to clarify, we already have this mirror:
  https://chromium.googlesource.com/aosp/platform/system/core/

we want to have the libsync/ subdir in the core/ git repo pulled out into its own git repo.  like we do today with:
  breakpad/breakpad/ -> breakpad/breakpad/src/
  chromium/src/ -> chromium/src/base/
  ...many others...

This is a lot of work :( We don't have that many instances of this, though: https://uberchromegw.corp.google.com/i/chromium.infra.cron/builders/ lists only 6 "gsubtreed-*" builders.
 
This involves:
1. Set up a new builder for gsubtreed
2. Initialize it on it aosp repo, which involves granting extra rights on it.
  Extra difficulty: it is a mirror, and hence permissions will be harder to set up :( 
3. Initialize it on aosp/platform/system/core/libsync repo.


So, are you sure this is a necessary thing to do?

the annoying thing for us is that the whole system/core/ repo is ~30k commits at 60+ MB, and libsync is <100KB (with <30 commits in 5+ years).  so adding that whole thing for such little return makes us sad, and we wouldn't want to track the full/active history.

if gsubtreed is a pita for you guys (i was hoping it was a bit easier), we could consider just rolling tarballs by hand (and adding a script to help automate it).  i think we'd be doing manual rolls anyways as we wouldn't want libsync changes to automatically show up on CrOS side w/out testing.

60+MB - I forgot how to count that low :)
 
gsubtreed is easy for not mirrors (though still some work, but 1 time only, so better than tar rolling, IMHO). But it's more PITA in your case: i have no idea yet how to make it store git notes in a GoB mirror.

I'll poke at this on Thursday. Meanwhile, 

+iannucci@ do you have thoughts about this?

I was hallucinating above. Gsubtreed won't be a PITA to set up, because it doesn't actually store anything in the main repo, and instead relies on footers of child repos only. So, all I need is a new builder. I can share the slave with some existing builder like say crashpad, given that your frequency of commits is low.

libsync repo created. Gsubtreed builder creation: https://chromium-review.googlesource.com/507128 

cleanup of gsubtreed recipe https://chrome-internal-review.googlesource.com/377349 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/bc4aedb9281865224a85b08442cdbee891ce2c07

commit bc4aedb9281865224a85b08442cdbee891ce2c07
Author: Andrii Shyshkalov <tandrii@chromium.org>
Date: Wed May 17 11:13:38 2017

Add gsubtreed builder for aosp/platform/system/core.

This will share the slave with infra because load is very small.

R=machenbach@chromium.org
Bug:  721838 
Change-Id: I3a332962b27562e37cfd37830829c96de243bf89
Reviewed-on: https://chromium-review.googlesource.com/507128
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

[modify] https://crrev.com/bc4aedb9281865224a85b08442cdbee891ce2c07/masters/master.chromium.infra.cron/slaves.cfg
[modify] https://crrev.com/bc4aedb9281865224a85b08442cdbee891ce2c07/masters/master.chromium.infra.cron/master.cfg

master restart https://chrome-internal-review.googlesource.com/377249 

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/infradata/master-manager/+/2ba97de7f48113931b2dcb2c02c1b32c0f5b2864

commit 2ba97de7f48113931b2dcb2c02c1b32c0f5b2864
Author: Andrii Shyshkalov <tandrii@google.com>
Date: Wed May 17 11:39:28 2017

All done. Enjoy: https://chromium.googlesource.com/aosp/platform/system/core/libsync

For posterity, new builder is here https://uberchromegw.corp.google.com/i/chromium.infra.cron/builders/gsubtreed-aosp-platform-system-core

it shares slave with infra gsubtreed, both of which have rare commits. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/1929522b5629bc18495781feaf794bef80db81f4

commit 1929522b5629bc18495781feaf794bef80db81f4
Author: Andrii Shyshkalov <tandrii@chromium.org>
Date: Thu May 18 13:06:53 2017

Nitfix follow up for https://chromium-review.googlesource.com/c/507128

TBR=machenbach@chromium.org
Bug:  721838 
Change-Id: Iafa5176d78d1bb4b6499fd42ac9bc081f839dd37
Reviewed-on: https://chromium-review.googlesource.com/508768
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

[modify] https://crrev.com/1929522b5629bc18495781feaf794bef80db81f4/masters/master.chromium.infra.cron/master.cfg

Thank you very much!

Hmm, how do we update our mirror so it would include latest from https://android.googlesource.com/platform/system/core/+/master/libsync/ ?

we need to push updates to https://chromium.googlesource.com/aosp/platform/system/core/.  just sync upstream to master.  which i've done now :).