Objective

To check if bazel actually fetches the tool dependent on the resolved toolchain

Observation

Bazel does only fetch the toolchain that is resolved

Learnings

Be as specific as possible with toolchain constraints. If atleast one constraint does match, that toolchain is selected.

This seems strange From the docs of constraint values:

In order for a platform to apply to a given environment, the environment must have at least the values in this list.

The toolchain resolution log shows that for bazel build product --platforms //:custom_platform The less specific toolchain is selected:

INFO: ToolchainResolution:     Type //:tool_toolchain: target platform //:custom_platform: Rejected toolchain //:tool_windows; mismatching values: windows
INFO: ToolchainResolution:   Type //:tool_toolchain: target platform //:custom_platform: execution @local_config_platform//:host: Selected toolchain //:tool_linux
INFO: ToolchainResolution: Target platform //:custom_platform: Selected execution platform @local_config_platform//:host, type //:tool_toolchain -> toolchain //:tool_linux

Given this scenario:

platform(
    name = "custom_platform",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
        ":value_a",
    ],
)

toolchain(
    name = "tool_toolchain_linux",
    exec_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    target_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    toolchain = ":tool_linux",
    toolchain_type = ":tool_toolchain",
)

toolchain(
    name = "tool_toolchain_linux_custom",
    exec_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    target_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
        "//:value_a",
    ],
    toolchain = ":tool_linux_custom",
    toolchain_type = ":tool_toolchain",
)

If a build with --platforms //:custom_platform one would assume that the toolchain selected with the compatible target platform does have atleast all constraint values defined in the platform.

The expectation is that bazel would select the toolchain tool_linux_custom

However bazel selects a toolchain that matches only a subset of the constraint_values

INFO: ToolchainResolution:     Type //:tool_toolchain: target platform //:custom_platform: Rejected toolchain //:tool_windows; mismatching values: windows
INFO: ToolchainResolution:   Type //:tool_toolchain: target platform //:custom_platform: execution @local_config_platform//:host: Selected toolchain //:tool_linux
INFO: ToolchainResolution: Target platform //:custom_platform: Selected execution platform @local_config_platform//:host, type //:tool_toolchain -> toolchain //:tool_linux

Only in this scenario the custom toolchain is selected. It is therefore not possible to combine the target platforms.

platform(
    name = "custom_platform",
    constraint_values = [
        ":value_a",
    ],
)

toolchain(
    name = "tool_toolchain_linux",
    exec_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    target_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    toolchain = ":tool_linux",
    toolchain_type = ":tool_toolchain",
)

toolchain(
    name = "tool_toolchain_linux_custom",
    exec_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    target_compatible_with = [
        "//:value_a",
    ],
    toolchain = ":tool_linux_custom",
    toolchain_type = ":tool_toolchain",
)

Seperating the os:linux and cpu constraint into a parent platform does yield the same seemingly wrong behaviour (tool_linux is selected instead of tool_linux_custom)

platform(
    name = "linux_platform",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
)

platform(
    name = "custom_platform",
    constraint_values = [
        ":value_a",
    ],
    parents = [":linux_platform"],
)