Subject: Re: [PATCH v2] fetch-pack: optionally save packs to disk

From: Jeff King <[email-masked]>
Date: Fri, 12 Jun 2015 17:25:26 -0400

On Fri, Jun 12, 2015 at 02:00:05PM -0400, Jeff King wrote:

> When I added GIT_TRACE_PACKET long ago, I had always intended to
> follow-up with a GIT_TRACE_PACKFILE. The former stops tracing when we
> get to the binary data, but I had intended the latter to store the pure
> on-the-wire packfile transmission for later debugging to. I never got
> around to it, but I think it is something like the patch below.

Here it is, a bit more cleaned up. I think this is a nice improvement,
and it does fix some minor issues in the existing GIT_TRACE_PACKET code.
But I don't think it will ever work for receive-pack, because we hand
off the socket directly to index-pack.

I can live with that. But another approach would be to make it easier to
capture the stdin of programs with GIT_TRACE, rather than just their
arguments. That makes it more generically usable (e.g., I have hacky
patches on our servers for capturing "pack-objects" input so I can
replay expensive fetch requests). As Augie noted, that's not a full
pack-file due to the --pack-header stuff. But given a full trace (which
has both the arguments and stdin), you could reconstruct the packfile,
which we could do as a post-processing step.

I dunno. I prefer introducing a nicely generic mechanism, but it would
probably be a little more irritating to use in practice.

I guess yet another way to do it would be to put the GIT_TRACE_PACK
intelligence into index-pack and unpack-objects (rather than
fetch-pack). Then it at least applies to both the pushing and fetching
sides.

Anyway, here are the patches I came up with:

  [1/3]: pkt-line: simplify starts_with checks in packet tracing
  [2/3]: pkt-line: tighten sideband PACK check when tracing
  [3/3]: pkt-line: support tracing verbatim pack contents

-Peff