libct/cg/sd: stop using regex, fix systemdVersionAtoi

Rewrite systemdVersionAtoi to not use regexp, and fix two issues:

1. It was returning 0 (rather than -1) for some errors.

2. The comment was saying that the input string is without quotes,
   while in fact it is.

Note the new function, similar to the old one, works on input either
with or without quotes. Amend the test to add test cases without quotes.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/common.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"math"
 	"os"
-	"regexp"
 	"strconv"
 	"strings"
 	"sync"
@@ -231,18 +230,22 @@ func systemdVersion(cm *dbusConnManager) int {
 	return version
 }
 
-func systemdVersionAtoi(verStr string) (int, error) {
-	// verStr should be of the form:
-	// "v245.4-1.fc32", "245", "v245-1.fc32", "245-1.fc32" (without quotes).
-	// The result for all of the above should be 245.
-	// Thus, we unconditionally remove the "v" prefix
-	// and then match on the first integer we can grab.
-	re := regexp.MustCompile(`v?([0-9]+)`)
-	matches := re.FindStringSubmatch(verStr)
-	if len(matches) < 2 {
-		return 0, fmt.Errorf("can't parse version %s: incorrect number of matches %v", verStr, matches)
+// systemdVersionAtoi extracts a numeric systemd version from the argument.
+// The argument should be of the form: "v245.4-1.fc32", "245", "v245-1.fc32",
+// "245-1.fc32" (with or without quotes). The result for all of the above
+// should be 245.
+func systemdVersionAtoi(str string) (int, error) {
+	// Unconditionally remove the leading prefix ("v).
+	str = strings.TrimLeft(str, `"v`)
+	// Match on the first integer we can grab.
+	for i := 0; i < len(str); i++ {
+		if str[i] < '0' || str[i] > '9' {
+			// First non-digit: cut the tail.
+			str = str[:i]
+			break
+		}
 	}
-	ver, err := strconv.Atoi(matches[1])
+	ver, err := strconv.Atoi(str)
 	if err != nil {
 		return -1, fmt.Errorf("can't parse version: %w", err)
 	}

libcontainer/cgroups/systemd/systemd_test.go

@@ -35,8 +35,11 @@ func TestSystemdVersion(t *testing.T) {
 		{`"v245.4-1.fc32"`, 245, false},
 		{`"241-1"`, 241, false},
 		{`"v241-1"`, 241, false},
-		{"NaN", 0, true},
-		{"", 0, true},
+		{`333.45"`, 333, false},
+		{`v321-0`, 321, false},
+		{"NaN", -1, true},
+		{"", -1, true},
+		{"v", -1, true},
 	}
 	for _, sdTest := range systemdVersionTests {
 		ver, err := systemdVersionAtoi(sdTest.verStr)