dimas-b commented on code in PR #1285:
URL: https://github.com/apache/polaris/pull/1285#discussion_r2096605152


##########
service/common/src/main/java/org/apache/polaris/service/catalog/iceberg/CatalogHandlerUtils.java:
##########
@@ -433,39 +446,174 @@ protected TableMetadata commit(TableOperations ops, 
UpdateTableRequest request)
               2.0 /* exponential */)
           .onlyRetryOn(CommitFailedException.class)
           .run(
-              taskOps -> {
+              (taskOps) -> {
                 TableMetadata base = isRetry.get() ? taskOps.refresh() : 
taskOps.current();
-                isRetry.set(true);
 
-                // validate requirements
+                TableMetadata.Builder metadataBuilder = 
TableMetadata.buildFrom(base);
+                TableMetadata newBase = base;
                 try {
-                  request.requirements().forEach(requirement -> 
requirement.validate(base));
+                  request.requirements().forEach((requirement) -> 
requirement.validate(base));
                 } catch (CommitFailedException e) {
-                  // wrap and rethrow outside of tasks to avoid unnecessary 
retry
-                  throw new ValidationFailureException(e);
+                  if (!isRollbackCompactionEnabled()) {
+                    throw new ValidationFailureException(e);
+                  }
+                  UpdateRequirement.AssertRefSnapshotID assertRefSnapshotId =
+                      findAssertRefSnapshotID(request);
+                  MetadataUpdate.SetSnapshotRef setSnapshotRef = 
findSetSnapshotRefUpdate(request);
+
+                  if (assertRefSnapshotId == null || setSnapshotRef == null) {
+                    // This implies the request was not trying to add a 
snapshot.
+                    throw new ValidationFailureException(e);
+                  }
+
+                  // snapshot-id the client expects the table 
current_snapshot_id
+                  long expectedCurrentSnapshotId = 
assertRefSnapshotId.snapshotId();
+                  // table current_snapshot_id.
+                  long currentSnapshotId = base.currentSnapshot().snapshotId();
+                  List<MetadataUpdate> metadataUpdates =
+                      generateUpdatesToRemoveNoopSnapshot(
+                          base,
+                          currentSnapshotId,
+                          expectedCurrentSnapshotId,
+                          setSnapshotRef.name());
+
+                  if (metadataUpdates == null || metadataUpdates.isEmpty()) {
+                    // Nothing can be done as this implies that there were not 
all
+                    // No-op snapshots (REPLACE) between 
expectedCurrentSnapshotId and
+                    // currentSnapshotId. hence re-throw the exception caught.
+                    throw new ValidationFailureException(e);
+                  }
+
+                  // Set back the ref we wanted to set, back to the snapshot-id
+                  // the client is expecting the table to be at.
+                  metadataBuilder.setBranchSnapshot(
+                      expectedCurrentSnapshotId, setSnapshotRef.name());
+
+                  // apply the remove snapshots update in the current metadata.
+                  // NOTE: we need to setRef to expectedCurrentSnapshotId 
first and then apply
+                  // remove, as otherwise the remove will drop the reference.
+                  // NOTE: we can skip removing the now orphan base. Its not a 
hard requirement.
+                  // just something good to do, and not leave for Remove 
Orphans.
+                  metadataUpdates.forEach((update -> 
update.applyTo(metadataBuilder)));
+                  // Ref rolled back update correctly to snapshot to be 
committed parent now.
+                  newBase = metadataBuilder.build();
+                  // move the lastSequenceNumber back, to apply snapshot 
properly on the
+                  // current-metadata Seq number are considered increasing 
monotonically
+                  // snapshot over snapshot, the client generates the manifest 
list and hence
+                  // the sequence number can't be changed for a snapshot the 
only possible option
+                  // then is to change the sequenceNumber tracked by 
metadata.json
+                  Class<?> clazz = newBase.getClass();
+                  try {
+                    Field field = clazz.getDeclaredField("lastSequenceNumber");
+                    field.setAccessible(true);

Review Comment:
   Well, my interpretation is this is that Iceberg code is not really reusable 
for our purpose here :shrug: 
   
   From my POV, dealing with metadata as JSON (or having a Polaris-specific 
POJO for it) is preferable to tweaking `private` fields in Iceberg code.



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