Regression: Kubelet fails on older distro Dockers

Changes broke compatibility with released versions of Docker on some
distributions like Fedora and RHEL (value 1.8.1.fc21 is in the wild).

pkg/kubelet/dockertools/manager.go

@@ -25,6 +25,7 @@ import (
 	"os"
 	"os/exec"
 	"path"
+	"regexp"
 	"strconv"
 	"strings"
 	"sync"
@@ -780,24 +781,33 @@ type dockerVersion struct {
 	*semver.Version
 }
 
+// Older versions of Docker could return non-semantically versioned values (distros like Fedora
+// included partial values such as 1.8.1.fc21 which is not semver). Force those values to be semver.
+var almostSemverRegexp = regexp.MustCompile(`^(\d+\.\d+\.\d+)\.(.*)$`)
+
+// newDockerVersion returns a semantically versioned docker version value
 func newDockerVersion(version string) (dockerVersion, error) {
 	sem, err := semver.NewVersion(version)
 	if err != nil {
-		return dockerVersion{}, err
+		matches := almostSemverRegexp.FindStringSubmatch(version)
+		if matches == nil {
+			return dockerVersion{}, err
+		}
+		sem, err = semver.NewVersion(strings.Join(matches[1:], "-"))
 	}
-	return dockerVersion{sem}, nil
+	return dockerVersion{sem}, err
 }
 
 func (r dockerVersion) Compare(other string) (int, error) {
-	v, err := semver.NewVersion(other)
+	v, err := newDockerVersion(other)
 	if err != nil {
 		return -1, err
 	}
 
-	if r.LessThan(*v) {
+	if r.LessThan(*v.Version) {
 		return -1, nil
 	}
-	if v.LessThan(*r.Version) {
+	if v.Version.LessThan(*r.Version) {
 		return 1, nil
 	}
 	return 0, nil

pkg/kubelet/dockertools/manager_test.go

@@ -115,6 +115,35 @@ func matchString(t *testing.T, pattern, str string) bool {
 	return match
 }
 
+func TestNewDockerVersion(t *testing.T) {
+	cases := []struct {
+		value string
+		out   string
+		err   bool
+	}{
+		{value: "1", err: true},
+		{value: "1.8", err: true},
+		{value: "1.8.1", out: "1.8.1"},
+		{value: "1.8.1.fc21", out: "1.8.1-fc21"},
+		{value: "1.8.1.fc21.other", out: "1.8.1-fc21.other"},
+		{value: "1.8.1-fc21.other", out: "1.8.1-fc21.other"},
+		{value: "1.8.1-beta.12", out: "1.8.1-beta.12"},
+	}
+	for _, test := range cases {
+		v, err := newDockerVersion(test.value)
+		switch {
+		case err != nil && test.err:
+			continue
+		case (err != nil) != test.err:
+			t.Errorf("error for %q: expected %t, got %v", test.value, test.err, err)
+			continue
+		}
+		if v.String() != test.out {
+			t.Errorf("unexpected parsed version %q for %q", v, test.value)
+		}
+	}
+}
+
 func TestSetEntrypointAndCommand(t *testing.T) {
 	cases := []struct {
 		name      string