comm: "- -" deadlocks

coreutils 0.10.0 (multi-call binary), latest commit

uutils comm hangs:

printf 'test' | ./comm - -

gnu comm behaves differently:

printf 'test' | gnucomm - -
test
gnucomm: -

gnu comm's behavior seems non-deterministic, with larger inputs it leaves stray leftovers and cuts lines based on glibc stdio buffer sizes.

Possible solutions would be to
a) replicate gnu comm's behavior or
b) reject "- -" as arguments

Hanging probably isn't the best behavior, but a POSIX conforming application can choose to do that if they want [1]:

If both file1 and file2 refer to standard input or to the same FIFO special, block special, or character special file, the results are undefined.

Also, that makes coreutils' behavior conformant.

Fixed with #15249

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