1$NetBSD: NOTES,v 1.2 1995/03/18 14:56:29 cgd Exp $
2
3POSIX and init:
4--------------
5
6POSIX.1 does not define 'init' but it mentions it in a few places.
7
8B.2.2.2, p205 line 873:
9
10	This is part of the extensive 'job control' glossary entry.
11	This specific reference says that 'init' must by default provide
12	protection from job control signals to jobs it starts --
13	it sets SIGTSTP, SIGTTIN and SIGTTOU to SIG_IGN.
14
15B.2.2.2, p206 line 889:
16
17	Here is a reference to 'vhangup'.  It says, 'POSIX.1 does
18	not specify how controlling terminal access is affected by
19	a user logging out (that is, by a controlling process
20	terminating).'  vhangup() is recognized as one way to handle
21	the problem.  I'm not clear what happens in Reno; I have
22	the impression that when the controlling process terminates,
23	references to the controlling terminal are converted to
24	references to a 'dead' vnode.  I don't know whether vhangup()
25	is required.
26
27B.2.2.2, p206 line 921:
28
29	Orphaned process groups bear indirectly on this issue.  A
30	session leader's process group is considered to be orphaned;
31	that is, it's immune to job control signals from the terminal.
32
33B.2.2.2, p233 line 2055:
34
35	'Historically, the implementation-dependent process that
36	inherits children whose parents have terminated without
37	waiting on them is called "init" and has a process ID of 1.'
38
39	It goes on to note that it used to be the case that 'init'
40	was responsible for sending SIGHUP to the foreground process
41	group of a tty whose controlling process has exited, using
42	vhangup().  It is now the responsibility of the kernel to
43	do this when the controlling process calls _exit().  The
44	kernel is also responsible for sending SIGCONT to stopped
45	process groups that become orphaned.  This is like old BSD
46	but entire process groups are signaled instead of individual
47	processes.
48
49	In general it appears that the kernel now automatically
50	takes care of orphans, relieving 'init' of any responsibility.
51	Specifics are listed on the _exit() page (p50).
52
53On setsid():
54-----------
55
56It appears that neither getty nor login call setsid(), so init must
57do this -- seems reasonable.  B.4.3.2 p 248 implies that this is the
58way that 'init' should work; it says that setsid() should be called
59after forking.
60
61Process group leaders cannot call setsid() -- another reason to
62fork!  Of course setsid() causes the current process to become a
63process group leader, so we can only call setsid() once.  Note that
64the controlling terminal acquires the session leader's process
65group when opened.
66
67Controlling terminals:
68---------------------
69
70B.7.1.1.3 p276: 'POSIX.1 does not specify a mechanism by which to
71allocate a controlling terminal.  This is normally done by a system
72utility (such as 'getty') and is considered ... outside the scope
73of POSIX.1.'  It goes on to say that historically the first open()
74of a tty in a session sets the controlling terminal.  P130 has the
75full details; nothing particularly surprising.
76
77The glossary p12 describes a 'controlling process' as the first
78process in a session that acquires a controlling terminal.  Access
79to the terminal from the session is revoked if the controlling
80process exits (see p50, in the discussion of process termination).
81
82Design notes:
83------------
84
85your generic finite state machine
86we are fascist about which signals we elect to receive,
87	even signals purportedly generated by hardware
88handle fatal errors gracefully if possible (we reboot if we goof!!)
89	if we get a segmentation fault etc., print a message on the console
90	and spin for a while before rebooting
91	(this at least decreases the amount of paper consumed :-)
92apply hysteresis to rapidly exiting gettys
93check wait status of children we reap
94	don't wait for stopped children
95don't use SIGCHILD, it's too expensive
96	but it may close windows and avoid races, sigh
97look for EINTR in case we need to change state
98init is responsible for utmp and wtmp maintenance (ick)
99	maybe now we can consider replacements?  maintain them in parallel
100	init only removes utmp and closes out wtmp entries...
101
102necessary states and state transitions (gleaned from the man page):
103	1: single user shell (with password checking?); on exit, go to 2
104	2: run rc script, on exit 0 check if init.root sysctl != "/", if it
105           differs then fork + chroot into the value of init.root and run
106           /etc/rc inside the chroot: on exit 0, go to 3; on exit N (error),
107           go to 1 (applies also to /etc/rc when init.root == "/")
108	3: read ttys file: on completion, go to 4.  If we did chroot in
109	   state 2, we chroot after forking each getty to the same dir
110	   (init.root is not re-read)
111	4: multi-user operation: on SIGTERM, go to 7; on SIGHUP, go to 5;
112		on SIGTSTP, go to 6
113	5: clean up mode (re-read ttys file, killing off controlling processes
114		on lines that are now 'off', starting them on lines newly 'on')
115		on completion, go to 4
116	6: boring mode (no new sessions); signals as in 4
117	7: death: send SIGHUP to all controlling processes, reap for 30 seconds,
118		then go to 1 (warn if not all processes died, i.e. wait blocks)
119Given the -s flag, we start at state 1; otherwise state 2
120