pool: stop automatic scrubber on repository read-only event

Motivation:
The external pool check can indicate that the pool repository should be
operated in a read-only mode. Such even can be triggered when under
laying filesystem run internal rebuild due to disk failure in a RAID
set. To reduce the load on the disk subsystem, it makes sense to disable
automatic scrub process. The admin still able do force checksum
validation.

Modification:
Update ChecksumScanner to listen repository fault events and disable
scrubber if running.

Result:
less load on pool's I/O subsystem during disk rebuilds.

```
18 Mar 2026 14:16:20 (pool_write) [] Read-only file system
18 Mar 2026 14:16:20 (pool_write) [] Pool mode changed to disabled(store,stage,p2p-client): Pool read-only: I/O test failed, READ_ONLY Error
18 Mar 2026 14:16:20 (PoolManager) [pool_write PoolManagerPoolUp] Pool pool_write changed from mode enabled  to disabled(store,stage,p2p-client).
18 Mar 2026 14:16:20 (pool_write) [] Scrubber is stopping due to repository fault event: I/O test failed, READ_ONLY Error
```

Acked-by: Dmitry Litvintsev
Target: master
Require-book: no
Require-notes: yes

Author: kofemann

modules/dcache/src/main/java/org/dcache/pool/classic/ChecksumScanner.java

@@ -38,6 +38,9 @@
 import java.util.concurrent.TimeUnit;
 import org.dcache.alarms.AlarmMarkerFactory;
 import org.dcache.alarms.PredefinedAlarm;
+import org.dcache.pool.FaultAction;
+import org.dcache.pool.FaultEvent;
+import org.dcache.pool.FaultListener;
 import org.dcache.pool.repository.ReplicaDescriptor;
 import org.dcache.pool.repository.ReplicaState;
 import org.dcache.pool.repository.Repository;
@@ -47,7 +50,7 @@
 import org.slf4j.LoggerFactory;
 
 public class ChecksumScanner
-      implements CellCommandListener, CellLifeCycleAware, CellSetupProvider, CellInfoProvider {
+      implements CellCommandListener, CellLifeCycleAware, CellSetupProvider, CellInfoProvider, FaultListener {
 
     private static final Logger LOGGER =
           LoggerFactory.getLogger(ChecksumScanner.class);
@@ -93,6 +96,7 @@ private void stopScrubber() {
 
     public void setRepository(Repository repository) {
         _repository = repository;
+        _repository.addFaultListener(this);
     }
 
     public void setChecksumModule(ChecksumModuleV1 csm) {
@@ -107,6 +111,17 @@ public void setPoolName(String poolName) {
         this.poolName = poolName;
     }
 
+    @Override
+    public void faultOccurred(FaultEvent event) {
+        // If the fault is not a log-only event, stop the scrubber to avoid
+        // further stressing the underlying storage. The scrubber will be restarted
+        // when the pool recovers.
+        if (event.getAction() != FaultAction.LOG && _scrubber.isActive()) {
+            LOGGER.warn("Scrubber is stopping due to repository fault event: {}", event.getMessage());
+            stopScrubber();
+        }
+    }
+
     private class FullScan extends Singleton {
 
         private volatile int _totalCount;