I think this is an instance of an out-of-date builder going into
service:  The last successful build used builder build304-m2.
The first failure used builder build110-m2.

I think this is the relevant portion of the failure logs:

info: A new version of repo is available


object 69034721e607ecbe8dad736c8ce07d91efdc8353
type commit
tag v1.12.37-cr1
tagger Mike Frysinger <vapier@chromium.org> 1476811349 -0400

Chromium-specific release

gpg: Signature made Tue 18 Oct 2016 10:22:29 AM PDT
gpg:                using RSA key DA03FD3916B500A8
gpg: Can't check signature: public key not found


warning: Skipped upgrade to unverified version

chromite in R56 depends on a new version of repo, but `repo selfupdate` is failing.

This was recently fixed on master by nxia@. I've created cherry-picks to R56.
https://chromium-review.googlesource.com/#/c/432916/
https://chromium-review.googlesource.com/#/c/432936/

@nxia: I think these two are the only CLs needed. Can you confirm?
Secondly, I think we should CP these to all active release branches. R57, R55?

This bug requires manual review: We are only 5 days from stable.
Please contact the milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), gkihumba@(cros), bustamante@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Assigning to nxia.
She is taking a look at the bug, and can land the CLs I CP'ed for the purpose.

https://chromium-review.googlesource.com/#/c/432916/ should solve the repo version issue here. 
https://chromium-review.googlesource.com/#/c/432936/ should solve repo init issues we saw in other builds. 
It doesn't hurt to merge both CLs.

I manually removed .repo/repo dir in /b/cbuild/shared_internal, the build should be fine now.

Please force samus-pre-flight-branch release-R56-9000.B if you want.

Re #8: I'm holding off of merging those CLs at this point. If the builder is fixed, I'd rather not merge anything on the day we're trying to branch stable.

https://uberchromegw.corp.google.com/i/chromeos_release/builders/samus-pre-flight-branch%20release-R56-9000.B/builds/197 clobbered the builder and seems to be doing OK.

Re#11 I didn't clobber the builder actually. I only deleted the .repo/repo dir, where stored the checked out repo tool.

That build has gotten past the sync stage. I'm going to call this a victory.

This issue has been approved for a merge. Please merge the fix to any appropriate branches as soon as possible!

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I'll skip the merge because clobbering the builder is an effective workaround, I don't expect to see this issue often, and I'd rather not merge into stable.

Removing all merge labels so that sheriffbot doesn't keep track of this bug as a merge candidate.