Description

• relates to c8d: Add platform parameter to history, save and load #48295
• relates to c8d/load: Don't ignore missing platform when requested #48718 (comment)

#48295 introduced --platform flags on docker image save and docker image load, but the client currently only accepts a single one. From #48718 (comment)

One thing I just realized is that we don't allow multiple platforms to be specified. Currently, specifying multiple platforms will result in the last one to be used;

$ docker image load --platform=linux/arm64/v8 --platform=linux/riscv64 -i ./alpine2.tar
Loaded image: alpine:latest

$ docker image ls --tree
IMAGE                   ID             DISK USAGE   CONTENT SIZE   IN USE
alpine:latest           beefdbd8a1da       10.6MB         3.37MB
├─ linux/amd64          33735bd63cf8           0B             0B
├─ linux/arm/v6         50f635c8b04d           0B             0B
├─ linux/arm/v7         f2f82d424957           0B             0B
├─ linux/arm64/v8       9cee2b382fe2           0B             0B
├─ linux/386            b3e87f642f5c           0B             0B
├─ linux/ppc64le        c7a6800e3dc5           0B             0B
├─ linux/riscv64        80cde017a105       10.6MB         3.37MB
└─ linux/s390x          2b5b26e09ca2           0B             0B

Part of that would be the CLI, which discards multiple values if only 1 is supported; the above produces (URL-decoded);

/v1.48/images/load?platform={"architecture":"arm64","os":"linux","variant":"v8"}&quiet=0

I wonder what it would take to allow multiple platforms? My use-case here would be that I have a image.tar that contains (say) 8 platforms, but I only need 2 (e.g., only need arm64 and amd64). In that case, I'd want to be able to load only those.

Do we need API changes to make that possible, or would the API already be ablee to handle multiple? :thinking_face: We didn't ship this feature yet, so there's still time to adjust the API (even if we don't support multiple yet, the API coud be able to handle it, just produce an error when multiple are passed).

We can support multiple platforms with the current API via multiple platform query fields, like:

localhost/images/load?platform={"architecture":"arm64","os":"linux","variant":"v8"}&platform={"architecture":"arm","os":"linux","variant":"v5"}`

We must make changes before we release v28 so that the API and client at least are ready for this.

• Update api/types/image.LoadOptions and api/types/image.SaveOptions to accept a []ocispec.Platform
• Update Client.ImageLoad() and Client.ImageSave() to handle multiple platforms
• Change the CLI to accept multiple --platform flags and/or a comma-separated list (--platform <platform>[,<platform>]) similar to how docker build allows this.
• Verify behavior on the daemon side

⚠️ if we cannot handle multiple platforms yet, it's OK to produce an error for now; the important bit is for the API (and Client) to be prepared.