[Bug debuginfod/25607] New: debuginfod-client: paranoid federation mode

["fche at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25607

            Bug ID: 25607
           Summary: debuginfod-client: paranoid federation mode
           Product: elfutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: debuginfod
          Assignee: unassigned at sourceware dot org
          Reporter: fche at redhat dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

It may be helpful to have a debuginfod_client option (env var? api?) that adds
a degree of paranoia to frontend debuginfod servers that simply federate out to
multiple mostly-trustworthy upstreams.  The present code simply launches the
outgoing curl requests to -all- upstream servers in $DEBUGINFOD_URLS, and the
first one that starts sending data back is selected as the winner.  The data
from the others is thrown away.

This is fine if all the servers are trustworthy, correct, etc. etc.

With a bit of extra code, we can detect some kinds of shenanigans or errors. 
One possibility is teaching the -client code to -not- choose the first upstream
server as the winner, but to wait for them all to send back a definite response
(subject to a timeout).  Then it can compare the responses from those servers
that claimed to know the buildid/$payload.  If there is a mismatch, signal a
loud error, as though there were a RAID-1 disagreement between mirrored disks.

With such checking logic in place, it is possible to proactively scan the
federated server tree by mechanically looking up all "interesting" buildids. 
It could permit the assembly of a semi-trusted tree of servers that still limit
client exposure to wrong data.

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