cmd/go: download 1.X.0 instead of 1.X during toolchain upgrade.

This CL modifies the download behavior when downloading a toolchain for 1.21+. Previously, Go would attempt to download 1.X when upgrading the toolchain which would cause the download to fail for 1.21+ since 1.X is an invalid toolchain. We will attempt to download 1.X.0 since that's likely what the user intended.

Additionally, we will also now provide a better error message when the
user provides a language version instead of a toolchain version for
1.21+.

Fixes #66175
Fixes #62278

Change-Id: I28f894290a19d8e3cd220e9d70aeca8f4447e5a1
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/580217
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -196,6 +196,13 @@ func Select() {
 			}
 			if gover.Compare(goVers, minVers) > 0 {
 				gotoolchain = "go" + goVers
+				// Starting with Go 1.21, the first released version has a .0 patch version suffix.
+				// Don't try to download a language version (sans patch component), such as go1.22.
+				// Instead, use the first toolchain of that language version, such as 1.22.0.
+				// See golang.org/issue/62278.
+				if gover.IsLang(goVers) && gover.Compare(goVers, "1.21") >= 0 {
+					gotoolchain += ".0"
+				}
 				gover.Startup.AutoGoVersion = goVers
 				gover.Startup.AutoToolchain = "" // in case we are overriding it for being too old
 			}
@@ -327,6 +334,10 @@ func Exec(gotoolchain string) {
 	dir, err := modfetch.Download(context.Background(), m)
 	if err != nil {
 		if errors.Is(err, fs.ErrNotExist) {
+			toolVers := gover.FromToolchain(gotoolchain)
+			if gover.IsLang(toolVers) && gover.Compare(toolVers, "1.21") >= 0 {
+				base.Fatalf("invalid toolchain: %s is a language version but not a toolchain version (%s.x)", gotoolchain, gotoolchain)
+			}
 			base.Fatalf("download %s for %s/%s: toolchain not available", gotoolchain, runtime.GOOS, runtime.GOARCH)
 		}
 		base.Fatalf("download %s: %v", gotoolchain, err)

src/cmd/go/testdata/script/gotoolchain_issue66175.txt

@@ -0,0 +1,104 @@
+env TESTGO_VERSION=go1.14
+
+# check for invalid toolchain in go.mod
+go mod init m
+go mod edit -go=1.14 -toolchain=go1.22
+! go version
+stderr 'go: invalid toolchain: go1.22 is a language version but not a toolchain version \(go1.22.x\)'
+
+rm go.mod
+go mod init m
+go mod edit -go=1.14 -toolchain=go1.21
+! go version
+stderr 'go: invalid toolchain: go1.21 is a language version but not a toolchain version \(go1.21.x\)'
+
+rm go.mod
+go mod init m
+go mod edit -go=1.14 -toolchain=go1.20
+! go version
+stderr 'go: downloading go1.20 '
+
+
+# check for invalid GOTOOLCHAIN
+env GOTOOLCHAIN=go1.14
+go version
+stdout 'go1.14'
+
+env GOTOOLCHAIN=go1.20
+! go version
+stderr 'go: downloading go1.20 '
+
+env GOTOOLCHAIN=go1.21
+! go version
+stderr 'go: invalid toolchain: go1.21 is a language version but not a toolchain version \(go1.21.x\)'
+
+env GOTOOLCHAIN=go1.22
+! go version
+stderr 'go: invalid toolchain: go1.22 is a language version but not a toolchain version \(go1.22.x\)'
+
+env GOTOOLCHAIN=go1.20+auto
+! go version
+stderr 'go: downloading go1.20 '
+
+env GOTOOLCHAIN=go1.21+auto
+! go version
+stderr 'go: invalid toolchain: go1.21 is a language version but not a toolchain version \(go1.21.x\)'
+
+env GOTOOLCHAIN=go1.22+auto
+! go version
+stderr 'go: invalid toolchain: go1.22 is a language version but not a toolchain version \(go1.22.x\)'
+
+env GOTOOLCHAIN=go1.21rc3
+! go version
+stderr 'go: downloading go1.21rc3 '
+
+env GOTOOLCHAIN=go1.22rc2
+! go version
+stderr 'go: downloading go1.22rc2 '
+
+env GOTOOLCHAIN=go1.66
+! go version
+stderr 'go: invalid toolchain: go1.66 is a language version but not a toolchain version \(go1.66.x\)'
+
+env GOTOOLCHAIN=go1.18beta2
+! go version
+stderr 'go: downloading go1.18beta2 '
+
+# go1.X is okay for path lookups
+env GOTOOLCHAIN=go1.20+path
+! go version
+stderr 'go: cannot find "go1.20" in PATH'
+
+env GOTOOLCHAIN=go1.21+path
+! go version
+stderr 'go: cannot find "go1.21" in PATH'
+
+env GOTOOLCHAIN=go1.22+path
+! go version
+stderr 'go: cannot find "go1.22" in PATH'
+
+# When a toolchain download takes place, download 1.X.0
+env GOTOOLCHAIN=auto
+rm go.mod
+go mod init m
+go mod edit -go=1.300 -toolchain=none
+! go version
+stderr 'go: downloading go1.300.0 '
+
+rm go.mod
+go mod init m
+go mod edit -go=1.21 -toolchain=none
+! go version
+stderr 'go: downloading go1.21.0 '
+
+rm go.mod
+go mod init m
+go mod edit -go=1.22 -toolchain=none
+! go version
+stderr 'go: downloading go1.22.0 '
+
+rm go.mod
+go mod init m
+go mod edit -go=1.15 -toolchain=none
+! go version
+stderr 'go: downloading go1.15 '