Truncate long service names to 63 characters

In some environments, the combination of cluster name and hostname
can get long enough that a service name exceeds 63 characters. This
is not allowed by the dns library, so we need to ensure that we never
pass it such long names.

This change shortens the name by hashing it to 32 characters, then
sandwiching that between the first 15 and last 16 characters of the
original name. This way the likely unique parts of the name are
preserved, but the total length of the name is shortened to an
allowable length. Something similar to kubernetes->k8s.

pkg/publisher/publisher.go

@@ -1,6 +1,7 @@
 package publisher
 
 import (
+	"crypto/sha256"
 	"fmt"
 	"net"
 	"sync"
@@ -14,7 +15,8 @@ var log = logrus.New()
 
 func Publish(ip net.IP, iface net.Interface, service Service, shutdown chan struct{}, waitGroup *sync.WaitGroup) (err error) {
 	defer waitGroup.Done()
-	svcEntry := zeroconf.NewServiceEntry(service.Name, service.SvcType, service.Domain)
+	svcName := truncateLongServiceName(service.Name)
+	svcEntry := zeroconf.NewServiceEntry(svcName, service.SvcType, service.Domain)
 	svcEntry.Port = service.Port
 	if ip.To4() != nil {
 		svcEntry.AddrIPv4 = append(svcEntry.AddrIPv4, ip)
@@ -105,6 +107,22 @@ func SetLogLevel(level logrus.Level) {
 	log.SetLevel(level)
 }
 
+// Service names cannot be longer than 63 characters. If we get a service name
+// that is longer, hash it to 32 characters, and sandwich that between the
+// first 15 and last 16 characters of the original name. This way we preserve
+// the likely unique parts of the name while ensuring it does not exceed
+// the 63 character limit.
+func truncateLongServiceName(serviceName string) (svcName string) {
+	svcName = serviceName
+	if len(svcName) > 63 {
+		value := sha256.Sum256([]byte(svcName))
+		hexValue := fmt.Sprintf("%x", value)[:32]
+		svcName = svcName[:15] + string(hexValue[:]) + svcName[len(svcName)-16:]
+		log.Printf("Truncating long service name '%s' to '%s'", serviceName, svcName)
+	}
+	return svcName
+}
+
 // networkInterfacer defines an interface for several net library functions. Production
 // code will forward to net library functions, and unit tests will override the methods
 // for testing purposes.

pkg/publisher/publisher_test.go

@@ -163,3 +163,23 @@ func (networkInterfaceWithInvalidAddr) Addrs(intf *net.Interface) ([]net.Addr, e
 func (networkInterfaceWithInvalidAddr) Interfaces() ([]net.Interface, error) {
 	return []net.Interface{upIntf}, nil
 }
+
+func TestLongNameTruncation(t *testing.T) {
+	testCases := []struct {
+		tcase    string
+		name     string
+		expected string
+	}{
+		{"short name", "master-0", "master-0"},
+		{"long name", "prefix-someverylongservicenamethatwouldcauseaproblemfordns-suffix", "prefix-somevery47540ff49304816bc5054bb4f3cc9a9clemfordns-suffix"},
+	}
+	for _, tc := range testCases {
+		result := truncateLongServiceName(tc.name)
+		if result != tc.expected {
+			t.Errorf("case[%v]: expected %v, got %v", tc.tcase, tc.expected, result)
+		}
+		if len(result) > 63 {
+			t.Errorf("case[%v]: Truncated name too long: '%v' is %d characters long", tc.tcase, result, len(result))
+		}
+	}
+}