cmd/go: update for go1 tag format

R=golang-dev, r, r
CC=golang-dev
https://golang.org/cl/5919048

src/cmd/go/get.go

@@ -11,6 +11,7 @@ import (
 	"go/build"
 	"os"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"strconv"
 	"strings"
@@ -323,9 +324,18 @@ func downloadPackage(p *Package) error {
 	return nil
 }
 
+// goTag matches go release tags such as go1 and go1.2.3.
+// The numbers involved must be small (at most 4 digits),
+// have no unnecessary leading zeros, and the version cannot
+// end in .0 - it is go1, not go1.0 or go1.0.0.
+var goTag = regexp.MustCompile(
+	`^go((0|[1-9][0-9]{0,3})\.)*([1-9][0-9]{0,3})$`,
+)
+
 // selectTag returns the closest matching tag for a given version.
 // Closest means the latest one that is not after the current release.
-// Version "release.rN" matches tags of the form "go.rN" (N being a decimal).
+// Version "goX" (or "goX.Y" or "goX.Y.Z") matches tags of the same form.
+// Version "release.rN" matches tags of the form "go.rN" (N being a floating-point number).
 // Version "weekly.YYYY-MM-DD" matches tags like "go.weekly.YYYY-MM-DD".
 func selectTag(goVersion string, tags []string) (match string) {
 	const rPrefix = "release.r"
@@ -349,6 +359,7 @@ func selectTag(goVersion string, tags []string) (match string) {
 			}
 		}
 	}
+
 	const wPrefix = "weekly."
 	if strings.HasPrefix(goVersion, wPrefix) {
 		p := "go.weekly."
@@ -362,5 +373,54 @@ func selectTag(goVersion string, tags []string) (match string) {
 			}
 		}
 	}
+
+	if goTag.MatchString(goVersion) {
+		v := goVersion
+		for _, t := range tags {
+			if !goTag.MatchString(t) {
+				continue
+			}
+			if cmpGoVersion(match, t) < 0 && cmpGoVersion(t, v) <= 0 {
+				match = t
+			}
+		}
+	}
+
 	return match
 }
+
+// cmpGoVersion returns -1, 0, +1 reporting whether
+// x < y, x == y, or x > y.
+func cmpGoVersion(x, y string) int {
+	// Malformed strings compare less than well-formed strings.
+	if !goTag.MatchString(x) {
+		return -1
+	}
+	if !goTag.MatchString(y) {
+		return +1
+	}
+
+	// Compare numbers in sequence.
+	xx := strings.Split(x[len("go"):], ".")
+	yy := strings.Split(y[len("go"):], ".")
+
+	for i := 0; i < len(xx) && i < len(yy); i++ {
+		// The Atoi are guaranteed to succeed
+		// because the versions match goTag.
+		xi, _ := strconv.Atoi(xx[i])
+		yi, _ := strconv.Atoi(yy[i])
+		if xi < yi {
+			return -1
+		} else if xi > yi {
+			return +1
+		}
+	}
+
+	if len(xx) < len(yy) {
+		return -1
+	}
+	if len(xx) > len(yy) {
+		return +1
+	}
+	return 0
+}

src/cmd/go/tag_test.go

@@ -18,6 +18,12 @@ var selectTagTestTags = []string{
 	"go.weekly.2011-10-12.1",
 	"go.weekly.2011-10-14",
 	"go.weekly.2011-11-01",
+	"go1",
+	"go1.0.1",
+	"go1.999",
+	"go1.9.2",
+	"go5",
+
 	// these should be ignored:
 	"release.r59",
 	"release.r59.1",
@@ -30,6 +36,14 @@ var selectTagTestTags = []string{
 	"go.f00",
 	"go!r60",
 	"go.1999-01-01",
+	"go.2x",
+	"go.20000000000000",
+	"go.2.",
+	"go.2.0",
+	"go2x",
+	"go20000000000000",
+	"go2.",
+	"go2.0",
 }
 
 var selectTagTests = []struct {
@@ -56,11 +70,21 @@ var selectTagTests = []struct {
 	{"weekly.2011-11-01", "go.weekly.2011-11-01"},
 	{"weekly.2014-01-01", "go.weekly.2011-11-01"},
 	{"weekly.3000-01-01", "go.weekly.2011-11-01"},
+	{"go1", "go1"},
+	{"go1.1", "go1.0.1"},
+	{"go1.998", "go1.9.2"},
+	{"go1.1000", "go1.999"},
+	{"go6", "go5"},
+
 	// faulty versions:
 	{"release.f00", ""},
 	{"weekly.1999-01-01", ""},
 	{"junk", ""},
 	{"", ""},
+	{"go2x", ""},
+	{"go200000000000", ""},
+	{"go2.", ""},
+	{"go2.0", ""},
 }
 
 func TestSelectTag(t *testing.T) {

src/cmd/go/vcs.go

@@ -71,7 +71,7 @@ var vcsHg = &vcsCmd{
 
 	// We allow both tag and branch names as 'tags'
 	// for selecting a version.  This lets people have
-	// a go.release.r60 branch and a go.1 branch
+	// a go.release.r60 branch and a go1 branch
 	// and make changes in both, without constantly
 	// editing .hgtags.
 	tagCmd: []tagCmd{