t: detect errors outside of test cases

We have recently merged a patch series that had a simple misspelling of
`test_expect_success`. Instead of making our tests fail though, this
typo went completely undetected and all of our tests passed, which is of
course unfortunate. This is a more general issue with our test suite:
all commands that run outside of a specific test case can fail, and if
we don't explicitly check for such failure then this failure will be
silently ignored.

Improve the status quo by enabling the errexit option so that any such
unchecked failures will cause us to abort immediately.

Note that for now, we only enable this option for Bash 5 and newer. This
is because other shells have wildly different behaviour, and older
versions of Bash (especially on macOS) are buggy. The list of enabled
shells may be extended going forward.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

ci/run-build-and-tests.sh

@@ -15,6 +15,11 @@ fedora-breaking-changes-musl|linux-breaking-changes)
 	MESONFLAGS="$MESONFLAGS -Drust=enabled"
 	;;
 linux-TEST-vars)
+	# Ubuntu uses Dash by default, but we only enable use of `set -e`
+	# when using Bash 5+. Ensure that we have at least one CI job that uses
+	# it.
+	export TEST_SHELL_PATH=/usr/bin/bash
+
 	export OPENSSL_SHA1_UNSAFE=YesPlease
 	export GIT_TEST_SPLIT_INDEX=yes
 	export GIT_TEST_FULL_IN_PACK_ARRAY=true

t/test-lib.sh

@@ -15,6 +15,15 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see https://www.gnu.org/licenses/ .
 
+# Enable the use of errexit so that any unexpected failures will cause us to
+# abort tests, even when outside of a specific test case. Note that we only
+# enable this on Bash 5 and newer, as `set -e` has wildly different behaviour
+# across shells. The list of allowed shells may be extended going forward.
+if test "${BASH_VERSINFO:=0}" -ge 5
+then
+	set -e
+fi
+
 # Test the binaries we have just built.  The tests are kept in
 # t/ subdirectory and are run in 'trash directory' subdirectory.
 if test -z "$TEST_DIRECTORY"