tests: add a special setup where prerequisites fail

As discussed in [1] there's a regression in the "pu" branch now
because a new test implicitly assumed that a previous test guarded by
a prerequisite had been run. Add a "GIT_TEST_FAIL_PREREQS" special
test setup where we'll skip (nearly) all tests guarded by
prerequisites, allowing us to easily emulate those platform where we
don't run these tests.

As noted in the documentation I'm adding I'm whitelisting the SYMLINKS
prerequisite for now. A lot of tests started failing if we lied about
not supporting symlinks. It's also unlikely that we'll have a failing
test due to a hard dependency on symlinks without that being the
obvious cause, so for now it's not worth the effort to make it work.

1. https://public-inbox.org/git/nycvar.QRO.7.76.6.1905131531000.44@tvgsbejvaqbjf.bet/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/t/README b/t/README
index 6404f33e19..9747971d58 100644
--- a/t/README
+++ b/t/README
@@ -334,6 +334,15 @@ 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
+useful for discovering issues with the tests where say a later test
+implicitly depends on an optional earlier test.
+
+There's a "FAIL_PREREQS" prerequisite that can be used to test for
+whether this mode is active, and e.g. skip some tests that are hard to
+refactor to deal with it. The "SYMLINKS" prerequisite is currently
+excluded as so much relies on it, but this might change in the future.
+
 GIT_TEST_GETTEXT_POISON=<non-empty?> turns all strings marked for
 translation into gibberish if non-empty (think "test -n"). Used for
 spotting those tests that need to be marked with a C_LOCALE_OUTPUT