internal/rosa: isolate patching helper

This is useful outside llvm as well.

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

internal/rosa/rosa.go

@@ -271,3 +271,38 @@ ln -vs ../usr/bin /work/bin
 		)}, paths)...,
 	)
 }
+
+// NewPatchedSource returns [pkg.Artifact] of source with patches applied. If
+// passthrough is true, source is returned as is for zero length patches.
+func (t Toolchain) NewPatchedSource(
+	name string,
+	source pkg.Artifact,
+	passthrough bool,
+	patches ...[2]string,
+) pkg.Artifact {
+	if passthrough && len(patches) == 0 {
+		return source
+	}
+
+	paths := make([]pkg.ExecPath, len(patches)+1)
+	for i, p := range patches {
+		paths[i+1] = pkg.Path(
+			AbsUsrSrc.Append(name+"-patches", p[0]+".patch"), false,
+			pkg.NewFile(p[0]+".patch", []byte(p[1])),
+		)
+	}
+	paths[0] = pkg.Path(AbsUsrSrc.Append(name), false, source)
+
+	aname := name + "-src"
+	script := `
+cp -r /usr/src/` + name + `/. /work/.
+chmod -R +w /work && cd /work
+`
+	if len(paths) > 1 {
+		script += `cat /usr/src/` + name + `-patches/* | patch -p 1`
+		aname += "-patched"
+	}
+	return t.New(aname, stage3Concat(t, []pkg.Artifact{},
+		t.Load(Patch),
+	), nil, nil, script, paths...)
+}