bazel: mkosi toolchain

Co-authored-by: Paul Meyer <49727155+katexochen@users.noreply.github.com>

WORKSPACE.bazel

@@ -18,6 +18,10 @@ nixpkgs_flake_package(
     package = "mkosi",
 )
 
+load("//bazel/mkosi:mkosi_configure.bzl", "find_system_mkosi")
+
+find_system_mkosi(name="constellation_mkosi")
+
 # Python toolchain
 load("//bazel/toolchains:python_deps.bzl", "python_deps")
 

bazel/mkosi/BUILD.bazel

@@ -0,0 +1,70 @@
+"""toolchain to wrap an mkosi binary.
+
+Type: @constellation//bazel/mkosi:toolchain_type
+
+Toolchains:
+- mkosi_missing_toolchain: provides a fallback toolchain for exec platforms
+  where mkosi might not be available.
+
+- mkosi_auto_toolchain: a toolchain that uses the installed mkosi. See
+  mkosi_configure.bzl%find_system_mkosi for usage.
+"""
+
+load(":toolchain.bzl", "is_mkosi_available", "mkosi_make_variables", "mkosi_toolchain")
+
+# Expose the availability of an actual mkosi as a config_setting, so we can
+# select() on it.
+config_setting(
+    name = "have_mkosi",
+    flag_values = {
+        ":is_mkosi_available": "1",
+    },
+    visibility = ["//visibility:public"],
+)
+
+# Expose the availability of an actual mkosi as a feature flag, so we can
+# create a config_setting from it.
+is_mkosi_available(
+    name = "is_mkosi_available",
+    visibility = ["//:__subpackages__"],
+)
+
+toolchain_type(
+    name = "toolchain_type",
+    visibility = ["//visibility:public"],
+)
+
+mkosi_make_variables(
+    name = "make_variables",
+    visibility = ["//visibility:public"],
+)
+
+# mkosi_missing_toolchain provides a fallback toolchain so that toolchain
+# resolution can succeed even on platforms that do not have a working mkosi.
+# If this toolchain is selected, the constraint ":have_mkosi" will not be
+# satistifed.
+mkosi_toolchain(
+    name = "no_mkosi",
+)
+
+toolchain(
+    name = "mkosi_missing_toolchain",
+    toolchain = ":no_mkosi",
+    toolchain_type = ":toolchain_type",
+)
+
+mkosi_toolchain(
+    name = "nix_mkosi",
+    label = "@mkosi//:bin/mkosi",
+)
+
+toolchain(
+    name = "mkosi_nix_toolchain",
+    exec_compatible_with = [
+        "@rules_nixpkgs_core//constraints:support_nix",
+    ],
+    toolchain = ":nix_mkosi",
+    toolchain_type = "@constellation//bazel/mkosi:toolchain_type",
+)
+
+exports_files(["mkosi_wrapper.sh.in"])

bazel/mkosi/BUILD.tpl

@@ -0,0 +1,13 @@
+# This content is generated by {GENERATOR}
+load("@constellation//bazel/mkosi:toolchain.bzl", "mkosi_toolchain")
+
+mkosi_toolchain(
+    name = "mkosi_auto",
+    path = "{MKOSI_PATH}",
+)
+
+toolchain(
+    name = "mkosi_auto_toolchain",
+    toolchain = ":mkosi_auto",
+    toolchain_type = "@constellation//bazel/mkosi:toolchain_type",
+)

