hinoka@ says this particular build does not have a swarming task id, which could be from the recent support of cancelling swarming builds. 

This build does have a swarming task ID, but the bug is that it did not bubble up to buildbucket.

This build is a buildbot build
https://apis-explorer.appspot.com/apis-explorer/?base=https://cr-buildbucket.appspot.com/_ah/api#p/buildbucket/v1/buildbucket.get?id=8961010014211749920&_h=1&


It cannot have a swarming task.

I don't think BuildInfo supports non-LUCI buildbucket build ids?

You're right... I missed it.

This is the Build:
https://uberchromegw.corp.google.com/i/chromiumos.tryserver/builders/factory/builds/20
Tue Dec 5 14:12:04 2017

It's not super recent.  Namely, it's before when when Milo started indexing the buildbucket build IDs of builds, so it's not accessable by buildinfo when querying by buildbucket ID.

A more recent build works:
https://luci-milo.appspot.com/rpcexplorer/services/milo.BuildInfo/Get?request={%20%20%20%20%22buildbucket%22:%20{%20%20%20%20%20%20%20%20%22id%22:%20%228950343528115513952%22%20%20%20%20}}