1There are six (normally) asynchronous actions about which vi cares: 2SIGHUP, SIGINT, SIGQUIT, SIGTERM, SIGTSTP and SIGWINCH. 3 4The assumptions: 5 1: The DB routines are not reentrant. 6 2: The curses routines may not be reentrant. 7 3: Neither DB nor curses will restart system calls. 8 9XXX 10Note, most C library functions don't restart system calls. So, we should 11*probably* start blocking around any imported function that we don't know 12doesn't make a system call. This is going to be a genuine annoyance... 13 14SIGHUP, SIGTERM 15 Used for file recovery. The DB routines can't be reentered, nor 16 can they handle interrupted system calls, so the vi routines that 17 call DB block signals. This means that DB routines could be 18 called at interrupt time, if necessary. 19 20SIGQUIT 21 Disabled by the signal initialization routines. Historically, ^\ 22 switched vi into ex mode, and we continue that practice. 23 24SIGWINCH: 25 The interrupt routine sets a global bit which is checked by the 26 key-read routine, so there are no reentrancy issues. This means 27 that the screen will not resize until vi runs out of keys, but 28 that doesn't seem like a problem. 29 30SIGINT and SIGTSTP are a much more difficult issue to resolve. Vi has 31to permit the user to interrupt long-running operations. Generally, a 32search, substitution or read/write is done on a large file, or, the user 33creates a key mapping with an infinite loop. This problem will become 34worse as more complex semantics are added to vi, especially things like 35making it a pure text widget. There are four major solutions on the table, 36each of which have minor permutations. 37 381: Run in raw mode. 39 40 The up side is that there's no asynchronous behavior to worry about, 41 and obviously no reentrancy problems. The down side is that it's easy 42 to misinterpret characters (e.g. :w big_file^Mi^V^C is going to look 43 like an interrupt) and it's easy to get into places where we won't see 44 interrupt characters (e.g. ":map a ixx^[hxxaXXX" infinitely loops in 45 historic implementations of vi). Periodically reading the terminal 46 input buffer might solve the latter problem, but it's not going to be 47 pretty. 48 49 Also, we're going to be checking for ^C's and ^Z's both, all over 50 the place -- I hate to litter the source code with that. For example, 51 the historic version of vi didn't permit you to suspend the screen if 52 you were on the colon command line. This isn't right. ^Z isn't a vi 53 command, it's a terminal event. (Dammit.) 54 552: Run in cbreak mode. There are two problems in this area. First, the 56 current curses implementations (both System V and Berkeley) don't give 57 you clean cbreak modes. For example, the IEXTEN bit is left on, turning 58 on DISCARD and LNEXT. To clarify, what vi WANTS is 8-bit clean, with 59 the exception that flow control and signals are turned on, and curses 60 cbreak mode doesn't give you this. 61 62 We can either set raw mode and twiddle the tty, or cbreak mode and 63 twiddle the tty. I chose to use raw mode, on the grounds that raw 64 mode is better defined and I'm less likely to be surprised by a curses 65 implementation down the road. The twiddling consists of setting ISIG, 66 IXON/IXOFF, and disabling some of the interrupt characters (see the 67 comments in cl_init.c). This is all found in historic System V (SVID 68 3) and POSIX 1003.1-1992, so it should be fairly portable. 69 70 The second problem is that vi permits you to enter literal signal 71 characters, e.g. ^V^C. There are two possible solutions. First, you 72 can turn off signals when you get a ^V, but that means that a network 73 packet containing ^V and ^C will lose, since the ^C may take effect 74 before vi reads the ^V. (This is particularly problematic if you're 75 talking over a protocol that recognizes signals locally and sends OOB 76 packets when it sees them.) Second, you can turn the ^C into a literal 77 character in vi, but that means that there's a race between entering 78 ^V<character>^C, i.e. the sequence may end up being ^V^C<character>. 79 Also, the second solution doesn't work for flow control characters, as 80 they aren't delivered to the program as signals. 81 82 Generally, this is what historic vi did. (It didn't have the curses 83 problems because it didn't use curses.) It entered signals following 84 ^V characters into the input stream, (which is why there's no way to 85 enter a literal flow control character). 86 873: Run in mostly raw mode; turn signals on when doing an operation the 88 user might want to interrupt, but leave them off most of the time. 89 90 This works well for things like file reads and writes. This doesn't 91 work well for trying to detect infinite maps. The problem is that 92 you can write the code so that you don't have to turn on interrupts 93 per keystroke, but the code isn't pretty and it's hard to make sure 94 that an optimization doesn't cover up an infinite loop. This also 95 requires interaction or state between the vi parser and the key 96 reading routines, as an infinite loop may still be returning keys 97 to the parser. 