Use -qualified flag when setting temporary breakpoint in start command

When using the "start" command, GDB puts a temporary breakpoint on the
"main" symbol (we literally invoke the tbreak command).  However, since
it does wild matching by default, it also puts a breakpoint on any C++
method or "main" function in a namespace.  For example, when debugging
GDB, it creates a total of 24 locations:

  (gdb) start
  Temporary breakpoint 1 at 0x198c1e9: main. (24 locations)

as there are a bunch of methods called main in the selftests, such as

  selftests::string_view::capacity_1::main()

If such method was called in the constructor of a global object, or a
function marked with the attribute "constructor", then we would stop at
the wrong place.  Also, this causes a few extra symtabs (those that
contain the "wrong" mains) to be expanded for nothing.

The dummiest, most straightforward solution is to add -qualified when
invoking tbreak.  With this patch, "start" creates a single-location
breakpoint, as expected.

I copied the start.exp test to start-cpp.exp and made it use a C++ test
file, which contains two main functions.  The new test verifies that the
output of "start" is the output we get when we set a single-location
breakpoint.

gdb/ChangeLog:

	* infcmd.c (run_command_1): Pass -qualified to tbreak when usind
	the "start" command.

gdb/testsuite/ChangeLog:

	* gdb.base/start-cpp.exp: New file.
	* gdb.base/start-cpp.cc: New file.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2019-04-09  Simon Marchi  <simon.marchi@efficios.com>
+
+	* infcmd.c (run_command_1): Pass -qualified to tbreak when usind
+	the "start" command.
+
 2019-04-08  Kevin Buettner  <kevinb@redhat.com>
 
 	* python/py-inferior.c (infpy_thread_from_thread_handle):

gdb/infcmd.c

@@ -604,7 +604,10 @@ run_command_1 (const char *args, int from_tty, enum run_how run_how)
 
   /* Insert temporary breakpoint in main function if requested.  */
   if (run_how == RUN_STOP_AT_MAIN)
-    tbreak_command (main_name (), 0);
+    {
+      std::string arg = string_printf ("-qualified %s", main_name ());
+      tbreak_command (arg.c_str (), 0);
+    }
 
   exec_file = get_exec_file (0);
 

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2019-04-09  Simon Marchi  <simon.marchi@efficios.com>
+
+	* gdb.base/start-cpp.exp: New file.
+	* gdb.base/start-cpp.cc: New file.
+
 2019-04-08  Kevin Buettner  <kevinb@redhat.com>
 
 	* gdb.python/py-thrhandle.exp: Adjust tests to call

gdb/testsuite/gdb.base/start-cpp.cc

@@ -0,0 +1,33 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2019 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/>.  */
+
+namespace foo
+{
+
+int
+main ()
+{
+  return 1;
+}
+
+} /* namespace foo */
+
+int
+main ()
+{
+  return foo::main ();
+}

gdb/testsuite/gdb.base/start-cpp.exp

@@ -0,0 +1,37 @@
+# Copyright 2005-2019 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/>.
+
+standard_testfile .cc
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+# This is a testcase specifically for the `start' GDB command.  For regular
+# stop-in-main goal in the testcases consider using `runto_main' instead.
+
+# In this C++ version of the test (as opposed to start.exp), we specifically
+# test that the temporary breakpoint created by the start command has a single
+# location, even if we have a function named "main" in a non-root namespace.
+
+# For C++ programs, "start" should stop in main().
+if { [gdb_start_cmd] < 0 } {
+    untested start
+    return -1
+}
+
+gdb_test "" \
+         "Temporary breakpoint $decimal at $hex: file.*main \\(\\) at .*start-cpp.cc:.*" \
+         "start"