Warn if add-symbol-file does not provide any symbols

A user suggested that add-symbol-file ought to warn if the file does
not in fact provide any symbols.  This seemed like a decent idea, so
this patch implements this idea.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* symfile.c (add_symbol_file_command): Remove obsolete comment.
	Warn if symbol file does not provide any symbols.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* gdb.base/symfile-warn.exp: New file.
	* gdb.base/symfile-warn.c: New file.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2019-06-14  Tom Tromey  <tromey@adacore.com>
+
+	* symfile.c (add_symbol_file_command): Remove obsolete comment.
+	Warn if symbol file does not provide any symbols.
+
 2019-06-14  Tom Tromey  <tromey@adacore.com>
 
 	* source.c (find_and_open_source): Respect basenames_may_differ.

gdb/symfile.c

@@ -2211,12 +2211,6 @@ set_objfile_default_section_offset (struct objfile *objf,
 
 /* This function allows the addition of incrementally linked object files.
    It does not modify any state in the target, only in the debugger.  */
-/* Note: ezannoni 2000-04-13 This function/command used to have a
-   special case syntax for the rombug target (Rombug is the boot
-   monitor for Microware's OS-9 / OS-9000, see remote-os9k.c). In the
-   rombug case, the user doesn't need to supply a text address,
-   instead a call to target_link() (in target.c) would supply the
-   value to use.  We are now discontinuing this type of ad hoc syntax.  */
 
 static void
 add_symbol_file_command (const char *args, int from_tty)
@@ -2358,6 +2352,9 @@ add_symbol_file_command (const char *args, int from_tty)
 
   objf = symbol_file_add (filename.get (), add_flags, &section_addrs,
 			  flags);
+  if (!objfile_has_symbols (objf) && objf->per_bfd->minimal_symbol_count <= 0)
+    warning (_("newly-added symbol file \"%s\" does not provide any symbols"),
+	     filename.get ());
 
   if (seen_offset)
     set_objfile_default_section_offset (objf, section_addrs, offset);

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2019-06-14  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.base/symfile-warn.exp: New file.
+	* gdb.base/symfile-warn.c: New file.
+
 2019-06-14  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.base/annotate-symlink.exp: New file.

gdb/testsuite/gdb.base/symfile-warn.c

@@ -0,0 +1,18 @@
+/* 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/>.  */
+
+/* Empty.  */

gdb/testsuite/gdb.base/symfile-warn.exp

@@ -0,0 +1,30 @@
+# 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/>.
+
+standard_testfile .c
+set ofile [standard_output_file ${testfile}.o]
+
+if {[gdb_compile "${srcdir}/${subdir}/${srcfile}" \
+	 $ofile object {debug}] != ""} {
+    untested "failed to compile"
+    return -1
+}
+
+clean_restart
+
+gdb_test "add-symbol-file $ofile 0" \
+    "Reading symbols from .*warning: newly-added symbol file.*does not provide any symbols" \
+    "add-symbol-file" \
+    "add symbol table from file \".*\" at.*\\(y or n\\) " "y"