tests: regression: separate bash traces from errors

The BASH_XTRACEFD variable can be used to redirect "set -x" traces
to a dedicated file. We can use it to split the execution trace
(what has actually happened) from the failure messages.

On a failing test this does provide improved clarity when debugging
interactively with "spread -debug". On non-interactive runs the now
shorter error list is also implicitly printed.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

tests/regression/apparmor/task.yaml

@@ -104,12 +104,13 @@ environment:
 artifacts:
     - bash.log
     - bash.err
+    - bash.trace
 execute: |
     # Run the shell script that is named after the spread variant we are running
     # now. The makefile runs them all sequentially via the "alltests" goal. Here
     # we can parallelize it through spread and also have a way to run precisely
     # the test we want, especially for debugging.
-    if ! bash -x "$SPREAD_VARIANT".sh >bash.log 2>bash.err; then
+    if ! BASH_XTRACEFD=42 bash -x "$SPREAD_VARIANT".sh >bash.log 2>bash.err 42>bash.trace; then
       for xfail in ${XFAIL:-}; do
         if [ "$SPREAD_SYSTEM" = "$xfail" ]; then
           echo "Ignoring expected failure of $SPREAD_TASK on $SPREAD_SYSTEM"
@@ -118,13 +119,17 @@ execute: |
       done
 
       echo "Test $SPREAD_VARIANT has unexpectedly failed"
-      echo "Test execution logs are in the files bash.{log.err} and are collected as artifacts"
+      echo "Test execution logs are in the files bash.{log,err,trace} and are collected as artifacts"
+      echo "Bash errors are listed below:"
+      cat bash.err
       exit 1
     else
       for xfail in ${XFAIL:-}; do
         if [ "$SPREAD_SYSTEM" = "$xfail" ]; then
           echo "Test $SPREAD_VARIANT has unexpectedly passed"
-          echo "Test execution logs are in the files bash.{log.err} and are collected as artifacts"
+          echo "Test execution logs are in the files bash.{log,err,trace} and are collected as artifacts"
+          echo "Bash errors are listed below:"
+          cat bash.err
           exit 1
         fi
       done