internal/rosa: toolchain abstraction

This provides a clean and easy to use API over toolchains. A toolchain is an opaque set of artifacts and environment fixups. Exported toolchains should be functionally indistinguishable from each other.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/rosa.go

@@ -4,6 +4,9 @@ package rosa
 import (
 	"log"
 	"runtime"
+	"slices"
+	"strconv"
+	"strings"
 
 	"hakurei.app/container/fhs"
 	"hakurei.app/internal/pkg"
@@ -48,3 +51,86 @@ func linuxArch() string {
 		panic("unsupported target " + runtime.GOARCH)
 	}
 }
+
+// Toolchain denotes the infrastructure to compile a [pkg.Artifact] on.
+type Toolchain uintptr
+
+const (
+	// toolchainBusybox denotes a busybox installation from the busyboxBin
+	// binary distribution. This is for decompressing unsupported formats.
+	toolchainBusybox Toolchain = iota
+)
+
+// lastIndexFunc is like [strings.LastIndexFunc] but for [slices].
+func lastIndexFunc[S ~[]E, E any](s S, f func(E) bool) (i int) {
+	if i = slices.IndexFunc(s, f); i < 0 {
+		return
+	}
+	if i0 := lastIndexFunc[S](s[i+1:], f); i0 >= 0 {
+		i = i0
+	}
+	return
+}
+
+// fixupEnviron fixes up PATH, prepends extras and returns the resulting slice.
+func fixupEnviron(env, extras []string, paths ...string) []string {
+	const pathPrefix = "PATH="
+	pathVal := strings.Join(paths, ":")
+
+	if i := lastIndexFunc(env, func(s string) bool {
+		return strings.HasPrefix(s, pathPrefix)
+	}); i < 0 {
+		env = append(env, pathPrefix+pathVal)
+	} else {
+		if len(env[i]) == len(pathPrefix) {
+			env[i] = pathPrefix + pathVal
+		} else {
+			env[i] += ":" + pathVal
+		}
+	}
+
+	return append(extras, env...)
+}
+
+// absCureScript is the absolute pathname [Toolchain.New] places the fixed-up
+// build script under.
+var absCureScript = fhs.AbsUsrBin.Append(".cure-script")
+
+// New returns a [pkg.Artifact] compiled on this toolchain.
+func (t Toolchain) New(
+	name string,
+	extra []pkg.Artifact,
+	checksum *pkg.Checksum,
+	env []string,
+	script string,
+
+	paths ...pkg.ExecPath,
+) pkg.Artifact {
+	var (
+		path    = AbsSystem.Append("bin", "busybox")
+		args    = []string{"hush", absCureScript.String()}
+		support []pkg.Artifact
+	)
+	switch t {
+	case toolchainBusybox:
+		support = slices.Concat([]pkg.Artifact{newBusyboxBin()}, extra)
+		env = fixupEnviron(env, nil, "/system/bin")
+
+	default:
+		panic("unsupported toolchain " + strconv.Itoa(int(t)))
+	}
+
+	return pkg.NewExec(
+		name, checksum, pkg.ExecTimeoutMax,
+		fhs.AbsRoot, env,
+		path, args,
+
+		slices.Concat([]pkg.ExecPath{pkg.Path(
+			fhs.AbsRoot, true,
+			support...,
+		), pkg.Path(
+			absCureScript, false,
+			pkg.NewFile(".cure-script", []byte("set -e\n"+script)),
+		)}, paths)...,
+	)
+}