Use platform transition in the oci_image_index rule

[moroten]

The current way of listing images makes Bazel build images for a platform that is not the configured target platform, defined by --platforms. This commit changes the behaviour and applies a transition so that the images are built for the configured target platform.

New way:

oci_image_index(
    name = "app",
    image = ":app_linux",
    platforms = [
        "@io_bazel_rules_go//go/toolchain:linux_amd64",
        "@io_bazel_rules_go//go/toolchain:linux_arm64",
    ]
)

Old way:

oci_image_index(
    name = "app",
    image = [
        ":app_linux_amd64",
        ":app_linux_arm64"
    ],
)

For backwards compatibility, the old way is still possible to use.

[alexeagle]

Our original intent here was to minimize the code in rules_oci that needs to understand what the user is doing, we liked that transitions occur explicitly. OTOH I do see how this syntax sugar makes it nicer.

Bazel build images for a platform that is not the configured target platform

I'm not sure I follow this existing bug you suggest - don't we build for the same platforms before and after this change? The same targets get transitioned in the same way?

[moroten]

Bazel build images for a platform that is not the configured target platform

I'm not sure I follow this existing bug you suggest - don't we build for the same platforms before and after this change? The same targets get transitioned in the same way?

Consider the following example:

cc_binary(
    name = "binary_linux_arm64",
)
pkg_tar(
    name = "app_tar_linux_arm64",
    srcs = [":binary_linux_arm64"],
)
oci_image(
    name = "app_image_linux_arm64",
    tars = [":app_tar_linux_arm64"],
)
oci_image_index(
    name = "app_index",
    images = [
        ":app_image_linux_arm64"
    ],
)

I would normally only have a target called binary_linux. Building binary_linux_arm64 with --platforms=//:linux_x86_64 is possible, but makes no sense as the result will be a binary binary_linux_arm64 will not be compatible with arm64. Adding target_compatible_with=[...arm64] will make all _arm64 targets unbuildable on x86_64, which is correct, but :app will also become unbuildable.

The idea here is that all the _linux_arm64 suffixes are removed and that bazel build :app_image --platforms=//:linux_x86_64 and bazel build :app_image --platforms=//:linux_arm64 produce images for respective OS, even if my host is Windows.

cc_binary(
    name = "binary",
)
pkg_tar(
    name = "app_tar",
    srcs = [":binary"],
)
oci_image(
    name = "app_image",
    tars = [":app_tar"],
)

The app_index target itself is actually compatible with any target platform, or at least multiple platforms, i.e. I expect to get the same output independent on --platforms (e.g. building on Windows or MacOS).

[thesayyn]

#228 (comment)

[moroten]

Let's keep the discussion about the plan forward in #228.

Are the failing tests due to this change? My feeling is that it is something unrelated. Anyway, some tests should be added for this new functionality.

[thesayyn]

Can we close this since we don't have to land a change for this?

[thesayyn]

Okay, i am going to close this for house keeping purposes. Thank you for putting time into this.