[Bug debuginfod/27673] [debuginfod] Handle source requests for same buildid more efficiently

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27673

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |UNCONFIRMED
     Ever confirmed|1                           |0

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Frank Ch. Eigler from comment #2)
> The subject line "handle sources ... from same buildid ... more efficiently"
> suggests a caching opportunity.  The debuginfod --fdcache-prefetch parameter
> is probably the gadget of interest here.  The opensuse operators could
> experiment with a much larger than default figure.  (You can test it locally
> by running debuinfod for yourself, against a local set of relevant -debug*
> RPMs.)

The time it takes for the client to see the response of the server to the
request consist of:
- time for request to travel to server (latency)
- time for server to react to request
- time for answer to travel back to client (latency again)

I've looked at the documentation of the option fdcache-prefetch, and AFAIU this
improves "time for server to react to request".

Instead, the focus of this PR is to hide latency (since that's where the
majority of the time is spent) by using more bandwidth.

Proposal a:

debuginfod:
- when receiving a source request and ENOENT, send as reply the list of
  available files for the buildid

client:
- when receiving a list of available files for a buildid, store it and
  use it to reply to source requests related to the buildid. That is,
  if the file is not in the list, reply with -2.  Otherwise, send a
  request to debuginfod, and expect it to succeed.

Proposal b:

debuginfod:
- when receiving a source request, send a package with the sources
  for that buildid to the client.

client:
- when receiving a package with the sources for a buildid, store them
  and use them to reply to source requests related to the buildid.

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