On Mon, Jun 26, 2023 at 6:12 PM Ahelenia Ziemiańska <nabijaczleweli@xxxxxxxxxxxxxxxxxx> wrote: > > On Mon, Jun 26, 2023 at 05:53:46PM +0300, Amir Goldstein wrote: > > > So is it really true that the only way to poll a pipe is a > > > sleep()/read(O_NONBLOCK) loop? > > I don't think so, but inotify is not the way. > So what is? What do the kernel developers recommend as a way to see if a > file is written to, and that file happens to be a pipe? > > FTR, I've opened the symmetric Debian#1039488: > https://bugs.debian.org/1039488 > against coreutils, since, if this is expected, and writing to a pipe > should not generate write events on that pipe, then tail -f is currently > broken on most systems. First of all, it is better to mention that you are trying to fix a real world use case when you are reporting a kernel misbehavior. What this makes me wonder is, if tail -f <fifo> is broken as you claim it is, how is it that decades go by without anyone noticing this problem? When looking at tail source code I see: /* Mark as '.ignore'd each member of F that corresponds to a pipe or fifo, and return the number of non-ignored members. */ static size_t ignore_fifo_and_pipe (struct File_spec *f, size_t n_files) { /* When there is no FILE operand and stdin is a pipe or FIFO POSIX requires that tail ignore the -f option. Since we allow multiple FILE operands, we extend that to say: with -f, ignore any "-" operand that corresponds to a pipe or FIFO. */ and it looks like tail_forever_inotify() is not being called unless there are non pipes: if (forever && ignore_fifo_and_pipe (F, n_files)) { The semantics of tail -f on a pipe input would be very odd, because the writer would need to close before tail can figure out which are the last lines. So honestly, we could maybe add IN_ACCESS/IN_MODIFY for the splice_pipe_to_pipe() case, but I would really like to know what the real use case is. Another observation is that splice(2) never used to report any inotify events at all until a very recent commit in v6.4 983652c69199 ("splice: report related fsnotify events") but this commit left out the splice_pipe_to_pipe() case. CC the author of the patch to ask why this case was left out and whether he would be interested in fixing that. Thanks, Amir.
