Re: [PATCH v2 3/4] Makefile: really use and document sha1collisiondetection by default

> When the sha1collisiondetection library was added and made the default
> in [1] we never updated the documentation added in [2] early in that
> series once the default was flipped to DC_SHA1 in [3]. Furthermore the
> INSTALL file has been claiming that we use OpenSSL by default since
> [4], and hadn't been updated for the sha1collisiondetection switch.
>
> The interaction between NO_APPLE_COMMON_CRYPTO and DC_SHA1 seems to
> have been missed in [3], so ever since DC_SHA1 was made the default
> we've still used Apple's CommonCrypto instead of
> sha1collisiondetection on Darwin and Mac OS X.
>
> Instead off all of this we now:
>
> * Don't have a DC_SHA1 know anymore (using it is an error), you need

>   to set NO_DC_SHA1 instead to use any optional *_SHA1 implementation.
> * Re-arranged the algorithm inclusion in hash.h to correspond to
>   NO_DC_SHA1, and "#error" if we have no defined SHA_*, rather than
>   silently picking block-sha1/sha1.h as a fallback.
> * Have an INSTALL that reflects reality. We were still claiming to use
>   OpenSSL's SHA-1 hashing by default.
> * Have Darwin and Mac OS X use sha1collisiondetection, like everywhere
>   else. There is still a NO_APPLE_COMMON_CRYPTO knob, but it's used for
>   things unrelated to SHA-1 (see [6]).
> * Have a rewritten discussion of SHA-1 and SHA-256 in the Makefile
>   which covers all of this.
>
> Let's also change the CI for "osx-clang" to test with the new
> APPLE_SHA1 knob ("osx-gcc" uses the new sha1collisiondetection
> default).
>
> In practice this will spot issues like the one noted in [7], 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.
>
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
> diff --git a/Makefile b/Makefile
> +ifdef DC_SHA1
> +$(error the DC_SHA1 flag is no longer used, and has become the default. Adjust your build scripts accordingly)
> +endif