NullPointerException thrown in monitor when visiting the External Compactions page

[DomGarguilo]

Describe the bug
When requesting external compaction data from the monitor server, the following code (added in ) is used to memoize and return that data:

accumulo/server/monitor/src/main/java/org/apache/accumulo/monitor/Monitor.java

Lines 686 to 711 in 767a68a

	private static class ExternalCompactionsSnapshot {
	public final RunningCompactions runningCompactions;
	public final Map<String,TExternalCompaction> ecRunningMap;
	private ExternalCompactionsSnapshot(Map<String,TExternalCompaction> ecRunningMap) {
	this.ecRunningMap = Collections.unmodifiableMap(ecRunningMap);
	this.runningCompactions = new RunningCompactions(ecRunningMap);
	}
	}
	private ExternalCompactionsSnapshot computeExternalCompactionsSnapshot() {
	if (coordinatorHost.isEmpty()) {
	throw new IllegalStateException(coordinatorMissingMsg);
	}
	var ccHost = coordinatorHost.orElseThrow();
	log.info("User initiated fetch of running External Compactions from " + ccHost);
	var client = getCoordinator(ccHost);
	TExternalCompactionList running;
	try {
	running = client.getRunningCompactions(TraceUtil.traceInfo(), getContext().rpcCreds());
	} catch (Exception e) {
	throw new IllegalStateException("Unable to get running compactions from " + ccHost, e);
	}
	return new ExternalCompactionsSnapshot(running.getCompactions());
	}

This code was added in #4867.

Versions (OS, Maven, Java, and others, as appropriate):

• Affected version(s) of this project: 2.1 and up

To Reproduce
Steps to reproduce the behavior (or a link to an example repository that reproduces the problem):

1. Start an instance of accumulo
2. Go to the external compactions page
3. look at the monitors logs

Here are some portions of the logs:

2024-11-22T15:21:46,486 [monitor.Monitor] INFO : User initiated fetch of running External Compactions from localhost:9132
2024-11-22T15:21:46,486 [external.ECResource] INFO : Got coordinator from monitor = Optional[localhost:9132]
2024-11-22T15:21:46,487 [server.HttpChannel] WARN : /rest/ec/running
jakarta.servlet.ServletException: java.lang.NullPointerException: Cannot invoke "Object.getClass()" because "m" is null
	at org.glassfish.jersey.servlet.WebComponent.serviceImpl(WebComponent.java:410) ~[jersey-container-servlet-core-3.0.9.jar:?]
	at org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:346) ~[jersey-container-servlet-core-3.0.9.jar:?]
	at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:358) ~[jersey-container-servlet-core-3.0.9.jar:?]
	at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:311) ~[jersey-container-servlet-core-3.0.9.jar:?]
...
Caused by: java.lang.NullPointerException: Cannot invoke "Object.getClass()" because "m" is null
	at java.base/java.util.Collections.unmodifiableMap(Collections.java:1476) ~[?:?]
	at org.apache.accumulo.monitor.Monitor$ExternalCompactionsSnapshot.<init>(Monitor.java:691) ~[accumulo-monitor-2.1.4-SNAPSHOT.jar:2.1.4-SNAPSHOT]
	at org.apache.accumulo.monitor.Monitor.computeExternalCompactionsSnapshot(Monitor.java:710) ~[accumulo-monitor-2.1.4-SNAPSHOT.jar:2.1.4-SNAPSHOT]
	at org.apache.accumulo.monitor.Monitor.lambda$new$1(Monitor.java:635) ~[accumulo-monitor-2.1.4-SNAPSHOT.jar:2.1.4-SNAPSHOT]
	at com.google.common.base.Suppliers$ExpiringMemoizingSupplier.get(Suppliers.java:261) ~[guava-33.0.0-jre.jar:?]
	at org.apache.accumulo.monitor.Monitor.getRunnningCompactions(Monitor.java:714) ~[accumulo-monitor-2.1.4-SNAPSHOT.jar:2.1.4-SNAPSHOT]
	at org.apache.accumulo.monitor.rest.compactions.external.ECResource.getRunning(ECResource.java:63) ~[accumulo-monitor-2.1.4-SNAPSHOT.jar:2.1.4-SNAPSHOT]
...

[DomGarguilo]

The following makes it so the exception is not thrown but I'm not sure if this is the best solution:

diff --git a/server/monitor/src/main/java/org/apache/accumulo/monitor/Monitor.java b/server/monitor/src/main/java/org/apache/accumulo/monitor/Monitor.java
index 80dd69b8cd..bd5111af47 100644
--- a/server/monitor/src/main/java/org/apache/accumulo/monitor/Monitor.java
+++ b/server/monitor/src/main/java/org/apache/accumulo/monitor/Monitor.java
@@ -688,7 +688,7 @@ public class Monitor extends AbstractServer implements HighlyAvailableService {
     public final Map<String,TExternalCompaction> ecRunningMap;
 
     private ExternalCompactionsSnapshot(Map<String,TExternalCompaction> ecRunningMap) {
-      this.ecRunningMap = Collections.unmodifiableMap(ecRunningMap);
+      this.ecRunningMap = ecRunningMap == null ? Collections.emptyMap() : Collections.unmodifiableMap(ecRunningMap);
       this.runningCompactions = new RunningCompactions(ecRunningMap);
     }
   }

[cshannon]

Looking at the code, that map having a null is valid so I think simply checking for null like the above example (or maybe using Optional) and just setting it to an immutable map should be fine in this case.