[PATCH 2/3] sha1dc: optionally use sha1collisiondetection as a submodule

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add an option to use the sha1collisiondetection library from the
submodule in sha1collisiondetection/ instead of in the copy in the
sha1dc/ directory.

This allows us to try out the submodule in sha1collisiondetection
without breaking the build for anyone who's not expecting them as we
work out any kinks.

This uses my own fork which integrates PR #36. See the preceding
commit ("sha1dc: update from my PR #36", 2017-06-27) for details.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 .gitmodules            |  4 ++++
 Makefile               | 12 ++++++++++++
 hash.h                 |  4 ++++
 sha1collisiondetection |  1 +
 4 files changed, 21 insertions(+)
 create mode 100644 .gitmodules
 create mode 160000 sha1collisiondetection

diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000000..2fea9996e9
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,4 @@
+[submodule "sha1collisiondetection"]
+	path = sha1collisiondetection
+	url = https://github.com/avar/sha1collisiondetection.git
+	branch = bigend-detect-solaris-again
diff --git a/Makefile b/Makefile
index b94cd5633c..f0cac1f246 100644
--- a/Makefile
+++ b/Makefile
@@ -162,6 +162,12 @@ all::
 # algorithm. This is slower, but may detect attempted collision attacks.
 # Takes priority over other *_SHA1 knobs.
 #
+# Define DC_SHA1_SUBMODULE in addition to DC_SHA1 to use the
+# sha1collisiondetection shipped as a submodule instead of the
+# non-submodule copy in sha1dc/. This is an experimental option used
+# by the git project to migrate to using sha1collisiondetection as a
+# submodule.
+#
 # Define OPENSSL_SHA1 environment variable when running make to link
 # with the SHA1 routine from openssl library.
 #
@@ -1448,8 +1454,14 @@ ifdef APPLE_COMMON_CRYPTO
 	BASIC_CFLAGS += -DSHA1_APPLE
 else
 	DC_SHA1 := YesPlease
+ifdef DC_SHA1_SUBMODULE
+	LIB_OBJS += sha1collisiondetection/lib/sha1.o
+	LIB_OBJS += sha1collisiondetection/lib/ubc_check.o
+	BASIC_CFLAGS += -DDC_SHA1_SUBMODULE
+else
 	LIB_OBJS += sha1dc/sha1.o
 	LIB_OBJS += sha1dc/ubc_check.o
+endif
 	BASIC_CFLAGS += \
 		-DSHA1_DC \
 		-DSHA1DC_NO_STANDARD_INCLUDES \
diff --git a/hash.h b/hash.h
index a11fc9233f..bef3e630a0 100644
--- a/hash.h
+++ b/hash.h
@@ -8,7 +8,11 @@
 #elif defined(SHA1_OPENSSL)
 #include <openssl/sha.h>
 #elif defined(SHA1_DC)
+#ifdef DC_SHA1_SUBMODULE
+#include "sha1collisiondetection/lib/sha1.h"
+#else
 #include "sha1dc/sha1.h"
+#endif
 #else /* SHA1_BLK */
 #include "block-sha1/sha1.h"
 #endif
diff --git a/sha1collisiondetection b/sha1collisiondetection
new file mode 160000
index 0000000000..56ab30c4c9
--- /dev/null
+++ b/sha1collisiondetection
@@ -0,0 +1 @@
+Subproject commit 56ab30c4c998e1e7f3075705087a2f0c4c4202d7
-- 
2.13.1.611.g7e3b11ae1