cells: ignore empty core domain uris propagated by zk

Motivation:
When Zookeeper updates core domain infos, dCache will first kill the existing cell tunnels
and then later try to read and parse the new value. If the new value is an empty
string (for whatever reason), parsing will fail, but a new connection will not be
established. The corresponding error in the log:

18 Nov 2024 08:45:00 (c-dcache-head-xxx03_messageDomain-AAYmVA1LtnA-AAYmVA16phA) [dcache-head-xxx03_messageDomain,9.2.21,CORE] Error while reading from tunnel: java.net.SocketExceptio>
18 Nov 2024 08:45:43 (c-dcache-head-xxx03_messageDomain-AAYnKxn40fA) [] Uncaught exception in thread TunnelConnector-dcache-head-xxx03_messageDomain
java.lang.NullPointerException: null
        at java.base/java.net.Socket.<init>(Socket.java:448)
        at java.base/java.net.Socket.<init>(Socket.java:264)
        at java.base/javax.net.DefaultSocketFactory.createSocket(SocketFactory.java:277)
        at dmg.cells.network.LocationManagerConnector.connect(LocationManagerConnector.java:64)
        at dmg.cells.network.LocationManagerConnector.run(LocationManagerConnector.java:94)
        at dmg.cells.nucleus.CellNucleus.lambda$wrapLoggingContext$2(CellNucleus.java:725)
        at java.base/java.lang.Thread.run(Thread.java:829)

Modification:
before killing existing tunnel check that ZK didn't propagate empty
data.

Result:
More roust cell communication

NOTE: a non empty invalid data still accepted!!!

Fixes: #7696
Acked-by: Lea Morschel
Target: master, 10.2, 10.1, 10.0, 9.2
Require-book: no
Require-notes: yes
(cherry picked from commit 30829c9)
Signed-off-by: Tigran Mkrtchyan <[email protected]>

modules/cells/src/main/java/dmg/cells/services/LocationManager.java

@@ -225,7 +225,7 @@ public CoreDomainInfo(byte[] bytes) {
                 }
             } catch (IOException ie) {
                 throw new IllegalArgumentException(
-                      "Failed deserializing LocationManager Cores as uri: {}", ie.getCause());
+                      "Failed deserializing LocationManager Cores as uri", ie);
             }
         }
 
@@ -513,6 +513,10 @@ public void close() {
         public void reset(Mode mode, State state) {
         }
 
+        private boolean hasNoData(ChildData data) {
+            return data == null || data.getData() == null || data.getData().length == 0;
+        }
+
         public void update(PathChildrenCacheEvent event) {
             LOGGER.info("{}", event);
             String cell;
@@ -525,12 +529,22 @@ public void update(PathChildrenCacheEvent event) {
                     }
                     break;
                 case CHILD_UPDATED:
-                    cell = connectors.remove(ZKPaths.getNodeFromPath(event.getData().getPath()));
+                    if (hasNoData(event.getData())) {
+                        LOGGER.warn("Ignoring empty data on UPDATED for {}", event.getData().getPath());
+                        break;
+                    }
+                    cell = connectors.remove(
+                          ZKPaths.getNodeFromPath(event.getData().getPath()));
                     if (cell != null) {
                         killConnector(cell);
                     }
                     // fall through
                 case CHILD_ADDED:
+                    if (hasNoData(event.getData())) {
+                        LOGGER.warn("Ignoring empty data on ADDED for {}", event.getData().getPath());
+                        break;
+                    }
+
                     //Log if the Core Domain Information received is incompatible with previous
                     CoreDomainInfo info = infoFromZKEvent(event);
                     String domain = ZKPaths.getNodeFromPath(event.getData().getPath());