Fix PR 21218: GDB dumps core when escaping newline in multi-line command

With commit 3b12939dfc ("Replace the sync_execution global with a
new enum prompt_state tristate"), GDB started aborting if you try
splitting an input line with a continuation char (backslash) while in
a multi-line command:

 (gdb) commands
 Type commands for breakpoint(s) 1, one per line.
 End with a line saying just "end".
 >print \

 (gdb) 1      # note "(gdb)" incorrectly printed here.
 >end

 readline: readline_callback_read_char() called with no handler!
 $

That abort is actually a symptom of an old problem introduced when
gdb_readline_wrapper was rewritten to use asynchronous readline, back
in 2007.  Note how the "(gdb)" prompt is printed above in the "(gdb)
1" line.  Clearly it shouldn't be there, but it already was before the
commit mentioned above.  Fixing that also fixes the readline abort
shown above.

The problem starts when command_line_input passes a NULL prompt to
gdb_readline_wrapper when it finds previous incomplete input due to a
backslash, trying to fetch more input without printing another ">"
secondary prompt.  That itself should not be a problem, because
passing NULL to gdb_readline_wrapper has the same meaning as passing a
pointer to empty string, since gdb_readline_wrapper exposes the same
interface as 'readline(char *)'.  However, gdb_readline_wrapper passes
the prompt argument directly to display_gdb_prompt, and for the
latter, a NULL prompt argument has a different meaning - it requests
printing the primary prompt.

Before commit 782a7b8ef9c096 (which rewrote gdb_readline_wrapper to
use asynchronous readline), GDB behaved like this:

 (gdb) commands
 [....]
 >print \
 1
 >end
 (gdb)

The above is what this commit restores GDB back to.

New test included.

gdb/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>

	PR cli/21218
	* top.c (gdb_readline_wrapper): Avoid passing NULL to
	display_gdb_prompt.
	(command_line_input): Add comment.

gdb/testsuite/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>
	    Jan Kratochvil  <jan.kratochvil@redhat.com>

	PR cli/21218
	* gdb.base/commands.exp (backslash_in_multi_line_command_test):
	New proc.
	(top level): Call it.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2017-03-08  Pedro Alves  <palves@redhat.com>
+
+	PR cli/21218
+	* top.c (gdb_readline_wrapper): Avoid passing NULL to
+	display_gdb_prompt.
+	(command_line_input): Add comment.
+
 2017-03-08  Pedro Alves  <palves@redhat.com>
 
 	PR tui/21216

gdb/testsuite/ChangeLog

@@ -1,3 +1,11 @@
+2017-03-08  Pedro Alves  <palves@redhat.com>
+	    Jan Kratochvil  <jan.kratochvil@redhat.com>
+
+	PR cli/21218
+	* gdb.base/commands.exp (backslash_in_multi_line_command_test):
+	New proc.
+	(top level): Call it.
+
 2017-03-08  Pedro Alves  <palves@redhat.com>
 
 	PR tui/21216

gdb/testsuite/gdb.base/commands.exp

@@ -1003,6 +1003,34 @@ proc_with_prefix redefine_backtrace_test {} {
     gdb_test "bt" "hibob" "execute bt command"
 }
 
+# Test an input line split with a continuation character (backslash)
+# while entering a multi-line command (in a secondary prompt).
+
+proc_with_prefix backslash_in_multi_line_command_test {} {
+    gdb_breakpoint "main"
+
+    gdb_test_multiple "commands" "commands" {
+	-re "End with a line saying just \"end\"\\.\r\n>$" {
+	    pass "commands"
+	}
+    }
+
+    set test "input line split with backslash"
+    send_gdb "print \\\nargc\n"
+    gdb_test_multiple "" $test {
+	-re "^print \\\\\r\nargc\r\n>$" {
+	    pass $test
+	}
+    }
+
+    gdb_test_no_output "end"
+
+    # Input any command, just to be sure the readline state is sane.
+    # In PR 21218, this would trigger the infamous:
+    # readline: readline_callback_read_char() called with no handler!
+    gdb_test "print 1" "" "run command"
+}
+
 gdbvar_simple_if_test
 gdbvar_simple_while_test
 gdbvar_complex_if_while_test
@@ -1027,5 +1055,6 @@ recursive_source_test
 if_commands_test
 error_clears_commands_left
 redefine_hook_test
+backslash_in_multi_line_command_test
 # This one should come last, as it redefines "backtrace".
 redefine_backtrace_test

gdb/top.c

@@ -1030,8 +1030,11 @@ gdb_readline_wrapper (const char *prompt)
   if (cleanup->target_is_async_orig)
     target_async (0);
 
-  /* Display our prompt and prevent double prompt display.  */
-  display_gdb_prompt (prompt);
+  /* Display our prompt and prevent double prompt display.  Don't pass
+     down a NULL prompt, since that has special meaning for
+     display_gdb_prompt -- it indicates a request to print the primary
+     prompt, while we want a secondary prompt here.  */
+  display_gdb_prompt (prompt != NULL ? prompt : "");
   if (ui->command_editing)
     rl_already_prompted = 1;
 
@@ -1307,6 +1310,9 @@ command_line_input (const char *prompt_arg, int repeat, char *annotation_suffix)
       if (cmd != NULL)
 	break;
 
+      /* Got partial input.  I.e., got a line that ends with a
+	 continuation character (backslash).  Suppress printing the
+	 prompt again.  */
       prompt = NULL;
     }