runtime: add a simple version number parser

This will be used to parse the Linux kernel versions, but this code is
generic and can be tested on its own.

For #35777.

Change-Id: If1df48d07250e5855dde45bc9d57c66f777b9fb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/209597
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/export_test.go

@@ -43,6 +43,8 @@ var PhysHugePageSize = physHugePageSize
 
 var NetpollGenericInit = netpollGenericInit
 
+var ParseRelease = parseRelease
+
 const PreemptMSupported = preemptMSupported
 
 type LFNode struct {

src/runtime/string.go

@@ -495,3 +495,37 @@ func gostringw(strw *uint16) string {
 	b[n2] = 0 // for luck
 	return s[:n2]
 }
+
+// parseRelease parses a dot-separated version number. It follows the
+// semver syntax, but allows the minor and patch versions to be
+// elided.
+func parseRelease(rel string) (major, minor, patch int, ok bool) {
+	// Strip anything after a dash or plus.
+	for i := 0; i < len(rel); i++ {
+		if rel[i] == '-' || rel[i] == '+' {
+			rel = rel[:i]
+			break
+		}
+	}
+
+	next := func() (int, bool) {
+		for i := 0; i < len(rel); i++ {
+			if rel[i] == '.' {
+				ver, ok := atoi(rel[:i])
+				rel = rel[i+1:]
+				return ver, ok
+			}
+		}
+		ver, ok := atoi(rel)
+		rel = ""
+		return ver, ok
+	}
+	if major, ok = next(); !ok || rel == "" {
+		return
+	}
+	if minor, ok = next(); !ok || rel == "" {
+		return
+	}
+	patch, ok = next()
+	return
+}

src/runtime/string_test.go

@@ -454,3 +454,34 @@ func TestAtoi32(t *testing.T) {
 		}
 	}
 }
+
+type parseReleaseTest struct {
+	in                  string
+	major, minor, patch int
+}
+
+var parseReleaseTests = []parseReleaseTest{
+	{"", -1, -1, -1},
+	{"x", -1, -1, -1},
+	{"5", 5, 0, 0},
+	{"5.12", 5, 12, 0},
+	{"5.12-x", 5, 12, 0},
+	{"5.12.1", 5, 12, 1},
+	{"5.12.1-x", 5, 12, 1},
+	{"5.12.1.0", 5, 12, 1},
+	{"5.20496382327982653440", -1, -1, -1},
+}
+
+func TestParseRelease(t *testing.T) {
+	for _, test := range parseReleaseTests {
+		major, minor, patch, ok := runtime.ParseRelease(test.in)
+		if !ok {
+			major, minor, patch = -1, -1, -1
+		}
+		if test.major != major || test.minor != minor || test.patch != patch {
+			t.Errorf("parseRelease(%q) = (%v, %v, %v) want (%v, %v, %v)",
+				test.in, major, minor, patch,
+				test.major, test.minor, test.patch)
+		}
+	}
+}