be54d6bc6
Denis Vlasenko
2008-10-27
Subject
ash: fix "while kill -0 $child; do true; done" looping forever.
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shell/ash.c+20 -46
| @@ -3762,48 +3762,6 @@ sprint_status(char *s, int status, int sigonly) | |
| return col; | |
| } | |
| - | /* |
| - | * Do a wait system call. If job control is compiled in, we accept |
| - | * stopped processes. If block is zero, we return a value of zero |
| - | * rather than blocking. |
| - | * |
| - | * System V doesn't have a non-blocking wait system call. It does |
| - | * have a SIGCLD signal that is sent to a process when one of it's |
| - | * children dies. The obvious way to use SIGCLD would be to install |
| - | * a handler for SIGCLD which simply bumped a counter when a SIGCLD |
| - | * was received, and have waitproc bump another counter when it got |
| - | * the status of a process. Waitproc would then know that a wait |
| - | * system call would not block if the two counters were different. |
| - | * This approach doesn't work because if a process has children that |
| - | * have not been waited for, System V will send it a SIGCLD when it |
| - | * installs a signal handler for SIGCLD. What this means is that when |
| - | * a child exits, the shell will be sent SIGCLD signals continuously |
| - | * until is runs out of stack space, unless it does a wait call before |
| - | * restoring the signal handler. The code below takes advantage of |
| - | * this (mis)feature by installing a signal handler for SIGCLD and |
| - | * then checking to see whether it was called. If there are any |
| - | * children to be waited for, it will be. |
| - | * |
| - | * If neither SYSV nor BSD is defined, we don't implement nonblocking |
| - | * waits at all. In this case, the user will not be informed when |
| - | * a background process until the next time she runs a real program |
| - | * (as opposed to running a builtin command or just typing return), |
| - | * and the jobs command may give out of date information. |
| - | */ |
| - | static int |
| - | waitproc(int wait_flags, int *status) |
| - | { |
| - | #if JOBS |
| - | if (doing_jobctl) |
| - | wait_flags |= WUNTRACED; |
| - | #endif |
| - | /* NB: _not_ safe_waitpid, we need to detect EINTR */ |
| - | return waitpid(-1, status, wait_flags); |
| - | } |
| - | |
| - | /* |
| - | * Wait for a process to terminate. |
| - | */ |
| static int | |
| dowait(int wait_flags, struct job *job) | |
| { | |
| @@ -3813,9 +3771,15 @@ dowait(int wait_flags, struct job *job) | |
| struct job *thisjob; | |
| int state; | |
| - | TRACE(("dowait(%d) called\n", wait_flags)); |
| - | pid = waitproc(wait_flags, &status); |
| - | TRACE(("wait returns pid=%d, status=%d\n", pid, status)); |
| + | TRACE(("dowait(0x%x) called\n", wait_flags)); |
| + | |
| + | /* Do a wait system call. If job control is compiled in, we accept |
| + | * stopped processes. wait_flags may have WNOHANG, preventing blocking. |
| + | * NB: _not_ safe_waitpid, we need to detect EINTR */ |
| + | pid = waitpid(-1, &status, |
| + | (doing_jobctl ? (wait_flags | WUNTRACED) : wait_flags)); |
| + | TRACE(("wait returns pid=%d, status=0x%x\n", pid, status)); |
| + | |
| if (pid <= 0) { | |
| /* If we were doing blocking wait and (probably) got EINTR, | |
| * check for pending sigs received while waiting. | |
| @@ -8923,7 +8887,10 @@ evalcommand(union node *cmd, int flags) | |
| /* Execute the command. */ | |
| switch (cmdentry.cmdtype) { | |
| default: | |
| + | |
| #if ENABLE_FEATURE_SH_NOFORK | |
| + | /* Hmmm... shouldn't it happen somewhere in forkshell() instead? |
| + | * Why "fork off a child process if necessary" doesn't apply to NOFORK? */ |
| { | |
| /* find_command() encodes applet_no as (-2 - applet_no) */ | |
| int applet_no = (- cmdentry.u.index - 2); | |
| @@ -8935,7 +8902,6 @@ evalcommand(union node *cmd, int flags) | |
| } | |
| } | |
| #endif | |
| - | |
| /* Fork off a child process if necessary. */ | |
| if (!(flags & EV_EXIT) || trap[0]) { | |
| INT_OFF; | |
| @@ -8963,6 +8929,12 @@ evalcommand(union node *cmd, int flags) | |
| } | |
| listsetvar(list, i); | |
| } | |
| + | /* Tight loop with builtins only: |
| + | * "while kill -0 $child; do true; done" |
| + | * will never exit even if $child died, unless we do this |
| + | * to reap the zombie and make kill detect that it's gone: */ |
| + | dowait(DOWAIT_NONBLOCK, NULL); |
| + | |
| if (evalbltin(cmdentry.u.cmd, argc, argv)) { | |
| int exit_status; | |
| int i = exception; | |
| @@ -8984,6 +8956,8 @@ evalcommand(union node *cmd, int flags) | |
| case CMDFUNCTION: | |
| listsetvar(varlist.list, 0); | |
| + | /* See above for the rationale */ |
| + | dowait(DOWAIT_NONBLOCK, NULL); |
| if (evalfun(cmdentry.u.func, argc, argv, flags)) | |
| goto raise; | |
| break; | |