pack-objects (mingw): demonstrate a segmentation fault with large deltas

There is a problem in the way 9ac3f0e5b3e4 (pack-objects: fix
performance issues on packing large deltas, 2018-07-22) initializes that
mutex in the `packing_data` struct. The problem manifests in a
segmentation fault on Windows, when a mutex (AKA critical section) is
accessed without being initialized. (With pthreads, you apparently do
not really have to initialize them?)

This was reported in https://github.com/git-for-windows/git/issues/1839.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/t/t5321-pack-large-objects.sh b/t/t5321-pack-large-objects.sh
new file mode 100755 (executable)
index 0000000..c36c66f
--- /dev/null
+++ b/t/t5321-pack-large-objects.sh
@@ -0,0 +1,32 @@
+#!/bin/sh
+#
+# Copyright (c) 2018 Johannes Schindelin
+#
+
+test_description='git pack-object with "large" deltas
+
+'
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-pack.sh
+
+# Two similar-ish objects that we have computed deltas between.
+A=01d7713666f4de822776c7622c10f1b07de280dc
+B=e68fe8129b546b101aee9510c5328e7f21ca1d18
+
+test_expect_success 'setup' '
+       clear_packs &&
+       {
+               pack_header 2 &&
+               pack_obj $A $B &&
+               pack_obj $B
+       } >ab.pack &&
+       pack_trailer ab.pack &&
+       git index-pack --stdin <ab.pack
+'
+
+test_expect_failure 'repack large deltas' '
+       printf "%s\\n" $A $B |
+       GIT_TEST_OE_DELTA_SIZE=2 git pack-objects tmp-pack
+'
+
+test_done