[PATCH v2 8/8] tests: make GIT_TEST_FAIL_PREREQS a boolean

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Change the GIT_TEST_FAIL_PREREQS variable from being "non-empty?" to
being a more standard boolean variable. I recently added the variable
in dfe1a17df9 ("tests: add a special setup where prerequisites fail",
2019-05-13), having to add another "non-empty?" special-case is what
prompted me to write the "git env--helper" utility being used here.

Converting this one is a bit tricky since we use it so early and
frequently in the guts of the test code itself, so let's set a
GIT_TEST_FAIL_PREREQS_INTERNAL which can be tested with the old "test
-n" for the purposes of the shell code, and change the user-exposed
and documented GIT_TEST_FAIL_PREREQS variable to a boolean.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 t/README                |  2 +-
 t/t0000-basic.sh        | 10 +++++-----
 t/test-lib-functions.sh |  2 +-
 t/test-lib.sh           | 25 +++++++++++++++++++++----
 4 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/t/README b/t/README
index 072c9854d1..60d5b77bcc 100644
--- a/t/README
+++ b/t/README
@@ -334,7 +334,7 @@ that cannot be easily covered by a few specific test cases. These
 could be enabled by running the test suite with correct GIT_TEST_
 environment set.
 
-GIT_TEST_FAIL_PREREQS<non-empty?> fails all prerequisites. This is
+GIT_TEST_FAIL_PREREQS=<boolean> fails all prerequisites. This is
 useful for discovering issues with the tests where say a later test
 implicitly depends on an optional earlier test.
 
diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index 31de7e90f3..e89438e619 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -726,7 +726,7 @@ donthaveit=yes
 test_expect_success DONTHAVEIT 'unmet prerequisite causes test to be skipped' '
 	donthaveit=no
 '
-if test -z "$GIT_TEST_FAIL_PREREQS" -a $haveit$donthaveit != yesyes
+if test -z "$GIT_TEST_FAIL_PREREQS_INTERNAL" -a $haveit$donthaveit != yesyes
 then
 	say "bug in test framework: prerequisite tags do not work reliably"
 	exit 1
@@ -747,7 +747,7 @@ donthaveiteither=yes
 test_expect_success DONTHAVEIT,HAVEIT 'unmet prerequisites causes test to be skipped' '
 	donthaveiteither=no
 '
-if test -z "$GIT_TEST_FAIL_PREREQS" -a $haveit$donthaveit$donthaveiteither != yesyesyes
+if test -z "$GIT_TEST_FAIL_PREREQS_INTERNAL" -a $haveit$donthaveit$donthaveiteither != yesyesyes
 then
 	say "bug in test framework: multiple prerequisite tags do not work reliably"
 	exit 1
@@ -763,7 +763,7 @@ test_expect_success !LAZY_TRUE 'missing lazy prereqs skip tests' '
 	donthavetrue=no
 '
 
-if test -z "$GIT_TEST_FAIL_PREREQS" -a "$havetrue$donthavetrue" != yesyes
+if test -z "$GIT_TEST_FAIL_PREREQS_INTERNAL" -a "$havetrue$donthavetrue" != yesyes
 then
 	say 'bug in test framework: lazy prerequisites do not work'
 	exit 1
@@ -779,7 +779,7 @@ test_expect_success LAZY_FALSE 'missing negative lazy prereqs will skip' '
 	havefalse=no
 '
 
-if test -z "$GIT_TEST_FAIL_PREREQS" -a "$nothavefalse$havefalse" != yesyes
+if test -z "$GIT_TEST_FAIL_PREREQS_INTERNAL" -a "$nothavefalse$havefalse" != yesyes
 then
 	say 'bug in test framework: negative lazy prerequisites do not work'
 	exit 1
@@ -790,7 +790,7 @@ test_expect_success 'tests clean up after themselves' '
 	test_when_finished clean=yes
 '
 
-if test -z "$GIT_TEST_FAIL_PREREQS" -a $clean != yes
+if test -z "$GIT_TEST_FAIL_PREREQS_INTERNAL" -a $clean != yes
 then
 	say "bug in test framework: basic cleanup command does not work reliably"
 	exit 1
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 527508c350..3fba71c358 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -309,7 +309,7 @@ test_unset_prereq () {
 }
 
 test_set_prereq () {
-	if test -n "$GIT_TEST_FAIL_PREREQS"
+	if test -n "$GIT_TEST_FAIL_PREREQS_INTERNAL"
 	then
 		case "$1" in
 		# The "!" case is handled below with
diff --git a/t/test-lib.sh b/t/test-lib.sh
index c45b0d2611..238ef62401 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1389,6 +1389,27 @@ yes () {
 	done
 }
 
+# The GIT_TEST_FAIL_PREREQS code hooks into test_set_prereq(), and
+# thus needs to be set up really early, and set an internal variable
+# for convenience so the hot test_set_prereq() codepath doesn't need
+# to call "git env--helper". Only do that work if needed by seeing if
+# GIT_TEST_FAIL_PREREQS is set at all.
+GIT_TEST_FAIL_PREREQS_INTERNAL=
+if test -n "$GIT_TEST_FAIL_PREREQS"
+then
+	if git env--helper --mode-bool --variable=GIT_TEST_FAIL_PREREQS \
+		--default=0 --exit-code --quiet
+	then
+		GIT_TEST_FAIL_PREREQS_INTERNAL=true
+		test_set_prereq FAIL_PREREQS
+	fi
+else
+	test_lazy_prereq FAIL_PREREQS '
+		git env--helper --mode-bool --variable=GIT_TEST_FAIL_PREREQS \
+			--default=0 --exit-code --quiet
+	'
+fi
+
 # Fix some commands on Windows
 uname_s=$(uname -s)
 case $uname_s in
@@ -1605,7 +1626,3 @@ test_lazy_prereq SHA1 '
 test_lazy_prereq REBASE_P '
 	test -z "$GIT_TEST_SKIP_REBASE_P"
 '
-
-test_lazy_prereq FAIL_PREREQS '
-	test -n "$GIT_TEST_FAIL_PREREQS"
-'
-- 
2.22.0.455.g172b71a6c5