[gdb/cli] Add ignore-errors command

While trying to reproduce a failing test-case from the testsuite on the
command line using a gdb command script, I ran into the problem that a command
failed which stopped script execution.

I could work around this by splitting the script at each error, but I realized
it would be nice if I could tell gdb to ignore the error.

A python workaround ignore-errors exists, mentioned here (
https://sourceware.org/legacy-ml/gdb/2010-06/msg00100.html ), which is
already supplied by distros like Fedora and openSUSE.

FTR, a more elaborate try-catch solution was posted here (
https://sourceware.org/bugzilla/show_bug.cgi?id=8487 ).

This patch adds native ignore-errors support (so no python needed).

So with this script:
...
$ cat script.gdb
ignore-errors run
echo here
...
we have:
...
$ gdb -q -batch -x script.gdb
No executable file specified.
Use the "file" or "exec-file" command.
here$
...

Note that quit is not caught:
...
$ gdb.sh -q
(gdb) ignore-errors quit
$
...
which is the same behaviour as with the python implementation.

gdb/ChangeLog:

2021-05-18  Tom de Vries  <tdevries@suse.de>

	* cli/cli-cmds.c (ignore_errors_command_completer)
	(ignore_errors_command): New function.
	(_initialize_cli_cmds): Add "ignore-errors" cmd.

---
 gdb/cli/cli-cmds.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index 0bf418e510e..c2c3ee49eb3 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -39,6 +39,7 @@
 #include "gdbsupport/filestuff.h"
 #include "location.h"
 #include "block.h"
+#include "event-top.h"
 
 #include "ui-out.h"
 #include "interps.h"
@@ -2249,6 +2250,34 @@ gdb_maint_setting_str_internal_fn (struct gdbarch *gdbarch,
 				 gdbarch);
 }
 
+/* Completer for "ignore-errors".  */
+
+static void
+ignore_errors_command_completer (cmd_list_element *ignore,
+				 completion_tracker &tracker,
+				 const char *text, const char * /*word*/)
+{
+  complete_nested_command_line (tracker, text);
+}
+
+/* Implementation of the ignore-errors command.  */
+
+static void
+ignore_errors_command (const char *args, int from_tty)
+{
+  try
+    {
+      execute_command (args, from_tty);
+    }
+  catch (const gdb_exception_error &ex)
+    {
+      exception_print (gdb_stderr, ex);
+
+      /* See also execute_gdb_command.  */
+      async_enable_stdin ();
+    }
+}
+
 void _initialize_cli_cmds ();
 void
 _initialize_cli_cmds ()
@@ -2618,4 +2647,12 @@ when GDB is started."), GDBINIT);
   c = add_cmd ("source", class_support, source_command,
 	       source_help_text, &cmdlist);
   set_cmd_completer (c, filename_completer);
+
+  const char *ignore_errors_help_text
+    = ("Execute a single command, ignoring all errors.\n"
+       "Only one-line commands are supported.\n"
+       "This is primarily useful in scripts.");
+  c = add_cmd ("ignore-errors", class_support, ignore_errors_command,
+	       ignore_errors_help_text, &cmdlist);
+  set_cmd_completer (c, ignore_errors_command_completer);
 }