Re: [PATCH 2/7] Makefile: optionally compile with both SHA1DC and SHA1_OPENSSL

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Junio,

On Thu, 30 Mar 2017, Junio C Hamano wrote:

> Johannes Schindelin <johannes.schindelin@gmx.de> writes:
> 
> > +#ifdef SHA1_DC_AND_OPENSSL
> > +void (*SHA1_Init_func)(SHA_CTX_union *ctx) = (void *)SHA1DCInit;
> > +void (*SHA1_Update_func)(SHA_CTX_union *ctx, const void *pointer, size_t size) =
> > +	(void *)git_SHA1DCUpdate;
> > +int (*SHA1_Final_func)(unsigned char sha1[20], SHA_CTX_union *ctx) =
> > +	(void *)git_SHA1DCFinal;
> > +
> > +void toggle_sha1dc(int enable)
> > +{
> > +	if (enable) {
> > +		SHA1_Init_func = (void *)SHA1DCInit;
> > +		SHA1_Update_func = (void *)git_SHA1DCUpdate;
> > +		SHA1_Final_func = (void *)git_SHA1DCFinal;
> > +	} else {
> > +		SHA1_Init_func = (void *)SHA1_Init;
> > +		SHA1_Update_func = (void *)SHA1_Update;
> > +		SHA1_Final_func = (void *)SHA1_Final;
> > +	}
> > +}
> > +#endif
> 
> As I understand that this is a demonstration series, the approach
> above is OK as an expedite way to illustrate one way how run-time
> switching could be done.  The approach however is not very thread
> friendly, though.

Indeed. However, the toggle is meant to be coarse, heavy-handed. I have to
protect my time as Git for Windows maintainer, so I have to keep this as
simple to maintain as possible while also benefitting my users. This
toggle is intended to be run very, very early in the cmd_main() functions.
As in: right when the config is read.

> > diff --git a/sha1dc/sha1.h b/sha1dc/sha1.h
> > index bd8bd928fb3..243c2fe0b6b 100644
> > --- a/sha1dc/sha1.h
> > +++ b/sha1dc/sha1.h
> > @@ -110,10 +110,26 @@ void git_SHA1DCFinal(unsigned char [20], SHA1_CTX *);
> >   */
> >  void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *data, unsigned long len);
> >  
> > +#ifdef SHA1_DC_AND_OPENSSL
> > +extern void toggle_sha1dc(int enable);
> > +
> > +typedef union {
> > +	SHA1_CTX dc;
> > +	SHA_CTX openssl;
> > +} SHA_CTX_union;
> 
> 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.

Thanks, at this stage it is pretty clear, though, that I will have to
maintain this. I am not willing to make this as configurable as you
suggested, as these patches will have to stay in git-for-windows/git.

Ciao,
Dscho