bazel/mkosi/mkosi_configure.bzl

@@ -0,0 +1,43 @@
+"""Repository rule to autoconfigure a toolchain using the system mkosi."""
+
+def _write_build(rctx, path):
+    if not path:
+        path = ""
+    rctx.template(
+        "BUILD",
+        Label("//bazel/mkosi:BUILD.tpl"),
+        substitutions = {
+            "{GENERATOR}": "@constellation//bazel/mkosi/mkosi_configure.bzl%find_system_mkosi",
+            "{MKOSI_PATH}": str(path),
+        },
+        executable = False,
+    )
+
+def _find_system_mkosi_impl(rctx):
+    mkosi_path = rctx.which("mkosi")
+    if rctx.attr.verbose:
+        if mkosi_path:
+            print("Found mkosi at '%s'" % mkosi_path)  # buildifier: disable=print
+        else:
+            print("No system mkosi found.")  # buildifier: disable=print
+    _write_build(rctx = rctx, path = mkosi_path)
+
+_find_system_mkosi = repository_rule(
+    implementation = _find_system_mkosi_impl,
+    doc = """Create a repository that defines an mkosi toolchain based on the system mkosi.""",
+    local = True,
+    environ = ["PATH"],
+    attrs = {
+        "verbose": attr.bool(
+            doc = "If true, print status messages.",
+        ),
+    },
+)
+
+def find_system_mkosi(name, verbose = False):
+    _find_system_mkosi(name = name, verbose = verbose)
+    native.register_toolchains(
+        "@constellation//bazel/mkosi:mkosi_nix_toolchain",
+        "@%s//:mkosi_auto_toolchain" % name,
+        "@constellation//bazel/mkosi:mkosi_missing_toolchain",
+    )

bazel/mkosi/toolchain.bzl

@@ -0,0 +1,73 @@
+"""toolchain to provide an mkosi binary."""
+
+TOOLCHAIN_TYPE = "@constellation//bazel/mkosi:toolchain_type"
+MAKE_VARIABLES = "@constellation//bazel/mkosi:make_variables"
+
+MkosiInfo = provider(
+    doc = """Information needed to invoke mkosi.""",
+    fields = {
+        "label": "Label of a target providing a mkosi binary",
+        "name": "The name of the toolchain",
+        "path": "Path to the mkosi binary",
+        "valid": "Is this toolchain valid and usable?",
+    },
+)
+
+def _mkosi_toolchain_impl(ctx):
+    if ctx.attr.label and ctx.attr.path:
+        fail("mkosi_toolchain: label and path are mutually exclusive")
+    valid = bool(ctx.attr.label) or bool(ctx.attr.path)
+
+    mkosi_info = MkosiInfo(
+        name = str(ctx.label),
+        valid = valid,
+        label = ctx.attr.label,
+        path = ctx.attr.path,
+    )
+    toolchain_info = platform_common.ToolchainInfo(
+        mkosi = mkosi_info,
+    )
+    return [toolchain_info]
+
+mkosi_toolchain = rule(
+    implementation = _mkosi_toolchain_impl,
+    attrs = {
+        "label": attr.label(
+            doc = "Label of a target providing a mkosi binary. Mutually exclusive with path.",
+            executable = True,
+            cfg = "exec",
+            allow_single_file = True,
+            default = None,
+        ),
+        "path": attr.string(
+            doc = "Path to the mkosi binary. Mutually exclusive with label.",
+        ),
+    },
+)
+
+def _mkosi_make_variables_impl(ctx):
+    info = ctx.toolchains[TOOLCHAIN_TYPE].mkosi
+    variables = {}
+    if info.valid:
+        binary_path = info.label[DefaultInfo].files_to_run.executable.path if info.label else info.path
+        variables["MKOSI"] = binary_path
+    return [platform_common.TemplateVariableInfo(variables)]
+
+mkosi_make_variables = rule(
+    doc = "Make variables for mkosi.",
+    implementation = _mkosi_make_variables_impl,
+    toolchains = [TOOLCHAIN_TYPE],
+)
+
+def _is_mkosi_available_impl(ctx):
+    toolchain = ctx.toolchains[TOOLCHAIN_TYPE]
+    available = toolchain and toolchain.mkosi.valid
+    return [config_common.FeatureFlagInfo(
+        value = ("1" if available else "0"),
+    )]
+
+is_mkosi_available = rule(
+    implementation = _is_mkosi_available_impl,
+    attrs = {},
+    toolchains = [TOOLCHAIN_TYPE],
+)