1.7.9 Missing SIGPIPE?

[Peter Rosin <peda at lysator dot liu dot se>]

Hi!

When I use bash to build pipelines, they sometimes don't finish but
instead some process remains running. Example:

$ tail -f -n 10000 log.txt | grep . | head -n 2

Almost instantly I get the expected two lines of output, but no prompt
back. I have to use ctrl-c. If I don't ctrl-c I can run pstree in
another terminal and see this:

$ pstree
?âââminttyâââbashâââtail
  ââminttyâââbashâââpstree

Now, all of the above is of course rather silly, but there seems to
be a few ingredients that are needed to trigger the bug. One is that
there has to be at least three processes in the pipeline, the other
that the pipeline should end not because the first process runs out
of data but because the last process (or perhaps one in the middle)
decides to end "prematurely". IIUC that should cascade a SIGPIPE
signal to the other involved processes, but that does not seem to
happen?

Note that this has little to do with the programs tail, grep and head
used in my example, it is possible to construct other pipelines that
behave similarly and I originally detected the problem with small
private tools of my own writing.

What I do as a workaround in my programs is that they exit on errors
when writing to stdout. But again, IIUC that should not be needed.

It also seems to depend on the content of log.txt, so I have uploaded
a log.txt that I can trigger the problem with every time [1].

Or are my expectations all wrong?

Cygcheck output attached. And for the record, it also reproduces with
the latest snapshot, i.e.:
$ uname -a
CYGWIN_NT-6.1-WOW64 peda-pc 1.7.10s(0.251/5/3) 20110829 15:53:50 i686 Cygwin

Cheers,
Peter

[1] http://www.lysator.liu.se/~peda/log.txt

Attachment: cygcheck.out
Description: Text document

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