test-lib: add GIT_SAN_OPTIONS, inherit [AL]SAN_OPTIONS

Change our ASAN_OPTIONS and LSAN_OPTIONS to set defaults for those
variables, rather than punting out entirely if we already have them in
the environment.

We want to take any user-provided settings over our own, but we can do
that by prepending our defaults to the variable. The libsanitizer
options parsing has "last option wins" semantics.

It's now possible to do e.g.:

    LSAN_OPTIONS=report_objects=1 ./t0006-date.sh

And not have the "report_objects=1" setting overwrite our sensible
default of "abort_on_error=1", but by prepending to the list we ensure
that:

    LSAN_OPTIONS=report_objects=1:abort_on_error=0 ./t0006-date.sh

Will take the desired "abort_on_error=0" over our default.

See b0f4c90 (t: support clang/gcc AddressSanitizer, 2014-12-08)
for the original pattern being altered here, and
85b81b3 (test-lib: set LSAN_OPTIONS to abort by default,
2017-09-05) for when LSAN_OPTIONS was added in addition to the
then-existing ASAN_OPTIONS.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/test-lib.sh

@@ -36,17 +36,33 @@ then
 fi
 GIT_BUILD_DIR="$TEST_DIRECTORY"/..
 
+# Prepend a string to a VAR using an arbitrary ":" delimiter, not
+# adding the delimiter if VAR or VALUE is empty. I.e. a generalized:
+#
+#	VAR=$1${VAR:+${1:+$2}$VAR}
+#
+# Usage (using ":" as the $2 delimiter):
+#
+#	prepend_var VAR : VALUE
+prepend_var () {
+	eval "$1=$3\${$1:+${3:+$2}\$$1}"
+}
+
+# If [AL]SAN is in effect we want to abort so that we notice
+# problems. The GIT_SAN_OPTIONS variable can be used to set common
+# defaults shared between [AL]SAN_OPTIONS.
+prepend_var GIT_SAN_OPTIONS : abort_on_error=1
+
 # If we were built with ASAN, it may complain about leaks
 # of program-lifetime variables. Disable it by default to lower
 # the noise level. This needs to happen at the start of the script,
 # before we even do our "did we build git yet" check (since we don't
 # want that one to complain to stderr).
-: ${ASAN_OPTIONS=detect_leaks=0:abort_on_error=1}
+prepend_var ASAN_OPTIONS : $GIT_SAN_OPTIONS
+prepend_var ASAN_OPTIONS : detect_leaks=0
 export ASAN_OPTIONS
 
-# If LSAN is in effect we _do_ want leak checking, but we still
-# want to abort so that we notice the problems.
-: ${LSAN_OPTIONS=abort_on_error=1}
+prepend_var LSAN_OPTIONS : $GIT_SAN_OPTIONS
 export LSAN_OPTIONS
 
 if test ! -f "$GIT_BUILD_DIR"/GIT-BUILD-OPTIONS