[PATCH 2/3] t5615: avoid re-using descriptor 4

File descriptors 3 and 4 are special in our test suite, as they link back to the test script's original stdout and stderr. Normally this isn't something tests need to worry about: they are free to clobber these descriptors for sub-commands without affecting the overall script.

But there's one very special thing about descriptor 4: since d88785e424 (test-lib: set BASH_XTRACEFD automatically, 2016-05-11), we ask bash to output "set -x" output to it by number. This goes to _any_ descriptor 4, even if it no longer points to the place it did when we set BASH_XTRACEFD.

But in t5615, we run a shell loop with descriptor 4 redirected. As a result, t5615 works with non-bash shells even with "-x". And it works with bash without "-x". But the combination of "bash t5615-alternate-env.sh -x" gets a test failure (because our "set -x" output pollutes one of the files).

We can fix this by using any descriptor _except_ the magical
4. So let's switch arbitrarily to using 5/6 in this loop,
not 3/4.

Signed-off-by: Jeff King <peff@peff.net>
---
I also considered trying to bump the "set -x" output descriptor to "9".
That just moves the problem around, but presumably scripts are less
likely to go that high. :)

It would also be possible to pick something insanely high, like "999". Many shells choke on descriptors higher than 9, but since the issue is related to BASH_XTRACEFD, we could make it a bash-only thing. I don't know if it's worth the trouble. I hate leaving a subtle "don't use descriptor 4 in a subshell or your script will break" hand-grenade like this lying around, but we do seem to have only one instance of it over the whole test suite.

 t/t5615-alternate-env.sh | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)