test-lib: correct and assert TEST_DIRECTORY overriding

Correct a misleading comment added by me in 62f5390 (test-lib:
Allow overriding of TEST_DIRECTORY, 2010-08-19), and add an assertion
that TEST_DIRECTORY cannot point to any directory except the "t"
directory in the top-level of git.git.

This assertion is in effect not new, since we'd already die if that
wasn't the case[1], but it and the updated commentary help to make
that clearer.

The existing comments were also on the wrong arms of the
"if". I.e. the "allow tests to override this" was on the "test -z"
arm. That came about due to a combination of 62f5390 and
85176d7 (test-lib.sh: convert $TEST_DIRECTORY to an absolute path,
2013-11-17).

Those earlier comments could be read as allowing the "$TEST_DIRECTORY"
to be some path outside of t/. As explained in the updated comment
that's impossible, rather it was meant for *tests* that ran outside of
t/, i.e. the "t0000-basic.sh" tests that use "lib-subtest.sh".

Those tests have a different working directory, but they set the
"TEST_DIRECTORY" to the same path for bootstrapping. The comments now
reflect that, and further comment on why we have a hard dependency on
this.

1. https://lore.kernel.org/git/[email protected]/

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

Author: avar

t/test-lib.sh

@@ -19,13 +19,20 @@
 # t/ subdirectory and are run in 'trash directory' subdirectory.
 if test -z "$TEST_DIRECTORY"
 then
-	# We allow tests to override this, in case they want to run tests
-	# outside of t/, e.g. for running tests on the test library
-	# itself.
-	TEST_DIRECTORY=$(pwd)
-else
 	# ensure that TEST_DIRECTORY is an absolute path so that it
 	# is valid even if the current working directory is changed
+	TEST_DIRECTORY=$(pwd)
+else
+	# The TEST_DIRECTORY will always be the path to the "t"
+	# directory in the git.git checkout. This is overridden by
+	# e.g. t/lib-subtest.sh, but only because its $(pwd) is
+	# different. Those tests still set "$TEST_DIRECTORY" to the
+	# same path.
+	#
+	# See use of "$GIT_BUILD_DIR" and "$TEST_DIRECTORY" below for
+	# hard assumptions about "$GIT_BUILD_DIR/t" existing and being
+	# the "$TEST_DIRECTORY", and e.g. "$TEST_DIRECTORY/helper"
+	# needing to exist.
 	TEST_DIRECTORY=$(cd "$TEST_DIRECTORY" && pwd) || exit 1
 fi
 if test -z "$TEST_OUTPUT_DIRECTORY"
@@ -35,6 +42,11 @@ then
 	TEST_OUTPUT_DIRECTORY=$TEST_DIRECTORY
 fi
 GIT_BUILD_DIR="$TEST_DIRECTORY"/..
+if test "$TEST_DIRECTORY" = "${TEST_DIRECTORY%/t}"
+then
+	echo "PANIC: Running in a $TEST_DIRECTORY that doesn't end in '/t'?" >&2
+	exit 1
+fi
 
 # Prepend a string to a VAR using an arbitrary ":" delimiter, not
 # adding the delimiter if VAR or VALUE is empty. I.e. a generalized: