subdaemon: better GDB support.

It was annoying me, so I made it much nicer to use.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/subdaemon.c

@@ -1,3 +1,4 @@
+#include <ccan/tal/str/str.h>
 #include <common/dev_disconnect.h>
 #include <common/status.h>
 #include <common/subdaemon.h>
@@ -24,11 +25,6 @@ static void status_backtrace_exit(void)
 	status_failed(STATUS_FAIL_INTERNAL_ERROR, "FATAL SIGNAL");
 }
 
-#if DEVELOPER
-extern volatile bool debugger_connected;
-volatile bool debugger_connected;
-#endif
-
 void subdaemon_setup(int argc, char *argv[])
 {
 	if (argc == 2 && streq(argv[1], "--version")) {
@@ -45,10 +41,11 @@ void subdaemon_setup(int argc, char *argv[])
 	/* From debugger, set debugger_spin to 0. */
 	for (int i = 1; i < argc; i++) {
 		if (streq(argv[i], "--debugger")) {
-			fprintf(stderr, "gdb -ex 'attach %u' -ex 'p debugger_connected=1' %s\n",
-				getpid(), argv[0]);
-			while (!debugger_connected)
-				usleep(10000);
+			char *cmd = tal_fmt(NULL, "${DEBUG_TERM:-gnome-terminal --} gdb -ex 'attach %u' %s &", getpid(), argv[0]);
+			fprintf(stderr, "Running %s\n", cmd);
+			system(cmd);
+			/* Continue in the debugger. */
+			kill(getpid(), SIGSTOP);
 		}
 		if (strstarts(argv[i], "--dev-disconnect=")) {
 			dev_disconnect_init(atoi(argv[i]

doc/HACKING.md

@@ -92,15 +92,15 @@ Debugging
 You can build c-lightning with DEVELOPER=1 to use dev commands listed in ``cli/lightning-cli help``. ``./configure --enable-developer`` will do that. You can log console messages with log_info() in lightningd and status_trace() in other subdaemons.
 
 You can debug crashing subdaemons with the argument
-`--dev-debugger=lightning_channeld`, where `channeld` is the subdaemon name.
-It will print out (to stderr) a command such as:
-
-    gdb -ex 'attach 22398' -ex 'p debugger_connected=1' \
-      lightningd/lightning_hsmd
-
-Run this command to start debugging.
-You may need to type `return` one more time to exit the infinite while
-loop, otherwise you can type `continue` to begin.
+`--dev-debugger=channeld`, where `channeld` is the subdaemon name.  It
+will run `gnome-terminal` by default with a gdb attached to the
+subdaemon when it starts.  You can change the terminal used by setting
+the `DEBUG_TERM` environment variable, such as `DEBUG_TERM="xterm -e"`
+or `DEBUG_TERM="konsole -e"`.
+
+It will also print out (to stderr) the gdb command for manual connection.  The
+subdaemon will be stopped (it sends itself a SIGSTOP); you'll need to
+`continue` in gdb.
 
 Database
 --------

lightningd/options.c

@@ -406,7 +406,7 @@ static void dev_register_opts(struct lightningd *ld)
 	opt_register_noarg("--dev-fail-on-subdaemon-fail", opt_set_bool,
 			   &ld->dev_subdaemon_fail, opt_hidden);
 	opt_register_arg("--dev-debugger=<subdaemon>", opt_subd_debug, NULL,
-			 ld, "Wait for gdb attach at start of <subdaemon>");
+			 ld, "Invoke gdb at start of <subdaemon>");
 	opt_register_arg("--dev-broadcast-interval=<ms>", opt_set_uintval,
 			 opt_show_uintval, &ld->config.broadcast_interval,
 			 "Time between gossip broadcasts in milliseconds");