Re: [RFC \ WISH] Add -o option to git-rev-list

From: Marco Costalba <mcostalba@gmail.com>
Date: Dec 10, 2006, 19:51 UTC
Message-ID: <e5bfff550612101151r5e77ddc0w8f61f36fc1f2920e@mail.gmail.com>
In-Reply-To: <Pine.LNX.4.64.0612101009040.12500@woody.osdl.org>

On 12/10/06, Linus Torvalds <torvalds@osdl.org> wrote:

Show 15 quoted lines

>
> Why don't you use the pipe and standard read()?
>
> Even if you use "popen()" and get a "FILE *" back, you can still do
>
>         int fd = fileno(file);
>
> and use the raw IO capabilities.
>
> The thing is, temporary files can actually be faster under Linux just
> because the Linux page-cache simply kicks ass. But it's not going to be
> _that_ big of a difference, and you need all that crazy "wait for rev-list
> to finish" and the "clean up temp-file on errors" etc crap, so there's no
> way it's a better solution.
>

Two things.

- memory use: the next natural step with files is, instead of loading
the file content in memory and *keep it there*, we could load one
chunk at a time, index the chunk and discard. At the end we keep in
memory only indexing info to quickly get to the data when needed, but
the big part of data stay on the file.

- This is probably my ignorance, but experimenting with popen() I
found I could not know *when* git-rev-list ends because both feof()
and ferror() give 0 after a fread() with git-rev-list already defunct.
Not having a reference to the process (it is hidden behind popen() ),
I had to check for 0 bytes read after a successful read (to avoid
racing in case I ask the pipe before the first data it's ready) to
know that job is finished and call pclose().