cmd/go: fix GOTOOLCHAIN parsing for +auto

The call from toolchain.Select to gover.FromToolchain was passing the
wrong argument but this was masked by gover.IsValid being a little bit
too lax.

Fix and test IsValid, which then breaks the existing gotoolchain_local
test, and then fix toolchain.Select to fix the test.

Fixes #61068.

Change-Id: I505ceb227457d6a51bd5e47f009b2fb1010c0d1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/508820
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/gover/gover.go

@@ -179,6 +179,9 @@ func parse(x string) version {
 	// Parse prerelease.
 	i := 0
 	for i < len(x) && (x[i] < '0' || '9' < x[i]) {
+		if x[i] < 'a' || 'z' < x[i] {
+			return version{}
+		}
 		i++
 	}
 	if i == 0 {

src/cmd/go/internal/gover/gover_test.go

@@ -95,6 +95,25 @@ var prevTests = []testCase1[string, string]{
 	{"1.40000000000000000", "1.39999999999999999"},
 }
 
+func TestIsValid(t *testing.T) { test1(t, isValidTests, "IsValid", IsValid) }
+
+var isValidTests = []testCase1[string, bool]{
+	{"1.2rc3", true},
+	{"1.2.3", true},
+	{"1.999testmod", true},
+	{"1.600+auto", false},
+	{"1.22", true},
+	{"1.21.0", true},
+	{"1.21rc2", true},
+	{"1.21", true},
+	{"1.20.0", true},
+	{"1.20", true},
+	{"1.19", true},
+	{"1.3", true},
+	{"1.2", true},
+	{"1", true},
+}
+
 type testCase1[In, Out any] struct {
 	in  In
 	out Out

src/cmd/go/internal/gover/toolchain.go

@@ -15,7 +15,7 @@ import (
 // FromToolchain returns the Go version for the named toolchain,
 // derived from the name itself (not by running the toolchain).
 // A toolchain is named "goVERSION".
-// A suffix after the VERSION introduced by a +, -, space, or tab is removed.
+// A suffix after the VERSION introduced by a -, space, or tab is removed.
 // Examples:
 //
 //	FromToolchain("go1.2.3") == "1.2.3"

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

@@ -131,7 +131,7 @@ func Select() {
 	} else {
 		min, suffix, plus := strings.Cut(gotoolchain, "+") // go1.2.3+auto
 		if min != "local" {
-			v := gover.FromToolchain(gotoolchain)
+			v := gover.FromToolchain(min)
 			if v == "" {
 				if plus {
 					base.Fatalf("invalid GOTOOLCHAIN %q: invalid minimum toolchain %q", gotoolchain, min)

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

@@ -34,9 +34,9 @@ env GOTOOLCHAIN=go1.600+auto
 go version
 stdout go1.600
 
-env GOTOOLCHAIN=go1.400+auto
+env GOTOOLCHAIN=go1.400.0+auto
 go version
-stdout go1.400
+stdout go1.400.0
 
 # GOTOOLCHAIN=version+path sets a minimum too.
 env GOTOOLCHAIN=go1.600+path