Merge branch 'jk/sanity' into maint

The description for SANITY prerequisite the test suite uses has
been clarified both in the comment and in the implementation.

* jk/sanity:
  test-lib: clarify and tighten SANITY

t/test-lib.sh

@@ -1054,20 +1054,28 @@ test_lazy_prereq NOT_ROOT '
 	test "$uid" != 0
 '
 
-# On a filesystem that lacks SANITY, a file can be deleted even if
-# the containing directory doesn't have write permissions, or a file
-# can be accessed even if the containing directory doesn't have read
-# or execute permissions, causing our tests that validate that Git
-# works sensibly in such situations.
+# SANITY is about "can you correctly predict what the filesystem would
+# do by only looking at the permission bits of the files and
+# directories?"  A typical example of !SANITY is running the test
+# suite as root, where a test may expect "chmod -r file && cat file"
+# to fail because file is supposed to be unreadable after a successful
+# chmod.  In an environment (i.e. combination of what filesystem is
+# being used and who is running the tests) that lacks SANITY, you may
+# be able to delete or create a file when the containing directory
+# doesn't have write permissions, or access a file even if the
+# containing directory doesn't have read or execute permissions.
+
 test_lazy_prereq SANITY '
 	mkdir SANETESTD.1 SANETESTD.2 &&
 
 	chmod +w SANETESTD.1 SANETESTD.2 &&
 	>SANETESTD.1/x 2>SANETESTD.2/x &&
 	chmod -w SANETESTD.1 &&
+	chmod -r SANETESTD.1/x &&
 	chmod -rx SANETESTD.2 ||
 	error "bug in test sript: cannot prepare SANETESTD"
 
+	! test -r SANETESTD.1/x &&
 	! rm SANETESTD.1/x && ! test -f SANETESTD.2/x
 	status=$?