Made of Bugs

Followup to "a Very Subtle Bug"

After my previous post got posted to reddit, there was a bunch of interesting discussion there about some details I'd handwaved over. This is a quick followup on some the investigation that various people carried out, and the conclusions they reached.

In the reddit thread, lacos/lbzip2 objected that in his experiments, he didn't see tar closing the input pipe before it was done reading the file, and so questioned where the SIGPIPE/EPIPE was coming from in the first place. I had actually done similar experiments with similar results, but I was still seeing the EPIPE, so I knew it could happen, but I couldn't totally explain why.

A friend of mine, David Benjamin, was curious enough to source-dive tar, and posted his results on his own blog. He discovered that by default, tar does not close the pipe after finding all the files it needs, because the tar archive format allows for later copies of the same file, which would supercede the previous ones. This explains why lacos and I saw tar reading to the end of a linux-2.6 tarball, even if we only asked for the first file.

He also discovered, however, that a typical tar file ends with a number of NUL blocks, which tar treats as end-of-file. And so tar will close the pipe after reading the first of these, which opens a narrow race condition whereby tar can potentially do so before gzip has written the remaining NUL blocks, resulting in a SIGPIPE.

Finally, the discussion inspired lacos to post a query clarifying tar's behavior with respect to SIGPIPE and closing the pipe early to the help-tar mailing list, which resulted in a brief thread that, among other things, revealed that the bug I posted about has been fixed in GNU tar as of last summer, by having tar reset the disposition of SIGPIPE to SIG_DFL before spawning a child. It was also pointed out that tar checks whether a filter subprocess is killed by SIGPIPE, and treats that as a success – so it's not actually necessary for a tar filter to handle SIGPIPE and exit cleanly, like gzip does.