build-ids, .debug_sup and other IDs (Was: [PATCH] Handle DWARF 5 separate debug sections)

[Matt Schulte]

Hi Mark,

My apologies for bringing this up so late. I was just re-reading this thread
while looking at how to find a .dwp for a given binary.

> But I was personally assuming we would extend it to also to other things like
> dwo IDs (which are again almost identical globally unique identifiers for
> files)

I'm concerned about using dwo IDs to index debuginfod. They are only 64-bits and
there will be many more dwo IDs than build ids or supplemental file ids since
there is 1 per compile unit.  Assuming dwo IDs are randomly distributed, once we
have ~600,000,000 dwo IDs we have a 1% chance of a collision. ~600,000,000 =
sqrt(2 * 2^64 * 0.01) (I think I did that math right but forgive me if not).

Maybe that's an ok number? (I tried to estimate the number of compile units in
one distro's release, but do not have a good way of doing that quickly)

What about using `/buildid/BUILDID/dwp` instead? This is not a perfect solution,
since (currently) no one puts the build-id into the *.dwp file, but it does get
around this collision problem.

-Matt