POSIX and init:
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POSIX.1 does not define 'init' but it mentions it in a few places.
B.2.2.2, p205 line 873:
This is part of the extensive 'job control' glossary entry.
This specific reference says that 'init' must by default provide
protection from job control signals to jobs it starts --
it sets SIGTSTP, SIGTTIN and SIGTTOU to SIG_IGN.
B.2.2.2, p206 line 889:
Here is a reference to 'vhangup'. It says, 'POSIX.1 does
not specify how controlling terminal access is affected by
a user logging out (that is, by a controlling process
terminating).' vhangup() is recognized as one way to handle
the problem. I'm not clear what happens in Reno; I have
the impression that when the controlling process terminates,
references to the controlling terminal are converted to
references to a 'dead' vnode. I don't know whether vhangup()
is required.
B.2.2.2, p206 line 921:
Orphaned process groups bear indirectly on this issue. A
session leader's process group is considered to be orphaned;
that is, it's immune to job control signals from the terminal.
B.2.2.2, p233 line 2055:
'Historically, the implementation-dependent process that
inherits children whose parents have terminated without
waiting on them is called "init" and has a process ID of 1.'
It goes on to note that it used to be the case that 'init'
was responsible for sending SIGHUP to the foreground process
group of a tty whose controlling process has exited, using
vhangup(). It is now the responsibility of the kernel to
do this when the controlling process calls _exit(). The
kernel is also responsible for sending SIGCONT to stopped
process groups that become orphaned. This is like old BSD
but entire process groups are signaled instead of individual
processes.
In general it appears that the kernel now automatically
takes care of orphans, relieving 'init' of any responsibility.
Specifics are listed on the _exit() page (p50).
On setsid():
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It appears that neither getty nor login call setsid(), so init must
do this -- seems reasonable. B.4.3.2 p 248 implies that this is the
way that 'init' should work; it says that setsid() should be called
after forking.
Process group leaders cannot call setsid() -- another reason to
fork! Of course setsid() causes the current process to become a
process group leader, so we can only call setsid() once. Note that
the controlling terminal acquires the session leader's process
group when opened.
Controlling terminals:
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B.7.1.1.3 p276: 'POSIX.1 does not specify a mechanism by which to
allocate a controlling terminal. This is normally done by a system
utility (such as 'getty') and is considered ... outside the scope
of POSIX.1.' It goes on to say that historically the first open()
of a tty in a session sets the controlling terminal. P130 has the
full details; nothing particularly surprising.
The glossary p12 describes a 'controlling process' as the first
process in a session that acquires a controlling terminal. Access
to the terminal from the session is revoked if the controlling
process exits (see p50, in the discussion of process termination).
Design notes:
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your generic finite state machine
we are fascist about which signals we elect to receive,
even signals purportedly generated by hardware
handle fatal errors gracefully if possible (we reboot if we goof!!)
if we get a segmentation fault etc., print a message on the console
and spin for a while before rebooting
(this at least decreases the amount of paper consumed :-)
apply hysteresis to rapidly exiting gettys
check wait status of children we reap
don't wait for stopped children
don't use SIGCHILD, it's too expensive
but it may close windows and avoid races, sigh
look for EINTR in case we need to change state
init is responsible for utmp and wtmp maintenance (ick)
maybe now we can consider replacements? maintain them in parallel
init only removes utmp and closes out wtmp entries...
necessary states and state transitions (gleaned from the man page):
1: single user shell (with password checking?); on exit, go to 2
2: rc script: on exit 0, go to 3; on exit N (error), go to 1
3: read ttys file: on completion, go to 4
4: multi-user operation: on SIGTERM, go to 7; on SIGHUP, go to 5;
on SIGTSTP, go to 6
5: clean up mode (re-read ttys file, killing off controlling processes
on lines that are now 'off', starting them on lines newly 'on')
on completion, go to 4
6: boring mode (no new sessions); signals as in 4
7: death: send SIGHUP to all controlling processes, reap for 30 seconds,
then go to 1 (warn if not all processes died, i.e. wait blocks)
Given the -s flag, we start at state 1; otherwise state 2