cmd/dist: set go version in bootstrap go.mod file

The commands to build the bootstrap toolchains and go commands are run
from modules created by two bootstrap go.mod files: one is used when
building toolchain1 and go_bootstrap, and the other is used for
toolchain2 and toolchain3, and the final build. Currently the first has
a go directive specifying go 1.20, and the second one does not have a go
directive at all. This affects the default GODEBUG setting when building
the final toolchain: the default GODEBUG value is based on the go
version of the go.mod file, and when the go.mod file does not have a
version it defaults to go1.16. We should set the go directive on the
bootstrap used for the second half of the builds to use the current go
verison from the std's go.mod file (which is the same as the version on
cmd's go.mod file).

The go.mod file used for the initial bootstrap should have a go
directive with the minimum version of the toolchain required for
bootstrapping. That version is the current version - 2 rounded down to
an even number.

For #64751
Fixes #68797

Change-Id: Ibdddf4bc36dc963291979d603c4f3fc55264f65b
Reviewed-on: https://go-review.googlesource.com/c/go/+/604799
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: matloob

src/cmd/dist/build.go

@@ -17,6 +17,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -261,8 +262,12 @@ func xinit() {
 	os.Unsetenv("GOFLAGS")
 	os.Setenv("GOWORK", "off")
 
+	// Create the go.mod for building toolchain2 and toolchain3. Toolchain1 and go_bootstrap are built with
+	// a separate go.mod (with a lower required go version to allow all allowed bootstrap toolchain versions)
+	// in bootstrapBuildTools.
+	modVer := goModVersion()
 	workdir = xworkdir()
-	if err := os.WriteFile(pathf("%s/go.mod", workdir), []byte("module bootstrap"), 0666); err != nil {
+	if err := os.WriteFile(pathf("%s/go.mod", workdir), []byte("module bootstrap\n\ngo "+modVer+"\n"), 0666); err != nil {
 		fatalf("cannot write stub go.mod: %s", err)
 	}
 	xatexit(rmworkdir)
@@ -441,6 +446,33 @@ func findgoversion() string {
 	return version
 }
 
+// goModVersion returns the go version declared in src/go.mod. This is the
+// go version to use in the go.mod building go_bootstrap, toolchain2, and toolchain3.
+// (toolchain1 must be built with requiredBootstrapVersion(goModVersion))
+func goModVersion() string {
+	goMod := readfile(pathf("%s/src/go.mod", goroot))
+	m := regexp.MustCompile(`(?m)^go (1.\d+)$`).FindStringSubmatch(goMod)
+	if m == nil {
+		fatalf("std go.mod does not contain go 1.X")
+	}
+	return m[1]
+}
+
+func requiredBootstrapVersion(v string) string {
+	minorstr, ok := strings.CutPrefix(v, "1.")
+	if !ok {
+		fatalf("go version %q in go.mod does not start with %q", v, "1.")
+	}
+	minor, err := strconv.Atoi(minorstr)
+	if err != nil {
+		fatalf("invalid go version minor component %q: %v", minorstr, err)
+	}
+	// Per go.dev/doc/install/source, for N >= 22, Go version 1.N will require a Go 1.M compiler,
+	// where M is N-2 rounded down to an even number. Example: Go 1.24 and 1.25 require Go 1.22.
+	requiredMinor := minor - 2 - minor%2
+	return "1." + strconv.Itoa(requiredMinor)
+}
+
 // isGitRepo reports whether the working directory is inside a Git repository.
 func isGitRepo() bool {
 	// NB: simply checking the exit code of `git rev-parse --git-dir` would

src/cmd/dist/build_test.go

@@ -24,3 +24,20 @@ func TestMustLinkExternal(t *testing.T) {
 		}
 	}
 }
+
+func TestRequiredBootstrapVersion(t *testing.T) {
+	testCases := map[string]string{
+		"1.22": "1.20",
+		"1.23": "1.20",
+		"1.24": "1.22",
+		"1.25": "1.22",
+		"1.26": "1.24",
+		"1.27": "1.24",
+	}
+
+	for v, want := range testCases {
+		if got := requiredBootstrapVersion(v); got != want {
+			t.Errorf("requiredBootstrapVersion(%v): got %v, want %v", v, got, want)
+		}
+	}
+}

src/cmd/dist/buildtool.go

@@ -151,7 +151,8 @@ func bootstrapBuildTools() {
 	xmkdirall(base)
 
 	// Copy source code into $GOROOT/pkg/bootstrap and rewrite import paths.
-	writefile("module bootstrap\ngo 1.20\n", pathf("%s/%s", base, "go.mod"), 0)
+	minBootstrapVers := requiredBootstrapVersion(goModVersion()) // require the minimum required go version to build this go version in the go.mod file
+	writefile("module bootstrap\ngo "+minBootstrapVers+"\n", pathf("%s/%s", base, "go.mod"), 0)
 	for _, dir := range bootstrapDirs {
 		recurse := strings.HasSuffix(dir, "/...")
 		dir = strings.TrimSuffix(dir, "/...")