[Bug debuginfod/29474] Server returns 404 for concurrent requests when leading to a same .rpm

["mliska at suse dot cz" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29474

--- Comment #2 from Martin Liska <mliska at suse dot cz> ---
So it's the prefetching feature as documented here:

  // 3) extract some number of prefetched entries (just into fdcache)
...
      if (r != 0) // stage 3
        {
          // NB: now we know we have a complete reusable file; make fdcache
          // responsible for unlinking it later.
          obatched(clog) << "INTERN " << b_source1 << " filename=" << fn <<
endl;
          fdcache.intern(b_source0, fn,
                         tmppath, archive_entry_size(e),
                         false); // prefetched ones go to the prefetch cache
          prefetch_count --;
          close (fd); // we're not saving this fd to make a mhd-response from!
          continue;
        }

So what's bad is that
handle_buildid_r_match begins with:

  // check for a match in the fdcache first
  int fd = fdcache.lookup(b_source0, b_source1);
  while (fd >= 0) // got one!; NB: this is really an if() with a possible
branch out to the end
    {
...
    }

which was false, but happens to be true once another thread inters the file :/

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