Junio C Hamano <gitster@xxxxxxxxx> writes:
> ...
> The use of union is a good ingredient for a solution. I would have
> chosen to do this slightly differently if I were doing it.
>
> typedef struct {
> int safe;
> union {
> SHA1_CTX_SAFE safe;
> SHA1_CTX_FAST fast;
> } u;
> } git_SHA_CTX;
>
> void git_SHA1_Init(git_SHA_CTX *ctx, int safe);
> void git_SHA1_Update(git_SHA_CTX *ctx, const void *, unsigned long);
> git_SHA1_Final(uchar [20], git_SHA_CTX *ctx);
>
> where SHA1_CTX_FAST may be chosen from the Makefile just like we
> currently choose platform_SHA_CTX. SHA1_CTX_SAFE could also be made
> configurable but it may be OK to hardcode it to refer to SHA1_CTX of
> DC's.
>
> As you already know, I am assuming that each codepath pretty much
> knows if it needs safe or fast one (e.g. the one used in csum-file.c
> knows it does not have to), so each git_SHA_CTX is told which one to
> use when it gets initialized.
And if we wanted to declare "git add" is always safe, we could still
do
int sha1_safety_global_override = -1; /* unspecified */
void git_SHA1_Init(git_SHA_CTX *ctx, int safe)
{
if (sha1_safety_global_override >= 0)
ctx->safe = sha1_safety_global_override;
else
ctx->safe = safe;
if (ctx->safe)
SHA1DCInit(&(ctx->u.safe));
else
platform_SHA1_Init(&(ctx->u.fast));
}
and then have cmd_add() in builtin/add.c to flip that global
override bit to say "this does not have to be safe". I personally
do not think it is a good idea, but I am showing that it is still
doable.
And as long as assignment to sha1_safety_global_override is done in
a thread-friendly way, such a scheme would be more thread-friendly
as a whole compared to the "toggle_sha1dc()" approach where each CTX
instance does not know which side of the union it is being used us
(which, if mixed-up, of course would lead to a funny behaviour).