test/common: Free stderr fd from tracing

Consider this:

foo() { echo bar >&2 ; }
set -x
foobar=$(foo 2>&1)
[[ $foobar == "bar" ]]

The above test will fail since $foobar will also include tracing
strings as stderr is where set -x redirects the output. Since in
some cases this may troublesome, replace stderr with a dedicated
fd allocated dynamically.

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: Ia6609e4463b371c07fab42d2bd291c9e43742df5
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/8158
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>

test/common/autotest_common.sh

@@ -1,5 +1,16 @@
 #!/usr/bin/env bash
 
+function xtrace_fd() {
+	if [[ -n $BASH_XTRACEFD && -e /proc/self/fd/$BASH_XTRACEFD ]]; then
+		# Close it first to make sure it's sane
+		exec {BASH_XTRACEFD}>&-
+	fi
+	exec {BASH_XTRACEFD}>&2
+
+	set -x
+	echo "Tracing to $BASH_XTRACEFD FD"
+}
+
 function xtrace_disable() {
 	if [ "$XTRACE_DISABLED" != "yes" ]; then
 		PREV_BASH_OPTS="$-"
@@ -1388,7 +1399,7 @@ if $SPDK_AUTOTEST_X; then
 	# explicitly enable xtraces, overriding any tracking information.
 	unset XTRACE_DISABLED
 	unset XTRACE_NESTING_LEVEL
-	set -x
+	xtrace_fd
 	xtrace_enable
 else
 	xtrace_restore