[2/3] pack-objects (mingw): demonstrate a segmentation fault with large deltas

Commit Message

Linus Arver via GitGitGadget Oct. 16, 2018, 9:02 p.m. UTC

From: Johannes Schindelin <[e-mail]>

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 <[e-mail]>
---
 t/t5321-pack-large-objects.sh | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)
 create mode 100755 t/t5321-pack-large-objects.sh

Comments

brian m. carlson Oct. 16, 2018, 11:43 p.m. UTC | #1

On Tue, Oct 16, 2018 at 02:02:50PM -0700, Johannes Schindelin via GitGitGadget wrote:
> From: Johannes Schindelin <[e-mail]>
> 
> 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 is a good catch.  You do have to initialize a pthread mutex, but on
amd64 Linux the default initializer is all zeros, so since we use a
static variable, it happens to coincidentally have the right value, so
we don't notice.

Thanks for fixing this.

Patch

diff --git a/t/t5321-pack-large-objects.sh b/t/t5321-pack-large-objects.sh
new file mode 100755
index 0000000000..c36c66fbb4
--- /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