copy hlopko@'s platforms examples to this repo

[aiuto]

I am just moving the code over in this PR. I still have to figure out where to place the index from https://docs.google.com/document/d/1UZaVcL08wePB41ATZHcxQV4Pu1YfA1RvvWm8FbZHuW8/edit#heading=h.r33iyda45sjm into the site (or a different site?).

[hlopko]

LGTM. Care to also migrate Julie's hlopko/bazel_platforms_examples#1?

[agoulti]

Sorry, just realized the whole thread was hidden.

I'd like to rework the examples a little bit before exposing them in a central place, since I'm afraid that they are occasionally misleading.

In the first example, HOST_CONSTRAINTS is used - and I don't think they'll do what it looks like they will do.
It creates a toolchain that would match a platform compatible with the host platform, but it doesn't guarantee that it selects the host platform.
For example, if you set linux_platform in the list of execution platforms, it will select that.
The toolchain, however, prints out pre-specified values and the example would seem to indicate that the host platform was still chosen.

• I'm not sure if this is a good example for using HOST_CONSTRAINTS (I don't actually know of a good case to ever use them in your platform).
• The fact that the examples print the strings hardcoded in the toolchain and not the actual platforms make this dangerous if people play with them in unexpected ways. I wonder if we could instead use the experimental Skylark API to print which platforms are actually selected.

[agoulti]

I can't find this file, am I looking in the wrong place?

[agoulti]

Is there a plan to handle examples that don't want to register this platform?

[rockwotj]

Is there work planned to merge this? These examples are super helpful but where very hard to find.

[agoulti]

Is there work planned to merge this? These examples are super helpful but where very hard to find.

See comments above. There's some issues with these examples.

• yolo_library is not examining the platform that got selected, but only prints what the toolchain gave it. So executing_on_cpu etc are the toolchain's "wishful thinking" and may or may not match the platform that's actually selected. This could lead to misunderstandings.

• the use of HOST_CONSTRAINTS is wrong. It seems to think that this would pick the host platform, where in reality it would pick any platform with the same constraints as host.

I don't think there's anyone actively working on this at the moment.

[aiuto]

Yes. The problem is that no one quite has the time to keep the samples running. Tyler: Now that you have found them, would you be willing to contribute by coming up with a PR brought up to current Bazel?

…

On Wed, Feb 26, 2020 at 11:00 AM agoulti ***@***.***> wrote: Is there work planned to merge this? These examples are super helpful but where very hard to find. See comments above. There's some issues with these examples. - yolo_library is not examining the platform that got selected, but only prints what the toolchain gave it. So executing_on_cpu etc are the toolchain's "wishful thinking" and may or may not match the platform that's actually selected. This could lead to misunderstandings. - the use of HOST_CONSTRAINTS is wrong. It seems to think that this would pick the host platform, where in reality it would pick any platform with the same constraints as host. I don't think there's anyone actively working on this at the moment. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#111?email_source=notifications&email_token=AAXHHHGTGRVHMKGM6ACTG6TRE2G2JA5CNFSM4ILNEQWKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOENAZYUI#issuecomment-591502417>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAXHHHFD4IRBNJTLBDUKVADRE2G2JANCNFSM4ILNEQWA> .