--> maruel@ to either advise on why this dimension key is not allowed, or to update swarming key regex to allow the usual language identifiers regex: r'^[a-zA-Z0-9\-\_\.][a-zA-Z\-\_\.]*$'

It's https://cs.chromium.org/chromium/infra/luci/appengine/swarming/server/config.py?l=35
r'^[a-zA-Z\-\_\.]+$'

i.e. we never supported numbers in the key. I guess we never thought of that.

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/infra_internal/skylab_inventory/+/f509659fbfd8b8a38385af9f7f32cfdd2bfba4c1

commit f509659fbfd8b8a38385af9f7f32cfdd2bfba4c1
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Tue Jan 22 18:47:26 2019

As noted in IM, Swarming regexp is easy to update, but there's copies in clients and buildbucket and starlark that would have to be updated, which would require the approval of Nodir and Vadim. I personally don't mind much something like r'^[a-zA-Z\-\_\.][0-9a-zA-Z\-\_\.]*$'.

I'm willing to make the software changes if peeps here are willing to review.

The real question is: Can we generalize the allowed dimensions gradually without breaking anything. i.e., I independently generalize the various places where this is checked but we don't actually push client binaries / service binaries for this bug (except swarming, which I need updated).

As and when these services / tools get pushed, users will be able to use the least restrictive definition of the dimension key on their path

e.g.: if you don't use buildbucket, you'll be able to use numbers once swarming is pushed. If you do use buildbucket, you'll have to wait until the next time buildbucket gets pushed. If you use a client tool which has the same check, you'll have to wait till that client binary is updated / update it yourself. And so forth.

Or is there a backwards dependency that may break buildbucket in a bad way (read: break more than the bots that advertise the more general dimensions) if swarming starts accepting numbers in dimensions and some bots start advertising such dimensions.

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/1d2490a8cb45dc7b04ed25e0ba95e2041406138e

commit 1d2490a8cb45dc7b04ed25e0ba95e2041406138e
Author: Vadim Shtayura <vadimsh@chromium.org>
Date: Wed Jan 23 02:59:14 2019

[lucicfg] Remove overzealous validation.

Validate only structural correctness (types, strings not empty, no duplication,
etc). Keep detailed validation against regexps and other similar limits to the
backend. That way when details change, we don't need to redeploy lucicfg too.

R=nodir@chromium.org, tandrii@chromium.org
BUG=924207

Change-Id: I2833cbfacf05299f24fb3885a662210c1e555331
Reviewed-on: https://chromium-review.googlesource.com/c/1429459
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>

[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/starlark/assets.gen.go
[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/starlark/stdlib/internal/luci/lib/acl.star
[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/starlark/stdlib/internal/luci/lib/swarming.star
[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/starlark/stdlib/internal/luci/lib/validate.star
[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/testdata/acl/entry.star
[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/testdata/misc/validators.star
[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/testdata/swarming/cache.star
[modify] https://crrev.com/1d2490a8cb45dc7b04ed25e0ba95e2041406138e/lucicfg/testdata/swarming/dimension.star