test-lib: set BASH_XTRACEFD automatically

Passing "-x" to a test script enables the shell's "set -x"
tracing, which can help with tracking down the command that
is causing a failure. Unfortunately, it can also _cause_
failures in some tests that redirect the stderr of a shell
function.  Inside the function the shell continues to
respect "set -x", and the trace output is collected along
with whatever stderr is generated normally by the function.

You can see an example of this by running:

  ./t0040-parse-options.sh -x -i

which will fail immediately in the first test, as it
expects:

  test_must_fail some-cmd 2>output.err

to leave output.err empty (but with "-x" it has our trace
output).

Unfortunately there isn't a portable or scalable solution to
this. We could teach test_must_fail to disable "set -x", but
that doesn't help any of the other functions or subshells.

However, we can work around it by pointing the "set -x"
output to our descriptor 4, which always points to the
original stderr of the test script. Unfortunately this only
works for bash, but it's better than nothing (and other
shells will just ignore the BASH_XTRACEFD variable).

The patch itself is a simple one-liner, but note the caveats
in the accompanying comments.

Automatic tests for our "-x" option may be a bit too meta
(and a pain, because they are bash-specific), but I did
confirm that it works correctly both with regular "-x" and
with "--verbose-only=1". This works because the latter flips
"set -x" off and on for particular tests (if it didn't, we
would get tracing for all tests, as going to descriptor 4
effectively circumvents the verbose flag).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -84,9 +84,9 @@ appropriately before running "make".
 
 -x::
 	Turn on shell tracing (i.e., `set -x`) during the tests
-	themselves. Implies `--verbose`. Note that this can cause
-	failures in some tests which redirect and test the
-	output of shell functions. Use with caution.
+	themselves. Implies `--verbose`. Note that in non-bash shells,
+	this can cause failures in some tests which redirect and test
+	the output of shell functions. Use with caution.
 
 -d::
 --debug::

t/test-lib.sh

@@ -322,6 +322,19 @@ else
 	exec 4>/dev/null 3>/dev/null
 fi
 
+# Send any "-x" output directly to stderr to avoid polluting tests
+# which capture stderr. We can do this unconditionally since it
+# has no effect if tracing isn't turned on.
+#
+# Note that this sets up the trace fd as soon as we assign the variable, so it
+# must come after the creation of descriptor 4 above. Likewise, we must never
+# unset this, as it has the side effect of closing descriptor 4, which we
+# use to show verbose tests to the user.
+#
+# Note also that we don't need or want to export it. The tracing is local to
+# this shell, and we would not want to influence any shells we exec.
+BASH_XTRACEFD=4
+
 test_failure=0
 test_count=0
 test_fixed=0