Re: [PATCH v3 1/2] add strbuf_set operations

[Jeremiah Mahler <jmmahler@gmail.com>]

Peff,

On Fri, Jun 13, 2014 at 03:15:50AM -0400, Jeff King wrote:
> On Thu, Jun 12, 2014 at 04:46:37PM -0700, Jeremiah Mahler wrote:
> 
> > >     Although strbuf_set() does make the code a bit easier to read
> > >     when strbufs are repeatedly re-used, re-using a variable for
> > >     different purposes is generally considered poor programming
> > >     practice. It's likely that heavy re-use of strbufs has been
> > >     tolerated to avoid multiple heap allocations, but that may be a
> > >     case of premature (allocation) optimization, rather than good
> > >     programming. A different ("better") way to make the code more
> > >     readable and maintainable may be to ban re-use of strbufs for
> > >     different purposes.
> > > 
> > > But I deleted it before sending because it's a somewhat tangential
> > > issue not introduced by your changes. However, I do see strbuf_set()
> > > as a Band-Aid for the problem described above, rather than as a useful
> > > feature on its own. If the practice of re-using strbufs (as a
> > > premature optimization) ever becomes taboo, then strbuf_set() loses
> > > its value.
> > > 
> > 
> > I am getting the feeling that I have mis-understood the purpose of
> > strbufs.  It is not just a library to use in place of char*.
> > 
> > If strbufs should only be added to and never reset a good test would be
> > to re-write builtin/remote.c without the use of strbuf_reset.
> > 
> > builtin/remote.c does re-use the buffers.  But it seems if a buffer is
> > used N times then to avoid a reset you would need N buffers.
> > 
> > But on the other hand I agree with your comment that re-using a variable
> > for different purposes is poor practice.
> > 
> > Now I am not even sure if I want my own patch :-)
> 
> I think reusing buffers like remote.c does makes things uglier and more
> confusing than necessary, and probably doesn't have any appreciable
> performance gain. We are saving a handful of allocations, and have such
> wonderful variable names as "buf2" (when is it OK to reuse that one,
> versus regular "buf"?).
> 
> A better reason I think is to reuse allocations across a loop, like:
> 
>   struct strbuf buf = STRBUF_INIT;
> 
>   for (i = 0; i < nr; i++) {
> 	strbuf_reset(&buf);
> 	strbuf_add(&buf, foo[i]);
> 	... do something with buf ...
>   }
>   strbuf_release(&buf);
> 
> You can write that as:
> 
>   for (i = 0; i < nr; i++) {
> 	struct strbuf buf = STRBUF_INIT;
> 	strbuf_add(&buf, foo[i]);
> 	... do something ...
> 	strbuf_release(&buf);
>   }
> 
> and it is definitely still a case of premature optimization. But:
> 
>   1. "nr" here may be very large, so the amortized benefits are greater
> 
>   2. It's only one call to strbuf_reset to cover many items. Not one
>      sprinkled every few lines.
> 
> You'll note that strbuf_getline uses a "set" convention (making it
> different from the rest of strbuf) to handle this looping case.
> 
> I don't have a problem with strbuf_set, but just peeking at remote.c, I
> think I'd rather see it cleaned up. It looks like one of the major uses
> is setting config variables. I wonder how hard it would be to make a
> git_config_set variant that took printf-style formats, and handled the
> strbuf itself.
> 
> -Peff

Improving remote.c sounds like a better direction than adding set
operations.  I will start looking in to it.

-- 
Jeremiah Mahler
jmmahler@gmail.com
http://github.com/jmahler