cmd/go: fix two-step toolchain upgrade through go install, GOTOOLCHAIN

If we do one upgrade because of a go install target's go.mod file,
we still might need a second upgrade to implement the GOTOOLCHAIN
minimum. Instead of allowing a two-step switch (which we were
cutting off anyway), skip the first step and go straight to the
GOTOOLCHAIN min upgrade.

Fixes golang#69051.

Change-Id: I16f060f473574d8b8f84c55fae2fd0cdabc8aa19
Reviewed-on: https://go-review.googlesource.com/c/go/+/637496
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/cmd/go/internal/toolchain/select.go

@@ -169,7 +169,7 @@ func Select() {
 	}
 
 	gotoolchain = minToolchain
-	if (mode == "auto" || mode == "path") && !goInstallVersion() {
+	if (mode == "auto" || mode == "path") && !goInstallVersion(minVers) {
 		// Read go.mod to find new minimum and suggested toolchain.
 		file, goVers, toolchain := modGoToolchain()
 		gover.Startup.AutoFile = file
@@ -549,7 +549,7 @@ func modGoToolchain() (file, goVers, toolchain string) {
 
 // goInstallVersion reports whether the command line is go install m@v or go run m@v.
 // If so, Select must not read the go.mod or go.work file in "auto" or "path" mode.
-func goInstallVersion() bool {
+func goInstallVersion(minVers string) bool {
 	// Note: We assume there are no flags between 'go' and 'install' or 'run'.
 	// During testing there are some debugging flags that are accepted
 	// in that position, but in production go binaries there are not.
@@ -708,7 +708,11 @@ func goInstallVersion() bool {
 	if errors.Is(err, gover.ErrTooNew) {
 		// Run early switch, same one go install or go run would eventually do,
 		// if it understood all the command-line flags.
-		SwitchOrFatal(ctx, err)
+		var s Switcher
+		s.Error(err)
+		if s.TooNew != nil && gover.Compare(s.TooNew.GoVersion, minVers) > 0 {
+			SwitchOrFatal(ctx, err)
+		}
 	}
 
 	return true // pkg@version found

src/cmd/go/testdata/script/gotoolchain_local.txt

@@ -197,6 +197,17 @@ go mod edit -go=1.501 -toolchain=none
 go version
 stdout go1.501
 
+# avoid two-step switch, first from install target requirement, then from GOTOOLCHAIN min
+# instead, just jump directly to GOTOOLCHAIN min
+env TESTGO_VERSION=go1.2.3
+env GODEBUG=toolchaintrace=1
+env GOTOOLCHAIN=go1.23.0+auto
+! go install rsc.io/fortune/[email protected]
+! stderr 'switching to go1.22.9'
+stderr 'using go1.23.0'
+env GODEBUG=
+env GOTOOLCHAIN=auto
+
 # go install m@v and go run m@v should ignore go.mod and use m@v
 env TESTGO_VERSION=go1.2.3
 go mod edit -go=1.999 -toolchain=go1.998