1# $Id: README.signal,v 10.1 1995/06/23 10:28:17 bostic Exp $
2
3There are six (normally) asynchronous actions about which vi cares:
4SIGHUP, SIGINT, SIGQUIT, SIGTERM, SIGTSTP and SIGWINCH.
5
6The assumptions:
7 1: The DB routines are not reentrant.
8 2: The curses routines may not be reentrant.
9 3: Neither DB nor curses will restart system calls.
10
11XXX
12Note, most C library functions don't restart system calls. So, we should
13*probably* start blocking around any imported function that we don't know
14doesn't make a system call. This is going to be a genuine annoyance...
15
16SIGHUP, SIGTERM
17 Used for file recovery. The DB routines can't be reentered, nor
18 can they handle interrupted system calls, so the vi routines that
19 call DB block signals. This means that DB routines could be
20 called at interrupt time, if necessary.
21
22SIGQUIT
23 Disabled by the signal initialization routines. Historically, ^\
24 switched vi into ex mode, and we continue that practice.
25
26SIGWINCH:
27 The interrupt routine sets a global bit which is checked by the
28 key-read routine, so there are no reentrancy issues. This means
29 that the screen will not resize until vi runs out of keys, but
30 that doesn't seem like a problem.
31
32SIGINT and SIGTSTP are a much more difficult issue to resolve. Vi has
33to permit the user to interrupt long-running operations. Generally, a
34search, substitution or read/write is done on a large file, or, the user
35creates a key mapping with an infinite loop. This problem will become
36worse as more complex semantics are added to vi, especially things like
37making it a pure text widget. There are four major solutions on the table,
38each of which have minor permutations.
39
401: Run in raw mode.
41
42 The up side is that there's no asynchronous behavior to worry about,
43 and obviously no reentrancy problems. The down side is that it's easy
44 to misinterpret characters (e.g. :w big_file^Mi^V^C is going to look
45 like an interrupt) and it's easy to get into places where we won't see
46 interrupt characters (e.g. ":map a ixx^[hxxaXXX" infinitely loops in
47 historic implementations of vi). Periodically reading the terminal
48 input buffer might solve the latter problem, but it's not going to be
49 pretty.
50
51 Also, we're going to be checking for ^C's and ^Z's both, all over
52 the place -- I hate to litter the source code with that. For example,
53 the historic version of vi didn't permit you to suspend the screen if
54 you were on the colon command line. This isn't right. ^Z isn't a vi
55 command, it's a terminal event. (Dammit.)
56
572: Run in cbreak mode. There are two problems in this area. First, the
58 current curses implementations (both System V and Berkeley) don't give
59 you clean cbreak modes. For example, the IEXTEN bit is left on, turning
60 on DISCARD and LNEXT. To clarify, what vi WANTS is 8-bit clean, with
61 the exception that flow control and signals are turned on, and curses
62 cbreak mode doesn't give you this.
63
64 We can either set raw mode and twiddle the tty, or cbreak mode and
65 twiddle the tty. I chose to use raw mode, on the grounds that raw
66 mode is better defined and I'm less likely to be surprised by a curses
67 implementation down the road. The twiddling consists of setting ISIG,
68 IXON/IXOFF, and disabling some of the interrupt characters (see the
69 comments in cl_init.c). This is all found in historic System V (SVID
70 3) and POSIX 1003.1-1992, so it should be fairly portable.
71
72 The second problem is that vi permits you to enter literal signal
73 characters, e.g. ^V^C. There are two possible solutions. First, you
74 can turn off signals when you get a ^V, but that means that a network
75 packet containing ^V and ^C will lose, since the ^C may take effect
76 before vi reads the ^V. (This is particularly problematic if you're
77 talking over a protocol that recognizes signals locally and sends OOB
78 packets when it sees them.) Second, you can turn the ^C into a literal
79 character in vi, but that means that there's a race between entering
80 ^V<character>^C, i.e. the sequence may end up being ^V^C<character>.
81 Also, the second solution doesn't work for flow control characters, as
82 they aren't delivered to the program as signals.
83
84 Generally, this is what historic vi did. (It didn't have the curses
85 problems because it didn't use curses.) It entered signals following
86 ^V characters into the input stream, (which is why there's no way to
87 enter a literal flow control character).
