This is causing the buildbot pubsub to fall behind
https://screenshot.googleplex.com/Sokyho9GFKx

Conjecture - This used to be caught by the case because saving a build returns ErrTooBig https://cs.chromium.org/chromium/infra/go/src/go.chromium.org/luci/milo/buildsource/buildbot/buildstore/build.go?q=%22entity+was+not+saved+because+it+was+too+large+to+store%22&sq=package:chromium&l=39
Which is caught here:
https://cs.chromium.org/chromium/infra/go/src/go.chromium.org/luci/milo/buildsource/buildbot/pubsub.go?type=cs&q=ErrTooBig&sq=package:chromium&l=274

I suspect since the buildentity save was wrapped in a transaction, it testing for err == ErrTooBig no longer works because the error was wrapped somehow.

https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/733723

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/b8969b77b3aa9bf08843cf4f2579444e6510c73f

commit b8969b77b3aa9bf08843cf4f2579444e6510c73f
Author: Nodir Turakulov <nodir@google.com>
Date: Mon Oct 23 21:48:46 2017

[milo] make buildstore.ErrTooBig an error tag

Datastore wraps errors returned by PropertyLoadSaver.Save, so ErrTooBig cannot
be used with direct comparison. Use an error tag instead.

Bug:  777475 
Change-Id: Id8a543065203226c48f01bbb939a1986b75e682a
Reviewed-on: https://chromium-review.googlesource.com/733723
Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>

[modify] https://crrev.com/b8969b77b3aa9bf08843cf4f2579444e6510c73f/milo/buildsource/buildbot/buildstore/build.go
[modify] https://crrev.com/b8969b77b3aa9bf08843cf4f2579444e6510c73f/milo/buildsource/buildbot/buildstore/master.go
[modify] https://crrev.com/b8969b77b3aa9bf08843cf4f2579444e6510c73f/milo/buildsource/buildbot/pubsub.go

graph is going down https://screenshot.googleplex.com/bDw7sbQsWY6