kevinrr888 commented on code in PR #4524:
URL: https://github.com/apache/accumulo/pull/4524#discussion_r1672930975


##########
core/src/main/java/org/apache/accumulo/core/fate/user/UserFateStore.java:
##########
@@ -151,6 +157,95 @@ protected void create(FateId fateId, FateKey fateKey) {
         + " and fateKey " + fateKey + " after " + maxAttempts + " attempts");
   }
 
+  @Override
+  public Optional<FateTxStore<T>> tryReserve(FateId fateId) {
+    // Create a unique FateReservation for this reservation attempt
+    FateReservation reservation = FateReservation.from(lockID, 
UUID.randomUUID());
+
+    FateMutator.Status status = 
newMutator(fateId).putReservedTx(reservation).tryMutate();
+    if (status.equals(FateMutator.Status.ACCEPTED)) {
+      return Optional.of(new FateTxStoreImpl(fateId, reservation));
+    } else if (status.equals(FateMutator.Status.UNKNOWN)) {
+      // If the status is UNKNOWN, this means an error occurred after the 
mutation was
+      // sent to the TabletServer, and it is unknown if the mutation was 
written. We
+      // need to check if the mutation was written and if it was written by 
this
+      // attempt at reservation. If it was written by this reservation attempt,
+      // we can return the FateTxStore since it was successfully reserved in 
this
+      // attempt, otherwise we return empty (was written by another reservation
+      // attempt or was not written at all).
+      try (Scanner scanner = context.createScanner(tableName, 
Authorizations.EMPTY)) {
+        scanner.setRange(getRow(fateId));
+        
scanner.fetchColumn(TxColumnFamily.RESERVATION_COLUMN.getColumnFamily(),
+            TxColumnFamily.RESERVATION_COLUMN.getColumnQualifier());
+        FateReservation persistedRes = scanner.stream()
+            .filter(entry -> 
FateReservation.isFateReservation(entry.getValue().toString()))
+            .map(entry -> 
FateReservation.from(entry.getValue().toString())).findFirst()

Review Comment:
   You're referring to MetaFateStore where I have that pattern correct? I don't 
think UserFateStore would have this issue but I could be wrong



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