Sounds great, in my uninformed opinion :-)

I'll take care of this.

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/build/+/59ab71bdb798d670e318052c3dee0cbbfb7ef789

commit 59ab71bdb798d670e318052c3dee0cbbfb7ef789
Author: sebmarchand <sebmarchand@google.com>
Date: Wed Aug 10 17:56:11 2016

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7ed420a42d6ea759e585510605888570caeb0677

commit 7ed420a42d6ea759e585510605888570caeb0677
Author: sebmarchand <sebmarchand@chromium.org>
Date: Wed Aug 10 20:16:23 2016

Add the MB config of the official win Clang bots.

These bots don't yet exist, I'm creating them in a separate CL.

BUG= 620849 

Review-Url: https://codereview.chromium.org/2222203003
Cr-Commit-Position: refs/heads/master@{#411130}

[modify] https://crrev.com/7ed420a42d6ea759e585510605888570caeb0677/tools/mb/mb_config.pyl

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/build/+/1fda2ee60becd1d572acef76edf34958765347e8

commit 1fda2ee60becd1d572acef76edf34958765347e8
Author: sebmarchand <sebmarchand@google.com>
Date: Thu Aug 11 15:43:03 2016

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/buildspec/+/cc7b1eee82fa0c70d52f11184211feacdbfe8d40

commit cc7b1eee82fa0c70d52f11184211feacdbfe8d40
Author: Sebastien Marchand <sebmarchand@chromium.org>
Date: Thu Aug 11 19:43:20 2016

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/release/+/80a02c0c0cb302d3214f7b664c0149cb4c1d3523

commit 80a02c0c0cb302d3214f7b664c0149cb4c1d3523
Author: Sebastien Marchand <sebmarchand@chromium.org>
Date: Thu Aug 11 19:46:47 2016

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/release/+/80a02c0c0cb302d3214f7b664c0149cb4c1d3523

commit 80a02c0c0cb302d3214f7b664c0149cb4c1d3523
Author: Sebastien Marchand <sebmarchand@chromium.org>
Date: Thu Aug 11 19:46:47 2016

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/build/+/c3437a6dc7813ab2557b6c8b4ab1e7c689393280

commit c3437a6dc7813ab2557b6c8b4ab1e7c689393280
Author: sebmarchand <sebmarchand@google.com>
Date: Thu Aug 11 20:06:31 2016

Looks like a lot of stuff happened here -- is this done?

We've got bots on the waterfall: https://uberchromegw.corp.google.com/i/official.desktop/waterfall

But they still need more configuring.

Yes sorry, it's 'almost' done but there's still some things to fix in the official build scripts. I'm OOO until Thursday, but I'll take care of this asap after that.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build.git/+/c2262bf0ef1655096485abfe32b850b1522c80c8

commit c2262bf0ef1655096485abfe32b850b1522c80c8
Author: Sebastien Marchand <sebmarchand@google.com>
Date: Thu Aug 25 15:58:33 2016

Make the IsClangWinBuild function GN friendly

R=thakis@chromium.org
BUG= 620849 

Review URL: https://codereview.chromium.org/2276363002 .

[modify] https://crrev.com/c2262bf0ef1655096485abfe32b850b1522c80c8/scripts/common/chromium_utils.py

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/build/+/ec998e9f7f3bf5fff0741450b971acc662032ff6

commit ec998e9f7f3bf5fff0741450b971acc662032ff6
Author: sebmarchand <sebmarchand@google.com>
Date: Thu Aug 25 18:15:58 2016

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/tools/build/+/ec998e9f7f3bf5fff0741450b971acc662032ff6

commit ec998e9f7f3bf5fff0741450b971acc662032ff6
Author: sebmarchand <sebmarchand@google.com>
Date: Thu Aug 25 18:15:58 2016

The official builder should be ready now, CC'ing waffles@ for the setup of the new Canary cohort. The tl;dr of this crbug is that we want to setup a new* Canary cohort (like the Win-Asan) with no users on it by default (rate of 0%) but with a way to opt-in (via a registry key). Could you help with that ? 

* Well... 2 new cohorts, win-clang and win64-clang

I can help.

How do we identify clang builds? Will they be in win-clang and win64-clang directories in the signed build outputs?

Yep, we should also make sure that we don't try to diff-update the non-clang users to a Clang build (they should always get a full installer, like for the PGO builds)

What about the problem of updating a Clang user with a non-Clang diff?

The way this used to work for PGO, before it became the default build, was that we just didn't create diffs of PGO builds. After it became the default build, we gated the diffs on the milestone (e.g. M53 or lower gets non-PGO diffs, M54 or higher get PGO diffs). I don't think there was any other way for omaha to know which diff to serve to which users, unless that has changed.

Updating a Clang user with a non-Clang diff takes forever, see  crbug.com/547142  .

So maybe we shouldn't produce the diffs for the Clang builds for now ?

Update: 54.0.2839.5 is the first Official Signed build, we should get a new official Canary every day from now on (most of the time it'll be a .2 build).

So we just need to setup the cohort now, we're almost done !

Done! You should now be able to run Clang canary on Windows by opting-in via registry key:

HKEY_CURRENT_USER\Software\Google\Update\ClientState\{4EA16AC7-FD5A-47C3-875B-DBF4A2008C20}\cohort → set "hint" (type: REG_SZ) to value "clang-optin"

Your channel preference will take effect on your next update (probably tomorrow morning).

Super awesome, thanks!

brucedawson@ says elsewhere that `reg add HKCU\Software\Google\Update\ClientState\{4EA16AC7-FD5A-47C3-875B-DBF4A2008C20}\cohort /v hint /t REG_SZ /d clang-optin /f` should do the needed registry modification.