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Re: CTL-z bug?

From: Eduardo A . Bustamante López
Subject: Re: CTL-z bug?
Date: Tue, 13 Jan 2015 22:59:50 -0600
User-agent: Mutt/1.5.23 (2014-03-12)

I think that the following text from the zshell's code is an interesting read
on the subject:

TL;DR: Another strategy is to "migrate" the while-loop to a child process the
moment that you hit ^Z, but, this is really hard (maybe impossible?) to do

(From Src/exec.c)

 * [...]
 * In most shells, if you do something like:
 *   cat foo | while read a; do grep $a bar; done
 * the shell forks and executes the loop in the sub-shell thus created.
 * In zsh this traditionally executes the loop in the current shell, which
 * is nice to have if the loop does something to change the shell, like
 * setting parameters or calling builtins.
 * Putting the loop in a sub-shell makes life easy, because the shell only
 * has to put it into the job-structure and then treats it as a normal
 * process. Suspending and interrupting is no problem then.
 * Some years ago, zsh either couldn't suspend such things at all, or
 * it got really messed up when users tried to do it. As a solution, we
 * implemented the list_pipe-stuff, which has since then become a reason
 * for many nightmares.
 * Pipelines like the one above are executed by the functions in this file
 * which call each other (and sometimes recursively). The one above, for
 * example would lead to a function call stack roughly like:
 *  execlist->execpline->execcmd->execwhile->execlist->execpline
 * (when waiting for the grep, ignoring execpline2 for now). At this time,
 * zsh has built two job-table entries for it: one for the cat and one for
 * the grep. If the user hits ^Z at this point (and jobbing is used), the
 * shell is notified that the grep was suspended. The list_pipe flag is
 * used to tell the execpline where it was waiting that it was in a pipeline
 * with a shell construct at the end (which may also be a shell function or
 * several other things). When zsh sees the suspended grep, it forks to let
 * the sub-shell execute the rest of the while loop. The parent shell walks
 * up in the function call stack to the first execpline. There it has to find
 * out that it has just forked and then has to add information about the sub-
 * shell (its pid and the text for it) in the job entry of the cat. The pid
 * is passed down in the list_pipe_pid variable.
 * But there is a problem: the suspended grep is a child of the parent shell
 * and can't be adopted by the sub-shell. So the parent shell also has to
 * keep the information about this process (more precisely: this pipeline)
 * by keeping the job table entry it created for it. The fact that there
 * are two jobs which have to be treated together is remembered by setting
 * the STAT_SUPERJOB flag in the entry for the cat-job (which now also
 * contains a process-entry for the whole loop -- the sub-shell) and by
 * setting STAT_SUBJOB in the job of the grep-job. With that we can keep
 * sub-jobs from being displayed and we can handle an fg/bg on the super-
 * job correctly. When the super-job is continued, the shell also wakes up
 * the sub-job. But then, the grep will exit sometime. Now the parent shell
 * has to remember not to try to wake it up again (in case of another ^Z).
 * It also has to wake up the sub-shell (which suspended itself immediately
 * after creation), so that the rest of the loop is executed by it.
 * But there is more: when the sub-shell is created, the cat may already
 * have exited, so we can't put the sub-shell in the process group of it.
 * In this case, we put the sub-shell in the process group of the parent
 * shell and in any case, the sub-shell has to put all commands executed
 * by it into its own process group, because only this way the parent
 * shell can control them since it only knows the process group of the sub-
 * shell. Of course, this information is also important when putting a job
 * in the foreground, where we have to attach its process group to the
 * controlling tty.
 * All this is made more difficult because we have to handle return values
 * correctly. If the grep is signaled, its exit status has to be propagated
 * back to the parent shell which needs it to set the exit status of the
 * super-job. And of course, when the grep is signaled (including ^C), the
 * loop has to be stopped, etc.
 * The code for all this is distributed over three files (exec.c, jobs.c,
 * and signals.c) and none of them is a simple one. So, all in all, there
 * may still be bugs, but considering the complexity (with race conditions,
 * signal handling, and all that), this should probably be expected.

I hope this explains why bash does what it does.

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