88
893: Run in mostly raw mode; turn signals on when doing an operation the
90 user might want to interrupt, but leave them off most of the time.
91
92 This works well for things like file reads and writes. This doesn't
93 work well for trying to detect infinite maps. The problem is that
94 you can write the code so that you don't have to turn on interrupts
95 per keystroke, but the code isn't pretty and it's hard to make sure
96 that an optimization doesn't cover up an infinite loop. This also
97 requires interaction or state between the vi parser and the key
98 reading routines, as an infinite loop may still be returning keys
99 to the parser.
100
101 Also, if the user inserts an interrupt into the tty queue while the
102 interrupts are turned off, the key won't be treated as an interrupt,
103 and requiring the user to pound the keyboard to catch an interrupt
104 window is nasty.
105
1064: Run in mostly raw mode, leaving signals on all of the time. Done
107 by setting raw mode, and twiddling the tty's termios ISIG bit.
108
109 This works well for the interrupt cases, because the code only has
110 to check to see if the interrupt flag has been set, and can otherwise
111 ignore signals. It's also less likely that we'll miss a case, and we
112 don't have to worry about synchronizing between the vi parser and the
113 key read routines.
114
115 The down side is that we have to turn signals off if the user wants
116 to enter a literal character (e.g. ^V^C). If the user enters the
117 combination fast enough, or as part of a single network packet,
118 the text input routines will treat it as a signal instead of as a
119 literal character. To some extent, we have this problem already,
120 since we turn off flow control so that the user can enter literal
121 XON/XOFF characters.
122
123 This is probably the easiest to code, and provides the smoothest
124 programming interface.
125
126There are a couple of other problems to consider.
127
128First, System V's curses doesn't handle SIGTSTP correctly. If you use the
129newterm() interface, the TSTP signal will leave you in raw mode, and the
130final endwin() will leave you in the correct shell mode. If you use the
131initscr() interface, the TSTP signal will return you to the correct shell
132mode, but the final endwin() will leave you in raw mode. There you have
133it: proof that drug testing is not making any significant headway in the
134computer industry. The 4BSD curses is deficient in that it does not have
135an interface to the terminal keypad. So, regardless, we have to do our
136own SIGTSTP handling.
137
138The problem with this is that if we do our own SIGTSTP handling, in either
139models #3 or #4, we're going to have to call curses routines at interrupt
140time, which means that we might be reentering curses, which is something we
141don't want to do.
142
143Second, SIGTSTP has its own little problems. It's broadcast to the entire
144process group, not sent to a single process. The scenario goes something
145like this: the shell execs the mail program, which execs vi. The user hits
146^Z, and all three programs get the signal, in some random order. The mail
147program goes to sleep immediately (since it probably didn't have a SIGTSTP
148handler in place). The shell gets a SIGCHLD, does a wait, and finds out
149that the only child in its foreground process group (of which it's aware)
150is asleep. It then optionally resets the terminal (because the modes aren't
151how it left them), and starts prompting the user for input. The problem is
152that somewhere in the middle of all of this, vi is resetting the terminal,
153and getting ready to send a SIGTSTP to the process group in order to put
154itself to sleep. There's a solution to all of this: when vi starts, it puts
155itself into its own process group, and then only it (and possible child
156processes) receive the SIGTSTP. This permits it to clean up the terminal
157and switch back to the original process group, where it sends that process
158group a SIGTSTP, putting everyone to sleep and waking the shell.
159
160Third, handing SIGTSTP asynchronously is further complicated by the child
161processes vi may fork off. If vi calls ex, ex resets the terminal and
162starts running some filter, and SIGTSTP stops them both, vi has to know
163when it restarts that it can't repaint the screen until ex's child has
164finished running. This is solveable, but it's annoying.
165
166Well, somebody had to make a decision, and this is the way it's going to be
167(unless I get talked out of it). SIGINT is handled asynchronously, so
168that we can pretty much guarantee that the user can interrupt any operation
169at any time. SIGTSTP is handled synchronously, so that we don't have to
170reenter curses and so that we don't have to play the process group games.
171^Z is recognized in the standard text input and command modes. (^Z should
172also be recognized during operations that may potentially take a long time.
173The simplest solution is probably to twiddle the tty, install a handler for
174SIGTSTP, and then restore normal tty modes when the operation is complete.)
175