[PATCH] Makefile: use sha1collisiondetection by default on OSX and Darwin

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

When the sha1collisiondetection library was added and made the default
in [1] the interaction with APPLE_COMMON_CRYPTO added in [2] and [3]
seems to have been missed. On modern OSX and Darwin we are able to use
Apple's CommonCrypto both for SHA-1, and as a generic (but partial)
OpenSSL replacement.

This left OSX and Darwin without protection against the SHAttered
attack when building Git in its default configuration.

Let's also use sha1collisiondetection on OSX, to do so we'll need to
split up the "APPLE_COMMON_CRYPTO" flag into that flag and a new
"APPLE_COMMON_CRYPTO_SHA1".

Let's also change the CI for "osx-clang" to test with the new
APPLE_COMMON_CRYPTO_SHA1 knob ("osx-gcc" uses the new
sha1collisiondetection default).

In practice this will spot issues like the one noted in [4], as
testing with just two backends should be enough to spot unportable
code. Ideally we'd have other CI jobs to test the various SHA-1
combinations, but for now we have better CI coverage than before.

1. e6b07da2780 (Makefile: make DC_SHA1 the default, 2017-03-17)
2. 4dcd7732db0 (Makefile: add support for Apple CommonCrypto facility, 2013-05-19)
3. 61067954ce1 (cache.h: eliminate SHA-1 deprecation warnings on Mac OS X, 2013-05-19)
4. 32205655dc7 (fsmonitor OSX: compile with DC_SHA1=YesPlease, 2022-10-19)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---

The 1st hunk here depends on the "base" topic which prepares the docs
for this small change:
https://lore.kernel.org/git/cover-v3-0.9-00000000000-20221020T223946Z-avarab@gmail.com/

But otherwise this applies on "master".

Junio: I see in the meantime you've queued your own
https://lore.kernel.org/git/cover-v3-0.9-00000000000-20221020T223946Z-avarab@gmail.com/;
which is currently in "seen".

That patch will merge smoothly with this one, both textually and
semantically, but if we have this it's all that's needed to flip the
default and give us pretty much the same OSX CI coverage.

"Pretty much" because an unstated effect of your patch is to disable
all use of the "Apple Common Crypto" library on "osx-clang", which
includes (but is not limited to) giving us another SHA-1 backend.

Range-diff:
1:  392fabdb456 < -:  ----------- fsmonitor OSX: compile with DC_SHA1=YesPlease
2:  7ae22276aa7 < -:  ----------- Makefile: create and use sections for "define" flag listing
3:  78ef8636c57 < -:  ----------- Makefile: really use and document sha1collisiondetection by default
4:  f1fb9775b33 < -:  ----------- Makefile: rephrase the discussion of *_SHA1 knobs
-:  ----------- > 1:  1f4e39be97b Makefile: use sha1collisiondetection by default on OSX and Darwin

 Makefile  | 10 ++++------
 ci/lib.sh |  3 +++
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index 36d6bffd1f1..fb4f240e28b 100644
--- a/Makefile
+++ b/Makefile
@@ -529,10 +529,8 @@ include shared.mak
 # Define BLK_SHA1 to make use of optimized C SHA-1 routines bundled
 # with git (in the block-sha1/ directory).
 #
-# Define NO_APPLE_COMMON_CRYPTO on OSX to opt-out of using the
-# "APPLE_COMMON_CRYPTO" backend for SHA-1, which is currently the
-# default on that OS. We'll define NO_APPLE_COMMON_CRYPTO on Mac OS
-# 10.4 or older ("Tiger", released in early 2005).
+# Define APPLE_COMMON_CRYPTO_SHA1 to use Apple's CommonCrypto for
+# SHA-1.
 #
 # === SHA-256 backend ===
 #
@@ -1873,7 +1871,7 @@ ifdef NO_POSIX_GOODIES
 	BASIC_CFLAGS += -DNO_POSIX_GOODIES
 endif
 
-ifdef APPLE_COMMON_CRYPTO
+ifdef APPLE_COMMON_CRYPTO_SHA1
 	# Apple CommonCrypto requires chunking
 	SHA1_MAX_BLOCK_SIZE = 1024L*1024L*1024L
 endif
@@ -1890,7 +1888,7 @@ ifdef BLK_SHA1
 	LIB_OBJS += block-sha1/sha1.o
 	BASIC_CFLAGS += -DSHA1_BLK
 else
-ifdef APPLE_COMMON_CRYPTO
+ifdef APPLE_COMMON_CRYPTO_SHA1
 	COMPAT_CFLAGS += -DCOMMON_DIGEST_FOR_OPENSSL
 	BASIC_CFLAGS += -DSHA1_APPLE
 else
diff --git a/ci/lib.sh b/ci/lib.sh
index 1b0cc2b57db..fda7e008f26 100755
--- a/ci/lib.sh
+++ b/ci/lib.sh
@@ -264,6 +264,9 @@ macos-latest)
 esac
 
 case "$jobname" in
+osx-clang)
+	MAKEFLAGS="$MAKEFLAGS APPLE_COMMON_CRYPTO_SHA1=Yes"
+	;;
 linux32)
 	CC=gcc
 	;;
-- 
2.38.0.1178.g509f5fa8ce0