From ab0cebd8830f6d48e49083a06de17a6a9e5d4d98 Mon Sep 17 00:00:00 2001 From: Simon McVittie Date: Thu, 16 Jun 2011 11:56:30 +0100 Subject: [PATCH] document how the various processes in dbus-launch interact Reviewed-by: Will Thompson Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39197 --- tools/dbus-launch.c | 41 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 41 insertions(+) diff --git a/tools/dbus-launch.c b/tools/dbus-launch.c index bbaac2c..7b1bb37 100644 --- a/tools/dbus-launch.c +++ b/tools/dbus-launch.c @@ -43,6 +43,47 @@ extern Display *xdisplay; #endif +/* PROCESSES + * + * If you are in a shell and run "dbus-launch myapp", here is what happens: + * + * shell [*] + * \- main() --exec--> myapp[*] + * \- "intermediate parent" + * \- bus-runner --exec--> dbus-daemon --fork + * \- babysitter[*] \- final dbus-daemon[*] + * + * Processes marked [*] survive the initial flurry of activity. + * + * If you run "dbus-launch --sh-syntax" then the diagram is the same, except + * that main() prints variables and exits 0 instead of exec'ing myapp. + * + * PIPES + * + * dbus-daemon --print-pid -> bus_pid_to_launcher_pipe -> main + * dbus-daemon --print-address -> bus_address_to_launcher_pipe -> main + * main -> bus_pid_to_babysitter_pipe -> babysitter + * + * The intermediate parent looks pretty useless at first glance. Its purpose + * is to avoid the bus-runner becoming a zombie: when the intermediate parent + * terminates, the bus-runner and babysitter are reparented to init, which + * reaps them if they have finished. We can't rely on main() to reap arbitrary + * children because it might exec myapp, after which it can't be relied on to + * reap its children. We *can* rely on main() to reap the intermediate parent, + * because that happens before it execs myapp. + * + * It's unclear why dbus-daemon needs to fork, but we explicitly tell it to + * for some reason, then wait for it. If we left it undefined, a forking + * dbus-daemon would get the parent process reparented to init and reaped + * when the intermediate parent terminated, and a non-forking dbus-daemon + * would get reparented to init and carry on there. + * + * myapp is exec'd by the process that initially ran main() so that it's + * the shell's child, so the shell knows how to do job control and stuff. + * This is desirable for the "dbus-launch an application" use-case, less so + * for the "dbus-launch a test suite in an isolated session" use-case. + */ + static char* machine_uuid = NULL; const char* -- 2.7.4