Add support for OS Features in the format

[dmcgowan]

Updates the os part of the format to include features after the os version. The guarantees that the format may fully represent the platform structure.

I propose we consider including the features in the format for 1.0 release, which we are trying to finalize.

Should we consider "/" or "+" as supported parts of OS features here? Right now the spec does not specify but we can define our own limitations. We could support it either by updating the parser (less backwards compatible) or URL encoding (ugly, but could be an edge case so who cares).

[tianon]

Does the format already support (non-OS) features?

(I don't think either are currently used for anything serious, but if we're going to support one we should probably support the other too.)

[dmcgowan]

Does the format already support (non-OS) features?

Which format are you referring to?

In OCI there are non-OS features reserved, but not sure if that will ever be defined or added to the struct. I don't know where the line would be drawn in this case in regards to OS feature vs non-OS feature. It could be a hardware feature stuffed in as an OS feature, but what really is the difference if any hardware feature ends up being visible through the OS anyway. This is more for completeness of avoid compatibility later on when we want to add it, as had been previously discussed and recently inquired about.

[kzys]

The corresponding part in the spec is (I believe) https://github.com/opencontainers/image-spec/blob/92353b0bee778725c617e7d57317b568a7796bd0/image-index.md#image-index-property-descriptions

This OPTIONAL property specifies an array of strings, each specifying a mandatory OS feature.

[kzys]

Should we consider "/" or "+" as supported parts of OS features here?

I'd like to disallow them and I don't want to have URL-encoding later. Let me check with OCI folks.

[kzys]

Opened opencontainers/image-spec#1237

[dmcgowan]

Marking this one as ready, OCI issue is there for discussion but the format is owned by containerd

[tianon]

As asked over in opencontainers/image-spec#1237, what's the use case? Is there a particular "feature" you're wanting to put in here / use?

[samuelkarp]

Do we consider platforms to be equal if the features are in a different order?

[dmcgowan]

It should be considered an unordered set, so yes. We can make the order deterministic here.

[dmcgowan]

@tianon firstly to indicate support for erofs from the runtime and requirement for erofs support in the image

[thaJeztah]

Curious; was there a specific reason for using + as delimiter? Also considering that version strings could potentially include +. Or at least, I recall the discussions (distribution/distribution#1201) around people asking for + to be accepted for tags, e.g. 1.0.0+1 (which for sure is not "related", but to illustrate where version could contain +)

I recall when we were discussing the addition of os-version, that I considered ways to expand that; #6 (comment)

Could we make it support both to allow future-expansion that we cannot (yet) foresee? Considering;

windows([version=]<version>[,key=value[,key=value]])

So version would be the default key, and windows(<version>) would be the short-form for windows(version=<version>). Both would already be accepted, but short form would be the commonly used form (at least until additional options are to be added).

So if the value between () does not have a [key]=, then consider the value to be the default (version). Trailing commas should not be allowed (nor should (un-escaped?) = and . be accepted as part of key or value), and the comma to only be used if multiple key/value tuples are included.

One thing we should look at (and perhaps as part of the OCI?) is to define limits; I recall a discussion on our Slack where Docker Hub failed to process some images, because some tool (I think it was https://github.com/unikraft/kraftkit) used the os.features field in the spec to store their kernel config, and Docker hub had a limit of 255 chars for that field?

Unfortunately, I don't have a reference to that image itself, but I found the content of the field;

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