Re: [RFC 1/4 v2] Implement a basic remote helper for svn in C.

[Florian Achleitner <florian.achleitner.2.6.31@gmail.com>]

On Tuesday 31 July 2012 15:43:57 Jonathan Nieder wrote:
> Florian Achleitner wrote:
> > I haven't tried that yet, nor do I remember anything where I've already
> > seen two processes writing to the same pipe.
> 
> It's a perfectly normal and well supported thing to do.

I played around with a little testprogram. It generally works.
I'm still not convinced that this doesn't cause more problems than it can 
solve.
The standard defines that write calls to pipe fds are atomic, i.e. data is not 
interleaved with data from other processes, if the data is less than PIPE_BUF 
[1].
We would need some kind of locking/synchronization to make it work for sure, 
while I believe it will work most of the time.

Currently it runs  like this:
transport-helper.c writes one or more 'import <ref>' lines, we don't know in 
advance how many and how long they are. Then it waits for fast-import to 
finish.

When the first line arrives at the remote-helper, it starts importing one line 
at a time, leaving the remaining lines in the pipe.
For importing it requires the data from fast-import, which would be mixed with 
import lines or queued at the end of them.

[1] 
http://pubs.opengroup.org/onlinepubs/009695399/functions/write.html