pool: include change explanation in replica monitoring events

Motivation:

EntryChangeEvent objects are sent to registered listeners when either a
replica's state changes, or when sticky flags are changed.  It is useful
to include the context to these changes to provide better logging.

Modification:

Add a why field member in EntryChangeEvent.  Update constructors to
populate this information.

Result:

No user or admin observable change, but EntryChangeEvent objects now
include an explanation describing why a change took place.

Target: master
Requires-notes: no
Requires-book: no
Patch: https://rb.dcache.org/r/11654/
Acked-by: Albert Rossi

modules/dcache/src/main/java/org/dcache/pool/repository/EntryChangeEvent.java

@@ -10,11 +10,13 @@ public class EntryChangeEvent
 {
     protected final CacheEntry oldEntry;
     protected final CacheEntry newEntry;
+    protected final String why;
 
-    public EntryChangeEvent(CacheEntry oldEntry, CacheEntry newEntry)
+    public EntryChangeEvent(String why, CacheEntry oldEntry, CacheEntry newEntry)
     {
         this.oldEntry = oldEntry;
         this.newEntry = newEntry;
+        this.why = why;
     }
 
     public PnfsId getPnfsId()
@@ -32,6 +34,11 @@ public CacheEntry getNewEntry()
         return newEntry;
     }
 
+    public String getWhy()
+    {
+        return why;
+    }
+
     public String toString()
     {
         return String.format("EntryChangeEvent [id=%s]", getPnfsId());

modules/dcache/src/main/java/org/dcache/pool/repository/ReplicaStoreCache.java

@@ -117,7 +117,7 @@ private synchronized ReplicaRecord get()
                 }
                 CacheEntry entry = new CacheEntryImpl(_record);
                 _stateChangeListener.stateChanged(
-                        new StateChangeEvent(entry, entry, NEW, _record.getState()));
+                        new StateChangeEvent("loading CacheEntry", entry, entry, NEW, _record.getState()));
             }
             return this;
         }
@@ -151,7 +151,7 @@ private void destroy(String why)
                 _inner.remove(_id);
                 _entries.remove(_id);
                 _stateChangeListener.stateChanged(
-                        new StateChangeEvent(entry, entry, entry.getState(), DESTROYED));
+                        new StateChangeEvent(why, entry, entry, entry.getState(), DESTROYED));
             } catch (DiskErrorCacheException | RuntimeException e) {
                 _faultListener.faultOccurred(
                         new FaultEvent("repository", FaultAction.DEAD, "Internal repository error", e));
@@ -318,7 +318,7 @@ public synchronized Collection<StickyRecord> removeExpiredStickyFlags() throws C
                 Collection<StickyRecord> removed = _record.removeExpiredStickyFlags();
                 if (!removed.isEmpty()) {
                     CacheEntryImpl newEntry = new CacheEntryImpl(_record);
-                    _stateChangeListener.stickyChanged(new StickyChangeEvent(oldEntry, newEntry));
+                    _stateChangeListener.stickyChanged(new StickyChangeEvent("sticky has expired", oldEntry, newEntry));
                 }
                 return removed;
             } catch (RuntimeException | DiskErrorCacheException e) {
@@ -357,7 +357,7 @@ public boolean setSticky(String owner, long validTill,
                                         if (changed) {
                                             CacheEntryImpl newEntry = new CacheEntryImpl(_record);
                                             _stateChangeListener.stickyChanged(
-                                                    new StickyChangeEvent(oldEntry, newEntry));
+                                                    new StickyChangeEvent(why, oldEntry, newEntry));
                                         }
                                         return changed;
                                     }
@@ -370,7 +370,7 @@ public Void setState(ReplicaState state) throws CacheException
                                             r.setState(state);
                                             CacheEntry newEntry = new CacheEntryImpl(_record);
                                             _stateChangeListener.stateChanged(
-                                                    new StateChangeEvent(oldEntry, newEntry, oldEntry.getState(),
+                                                    new StateChangeEvent(why, oldEntry, newEntry, oldEntry.getState(),
                                                                          newEntry.getState()));
                                         }
                                         return null;

modules/dcache/src/main/java/org/dcache/pool/repository/StateChangeEvent.java

@@ -9,9 +9,9 @@ public class StateChangeEvent extends EntryChangeEvent
     private final ReplicaState _oldState;
     private final ReplicaState _newState;
 
-    public StateChangeEvent(CacheEntry oldEntry, CacheEntry newEntry, ReplicaState oldState, ReplicaState newState)
+    public StateChangeEvent(String why, CacheEntry oldEntry, CacheEntry newEntry, ReplicaState oldState, ReplicaState newState)
     {
-        super(oldEntry, newEntry);
+        super(why, oldEntry, newEntry);
         _oldState = oldState;
         _newState = newState;
     }

modules/dcache/src/main/java/org/dcache/pool/repository/StickyChangeEvent.java

@@ -6,9 +6,9 @@
  */
 public class StickyChangeEvent extends EntryChangeEvent
 {
-    public StickyChangeEvent(CacheEntry oldEntry, CacheEntry newEntry)
+    public StickyChangeEvent(String why, CacheEntry oldEntry, CacheEntry newEntry)
     {
-        super(oldEntry, newEntry);
+        super(why, oldEntry, newEntry);
     }
 
     public String toString()