internal/rosa: use self-hosted stage0

This removes the bootstrap dependency on Gentoo stage3 tarball.

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

cmd/mbf/main.go

@@ -111,15 +111,31 @@ func main() {
 
 	{
 		var (
+			flagGentoo   string
+			flagChecksum string
+
 			flagStage0 bool
 		)
 		c.NewCommand(
 			"stage3",
 			"Check for toolchain 3-stage non-determinism",
 			func(args []string) (err error) {
-				_, _, _, stage1 := (rosa.Std - 2).NewLLVM()
-				_, _, _, stage2 := (rosa.Std - 1).NewLLVM()
-				_, _, _, stage3 := rosa.Std.NewLLVM()
+				std := rosa.Std
+				if flagGentoo != "" {
+					std -= 3 // magic number to discourage misuse
+
+					var checksum pkg.Checksum
+					if len(flagChecksum) != 0 {
+						if err = pkg.Decode(&checksum, flagChecksum); err != nil {
+							return
+						}
+					}
+					rosa.SetGentooStage3(flagGentoo, checksum)
+				}
+
+				_, _, _, stage1 := (std - 2).NewLLVM()
+				_, _, _, stage2 := (std - 1).NewLLVM()
+				_, _, _, stage3 := std.NewLLVM()
 				var (
 					pathname *check.Absolute
 					checksum [2]unique.Handle[pkg.Checksum]
@@ -153,7 +169,7 @@ func main() {
 
 				if flagStage0 {
 					if pathname, _, err = cache.Cure(
-						rosa.Std.Load(rosa.Stage0),
+						std.Load(rosa.Stage0),
 					); err != nil {
 						return err
 					}
@@ -163,6 +179,16 @@ func main() {
 				return
 			},
 		).
+			Flag(
+				&flagGentoo,
+				"gentoo", command.StringFlag(""),
+				"Bootstrap from a Gentoo stage3 tarball",
+			).
+			Flag(
+				&flagChecksum,
+				"checksum", command.StringFlag(""),
+				"Checksum of Gentoo stage3 tarball",
+			).
 			Flag(
 				&flagStage0,
 				"stage0", command.BoolFlag(false),