Inside the GerritMeta.LabelVotes method, there's code like:

oldCommit := m.CL.Project.GitCommit(lastCommit)
newCommit := m.CL.Project.GitCommit(commit)
if !oldCommit.SameDiffStat(newCommit) {

Since it iterates over known patch sets of a given Gerrit CL, it's expected both oldCommit and newCommit will be non-nil, otherwise it'd panic. The current version of GerritMeta.LabelVotes is only used on the https://dev.golang.org/reviews page, and it's expected to always work, so it doesn't currently return an error.

Issue #47695 showed, at least one time, an instance where that wasn't the case. It hasn't happened again. My current best understanding is that it may have been some race, possibly in Gerrit itself, but it's not clear if it was a one-off problem that was resolved, or may continue to happen again.

This is a tracking issue to decide how to best handle this, especially if it becomes a problem again.

My current preference is to deprecate/remove GerritMeta.LabelVotes from x/build/maintner and move its implementation to x/build/devapp, the only place where it's used. The method seems to rely on too many heuristics and doesn't fit with the rest of maintner API; if we can't make it work well in the general case, we shouldn't try to offer it.