Thanks for the quick turnaround. Let's see if I understand. The key part is here:

      case SIGTERM :
        if (flagprotect) break ;
      case SIGHUP :
        handle_stdin = &last_stdin ;
        if (!indata.len) { prepare_to_exit() ; e = 1 ; }
        break ;

The risk is that last_stdin() will try to read stdin after prepare_to_exit() is called.

However, prepare_to_exit() is only called if !indata.len, and causes handle_signals() to return 1.

The caller of handle_signals() is:

      if (x[0].revents & (IOPAUSE_READ | IOPAUSE_EXCEPT) && handle_signals()) continue ;

This causes the next iteration of the event loop to set xindex0 zero because flagexiting was set by prepare_to_exit():

      if (!flagexiting && !(flagblock && r))
        x[j].fd = 0 ;
        x[j].events = IOPAUSE_READ ;
        xindex0 = j++ ;
      else xindex0 = 0 ;


It did cross my mind that another approach is to trade state space complexity for resource usage by creating an additional file descriptor to /dev/null during the initialisation path. With this additional file descriptor, either:

a) have prepare_to_exit() either dup2() this to stdin, or

b) use an int inputfd = 0 to earmark the corresponding input source and overwrite this to instead indicate the /dev/null file descriptor.


On 2021-12-24 00:46, Laurent Bercot wrote:

 So indeed, when an exit signal was received at the same time stdin was
readable (unless your producer is spamming logs, that's a rare event,
which is why I never saw it), prepare_to_exit() was called but the
xindex0 marker was not updated and the remainder of the iteration still
called *handle_stdin, yielding read errors.

 Now, when handle_signals() calls prepare_to_exit(), the event loop
is restarted, so we're never handling events in an obsolete state.

 Please check the latest s6 git head and tell me if it works for you.

 The mistake of not restarting the event loop right away on state change
is something I became aware of and stopped making in later software, but
I never thought of going back and checking whether s6-log had it.


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