bug#28417: ptx does not terminate when matching a zero length sentence regex

[Julian Büning]

observed behavior:

$ echo | ptx -S $ &
[1] 1000
$ jobs
[1]+  Running                 echo | ptx -S $ &

expected behavior:

$ echo | ptx -S $ &
[1] 1000
[1]+  Done                    echo | ptx -S $

ptx does not terminate in case the specified sentence regex can be
matched but has a match of length zero on input that is non-empty.

The following test cases show the same behavior:
$ echo | ptx -S ^
$ echo | ptx -S "a*"
$ echo | ptx -S "\(\)"
$ echo test | ptx -S "\n*"
$ echo foo > non_empty; ptx non_empty -S $
...

In ptx.c, find_occurs_in_text() calls re_search() and uses the length of
a match (which is falsely assumed to be greater than zero) to advance a
cursor through the input. For a match length of zero, the cursor is
never advanced.

When switching on the results of re_search(), a case 0 could be added.
One possible fix would be to then abort with an error message.

We found this behavior in version 8.27 and can reproduce it in version
8.25 as well as version 8.28.

This behavior was found using Symbolic Execution techniques developed in
the course of the SYMBIOSYS research project at COMSYS, RWTH Aachen
University. This research is supported by the European Research Council
(ERC) under the EU's Horizon 2020 Research and Innovation Programme
grant agreement n. 647295 (SYMBIOSYS).

Best regards,
Julian