98 99 Also, if the user inserts an interrupt into the tty queue while the 100 interrupts are turned off, the key won't be treated as an interrupt, 101 and requiring the user to pound the keyboard to catch an interrupt 102 window is nasty. 103 1044: Run in mostly raw mode, leaving signals on all of the time. Done 105 by setting raw mode, and twiddling the tty's termios ISIG bit. 106 107 This works well for the interrupt cases, because the code only has 108 to check to see if the interrupt flag has been set, and can otherwise 109 ignore signals. It's also less likely that we'll miss a case, and we 110 don't have to worry about synchronizing between the vi parser and the 111 key read routines. 112 113 The down side is that we have to turn signals off if the user wants 114 to enter a literal character (e.g. ^V^C). If the user enters the 115 combination fast enough, or as part of a single network packet, 116 the text input routines will treat it as a signal instead of as a 117 literal character. To some extent, we have this problem already, 118 since we turn off flow control so that the user can enter literal 119 XON/XOFF characters. 120 121 This is probably the easiest to code, and provides the smoothest 122 programming interface. 123 124There are a couple of other problems to consider. 125 126First, System V's curses doesn't handle SIGTSTP correctly. If you use the 127newterm() interface, the TSTP signal will leave you in raw mode, and the 128final endwin() will leave you in the correct shell mode. If you use the 129initscr() interface, the TSTP signal will return you to the correct shell 130mode, but the final endwin() will leave you in raw mode. There you have 131it: proof that drug testing is not making any significant headway in the 132computer industry. The 4BSD curses is deficient in that it does not have 133an interface to the terminal keypad. So, regardless, we have to do our 134own SIGTSTP handling. 135 136The problem with this is that if we do our own SIGTSTP handling, in either 137models #3 or #4, we're going to have to call curses routines at interrupt 138time, which means that we might be reentering curses, which is something we 139don't want to do. 140 141Second, SIGTSTP has its own little problems. It's broadcast to the entire 142process group, not sent to a single process. The scenario goes something 143like this: the shell execs the mail program, which execs vi. The user hits 144^Z, and all three programs get the signal, in some random order. The mail 145program goes to sleep immediately (since it probably didn't have a SIGTSTP 146handler in place). The shell gets a SIGCHLD, does a wait, and finds out 147that the only child in its foreground process group (of which it's aware) 148is asleep. It then optionally resets the terminal (because the modes aren't 149how it left them), and starts prompting the user for input. The problem is 150that somewhere in the middle of all of this, vi is resetting the terminal, 151and getting ready to send a SIGTSTP to the process group in order to put 152itself to sleep. There's a solution to all of this: when vi starts, it puts 153itself into its own process group, and then only it (and possible child 154processes) receive the SIGTSTP. This permits it to clean up the terminal 155and switch back to the original process group, where it sends that process 156group a SIGTSTP, putting everyone to sleep and waking the shell. 157 158Third, handing SIGTSTP asynchronously is further complicated by the child 159processes vi may fork off. If vi calls ex, ex resets the terminal and 160starts running some filter, and SIGTSTP stops them both, vi has to know 161when it restarts that it can't repaint the screen until ex's child has 162finished running. This is solveable, but it's annoying. 163 164Well, somebody had to make a decision, and this is the way it's going to be 165(unless I get talked out of it). SIGINT is handled asynchronously, so 166that we can pretty much guarantee that the user can interrupt any operation 167at any time. SIGTSTP is handled synchronously, so that we don't have to 168reenter curses and so that we don't have to play the process group games. 169^Z is recognized in the standard text input and command modes. (^Z should 170also be recognized during operations that may potentially take a long time. 171The simplest solution is probably to twiddle the tty, install a handler for 172SIGTSTP, and then restore normal tty modes when the operation is complete.) 173