Handle case where
	current_top_target returns nullptr.  Return bool.

gdb/testsuite/ChangeLog
2018-11-08  Tom Tromey  <tom@tromey.com>

	PR gdb/23555:
	PR gdb/23838:
	* gdb.base/warning.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2018-11-08  Tom Tromey  <tom@tromey.com>
+
+	PR gdb/23555:
+	PR gdb/23838:
+	* target.h (target_supports_terminal_ours): Return bool.
+	* target.c (target_supports_terminal_ours): Handle case where
+	current_top_target returns nullptr.  Return bool.
+
 2018-11-08  Joel Brobecker  <brobecker@adacore.com>
 
 	* aarch64-tdep.c (aapcs_is_vfp_call_or_return_candidate_1):

gdb/target.c

@@ -584,10 +584,16 @@ target_terminal::info (const char *arg, int from_tty)
 
 /* See target.h.  */
 
-int
+bool
 target_supports_terminal_ours (void)
 {
-  return current_top_target ()->supports_terminal_ours ();
+  /* This can be called before there is any target, so we must check
+     for nullptr here.  */
+  target_ops *top = current_top_target ();
+
+  if (top == nullptr)
+    return false;
+  return top->supports_terminal_ours ();
 }
 
 static void

gdb/target.h

@@ -1577,7 +1577,7 @@ extern int target_remove_breakpoint (struct gdbarch *gdbarch,
 /* Return true if the target stack has a non-default
   "terminal_ours" method.  */
 
-extern int target_supports_terminal_ours (void);
+extern bool target_supports_terminal_ours (void);
 
 /* Kill the inferior process.   Make it go away.  */
 

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2018-11-08  Tom Tromey  <tom@tromey.com>
+
+	PR gdb/23555:
+	PR gdb/23838:
+	* gdb.base/warning.exp: New file.
+
 2018-11-08  Jan Beulich  <jbeulich@suse.com>
 
 	* testsuite/gdb.arch/i386-avx512.c,

gdb/testsuite/gdb.base/warning.exp

@@ -0,0 +1,36 @@
+# Copyright 2018 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test that an early warning does not cause a crash.
+
+if {[is_remote host]} {
+    unsupported "warning.exp can only run on local host"
+    return
+}
+
+set tname [standard_temp_file warning]
+set fd [open $tname w]
+puts $fd "anything"
+close $fd
+
+set save $INTERNAL_GDBFLAGS
+set INTERNAL_GDBFLAGS "-nw -nx -data-directory $tname"
+
+gdb_start
+
+# Make sure gdb started up.
+gdb_test "echo 23\\n" "23"
+
+set INTERNAL_GDBFLAGS $save