Fix completing an empty string

Earlier while working on the big completer rework series, I managed to
break

 (gdb) [TAB]

locally, and make GDB crash, but only notice a few weeks down the
road, because we have no test for that...

I also noticed that:

 (gdb)     [TAB]

didn't work (didn't show all commands as matches), even though
entering a command with leading whitespace works:

 (gdb)     help

This commit fixes the latter and adds a testcase that covers both
issues.

The gdb.base/completion.exp change is necessary because the new test
has a file name that also starts with "gdb.base/complet", making that
particular test ambiguous.  Adding another letter disambiguates.

gdb/ChangeLog:
2017-11-25   Pedro Alves  <palves@redhat.com>

	* completer.c (complete_line_internal_1): Skip spaces until the
	start of the command.

gdb/testsuite/ChangeLog:
2017-11-25   Pedro Alves  <palves@redhat.com>

	* gdb.base/complete-empty.exp: New file.
	* gdb.base/completion.exp: Adjust.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-25   Pedro Alves  <palves@redhat.com>
+
+	* completer.c (complete_line_internal_1): Skip spaces until the
+	start of the command.
+
 2017-11-24  Pedro Alves  <palves@redhat.com>
 
 	* cp-support.c (cp_symbol_name_matches_1): New, factored out from

gdb/completer.c

@@ -1270,10 +1270,13 @@ complete_line_internal_1 (completion_tracker &tracker,
       word = tmp_command + point - strlen (text);
     }
 
-  if (point == 0)
+  /* Move P up to the start of the command.  */
+  p = skip_spaces (p);
+
+  if (*p == '\0')
     {
-      /* An empty line we want to consider ambiguous; that is, it
-	 could be any command.  */
+      /* An empty line is ambiguous; that is, it could be any
+	 command.  */
       c = CMD_LIST_AMBIGUOUS;
       result_list = 0;
     }

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-25   Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/complete-empty.exp: New file.
+	* gdb.base/completion.exp: Adjust.
+
 2017-11-25  Pedro Alves  <palves@redhat.com>
 
 	* gdb.linespec/cpls-ops.cc: New file.

gdb/testsuite/gdb.base/complete-empty.exp

@@ -0,0 +1,44 @@
+# Copyright 2017 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/>.
+
+# This file is part of the gdb testsuite.
+
+load_lib completion-support.exp
+
+gdb_exit
+gdb_start
+
+# Start of tests.
+
+# Test TAB with no input.
+proc_with_prefix empty-input-line {} {
+    # Set max-completions to 1 to avoid having to hardcode a set of
+    # command names.
+    gdb_test_no_output "set max-completions 1"
+
+    # Given the completion limit, this completes to the command with
+    # the lowest alphanumeric sort, which is, and is likely to remain,
+    # "!".
+    test_gdb_complete_unique "" "!" " " 1
+
+    # Same, but with some leading whitespace.
+    test_gdb_complete_unique "   " "   !" " " 1
+}
+
+proc test_driver {} {
+    empty-input-line
+}
+
+test_driver

gdb/testsuite/gdb.base/completion.exp

@@ -722,13 +722,13 @@ gdb_test "complete file ./gdb.base/compl" \
     "file ./gdb.base/completion\\.exp.*" \
     "complete-command 'file ./gdb.base/compl'"
 
-set test "complete 'file ./gdb.base/complet'"
-send_gdb "file ./gdb.base/complet\t"
+set test "complete 'file ./gdb.base/completi'"
+send_gdb "file ./gdb.base/completi\t"
 gdb_test_multiple "" "$test" {
     -re "^file ./gdb.base/completion\\.exp $" {
 	send_gdb "\n"
 	# Ignore the exact error message.
-	gdb_test_multiple "" "complete 'file ./gdb.base/complet'" {
+	gdb_test_multiple "" "complete 'file ./gdb.base/completi'" {
 	    -re "\r\nA program is being debugged already\\.\[\r\n\]+Are you sure you want to change the file\\? \\(y or n\\) $" {
 		send_gdb "n\n"
 		exp_continue