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[PATCH 2/2] test-lib: exhaustively insert non-alnum ASCII into the TRASH_DIRECTORY name

Change the test library to insert non-alphanumeric ASCII characters
into the TRASH_DIRECTORY name, that's the directory the test library
creates, chdirs to and runs each individual test from.

Unless test_fails_on_unusual_directory_names=1 is declared before
importing test-lib.sh (and if perl isn't available on the system), the
trash directory will contain every non-alphanumeric character in
ASCII, in order.

This includes all the control characters, !, [], {} etc. the "."
character isn't included because it's already in the directory name,
and nor is "/" for obvious reasons, although that would actually work,
we'd just create a subdirectory, which would make the tests harder to
inspect when they fail.i

This change is inspired by the "submodule: prevent backslash expantion
in submodule names" patch[1]. If we'd had backslashes in the
TRASH_DIRECTORY all along that bug would have been fixed a long time
ago. This will flag such issues by marking tests that currently fail
with "test_fails_on_unusual_directory_names=1", ensure that new tests
aren't added unless a discussion is had about why the code can't
handle unusual pathnames, and prevent future regressions.

1. <20170407172306.172673-1-bmwill@xxxxxxxxxx>
 t/README      | 12 ++++++++++++
 t/test-lib.sh |  4 ++++
 2 files changed, 16 insertions(+)

diff --git a/t/README b/t/README
index ab386c3681..314dd40221 100644
--- a/t/README
+++ b/t/README
@@ -345,6 +345,18 @@ assignment to variable 'test_description', like this:
 	This test registers the following structure in the cache
 	and tries to run git-ls-files with option --frotz.'
+By default the tests will be run from a directory with a highly
+unusual filename that includes control characters, a newline, various
+punctuation etc., this is done to smoke out any bugs related to path
+handling. If for whatever reason the tests can't deal with such
+unusual path names, set:
+    test_fails_on_unusual_directory_names=1
+Before sourcing 'test-lib.sh' as described below. This option is
+mainly intended to grandfather in existing broken tests & code, and
+should usually not be used in new code, instead your tests or code
+probably need fixing.
 Source 'test-lib.sh'
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 13b5696822..089ff5ac7d 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -914,6 +914,10 @@ fi
 # Test repository
 TRASH_DIRECTORY="trash directory.$(basename "$0" .sh)"
+if test -z "$test_fails_on_unusual_directory_names" -a "$(perl -e 'print 1+1' 2>/dev/null)" = "2"
+   TRASH_DIRECTORY="$TRASH_DIRECTORY.$(perl -e 'print join q[], grep { /[^[:alnum:]]/ and !m<[./]> } map chr, 0x01..0x7f')"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 /*) ;; # absolute path is good