Commit ab0cebd8 authored by Simon McVittie's avatar Simon McVittie

document how the various processes in dbus-launch interact

Reviewed-by: 's avatarWill Thompson <will.thompson@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39197
parent faf0c88d
......@@ -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*
......
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