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

On Saturday 28 July 2012 02:00:31 Jonathan Nieder wrote:

Thanks for explaining.  Now we've discussed a few different approproaches,
none of which is perfect.

a. use --cat-blob-fd, no FIFO

   Doing this unconditionally would break platforms that don't support
   --cat-blob-fd=(descriptor >2), like Windows, so we'd have to:

   * Make it conditional --- only do it (1) we are not on Windows and
     (2) the remote helper requests backflow by advertising the
     import-bidi capability.

   * Let the remote helper know what's going on by using
     "import-bidi" instead of "import" in the command stream to
     initiate the import.

Generally I like your prefered solution.
I think there's one problem:
The pipe needs to be created before the fork, so that the fd can be inherited. 
There is no way of creating it if the remote-helper advertises a capability, 
because it is already forked then. This would work with fifos, though.

We could:
- add a capability: bidi-import. 
- make transport-helper create a fifo if the helper advertises it.
- add a command for remote-helpers, like 'bidi-import <pipename>' that makes 
the remote helper open the fifo at <pipename> and use it.
- fast-import is forked after the helper, so we do already know if there will 
be a back-pipe. If yes, open it in transport-helper and pass the fd as command 
line argument cat-blob-fd. 

--> fast-import wouldn't need to be changed, but we'd use a fifo, and we get 
rid of the env-vars.
(I guess it could work on windows too).

What do you think?

b. use envvars to pass around FIFO path

   This complicates the fast-import interface and makes debugging hard.
   It would be nice to avoid this if we can, but in case we can't, it's
   nice to have the option available.

c. transport-helper.c uses FIFO behind the scenes.

   Like (a), except it would require a fast-import tweak (boo) and
   would work on Windows (yea)

d. use --cat-blob-fd with FIFO

   Early scripted remote-svn prototypes did this to fulfill "fetch"
   requests.

   It has no advantage over "use --cat-blob-fd, no FIFO" except being
   easier to implement as a shell script.  I'm listing this just for
   comparison; since (a) looks better in every way, I don't see any
   reason to pursue this one.

Since avoiding deadlocks with bidirectional communication is always a
little subtle, it would be nice for this to be implemented once in
transport-helper.c rather than each remote helper author having to
reimplement it again.  As a result, my knee-jerk ranking is a > c >
b > d.

Sane?
Jonathan