Everything under /go/sumdb/ was passed on to the module upstream. But
proxy.golang.org doesn't serve the checksum database, so those requests
always came back 404 and every client had to talk to sum.golang.org itself.
The Go handler now serves /sumdb/{name}/ for the databases in
upstream.go_sumdbs, which defaults to sum.golang.org. Lookups, latest and
tiles come from the database through the metadata cache. Complete tiles
never change, so once they're cached we don't ask again. Unknown names and
paths that aren't part of the protocol get a 404 without going upstream.
If the database answers 404 we pass on up to 4 KiB of its text, because the
go command prints it and it tells you why a version is missing. For that the
not-found error from the metadata fetch now carries the start of the body.
Other handlers still just see ErrUpstreamNotFound.
