rdblue commented on code in PR #4578:
URL: https://github.com/apache/iceberg/pull/4578#discussion_r884929386


##########
core/src/main/java/org/apache/iceberg/RemoveSnapshots.java:
##########
@@ -161,21 +174,89 @@ public List<Snapshot> apply() {
 
   private TableMetadata internalApply() {
     this.base = ops.refresh();
+    if (base.snapshots().isEmpty()) {
+      return base;
+    }
 
     Set<Long> idsToRetain = Sets.newHashSet();
-    List<Long> ancestorIds = SnapshotUtil.ancestorIds(base.currentSnapshot(), 
base::snapshot);
-    if (minNumSnapshots >= ancestorIds.size()) {
-      idsToRetain.addAll(ancestorIds);
-    } else {
-      idsToRetain.addAll(ancestorIds.subList(0, minNumSnapshots));
+    // Identify refs that should be removed
+    Map<String, SnapshotRef> retainedRefs = computeRetainedRefs(base.refs());
+    Map<Long, List<String>> retainedIdToRefs = Maps.newHashMap();
+    for (Map.Entry<String, SnapshotRef> retainedRefEntry : 
retainedRefs.entrySet()) {
+      long snapshotId = retainedRefEntry.getValue().snapshotId();
+      retainedIdToRefs.putIfAbsent(snapshotId, Lists.newArrayList());
+      retainedIdToRefs.get(snapshotId).add(retainedRefEntry.getKey());
+    }
+
+    for (long idToRemove : idsToRemove) {
+      List<String> refsForId = retainedIdToRefs.get(idToRemove);
+      Preconditions.checkArgument(refsForId == null,
+          "Cannot expire %s. Still referenced by refs: %s", idToRemove, 
refsForId);
     }
 
-    TableMetadata updateMeta = base.removeSnapshotsIf(snapshot ->
-        idsToRemove.contains(snapshot.snapshotId()) ||
-        (snapshot.timestampMillis() < expireOlderThan && 
!idsToRetain.contains(snapshot.snapshotId())));
-    List<Snapshot> updateSnapshots = updateMeta.snapshots();
-    List<Snapshot> baseSnapshots = base.snapshots();
-    return updateSnapshots.size() != baseSnapshots.size() ? updateMeta : base;
+    idsToRetain.addAll(retainedIdToRefs.keySet());
+    Set<Long> branchSnapshotsToRetain = 
computeAllBranchSnapshotsToRetain(retainedRefs);
+    idsToRetain.addAll(branchSnapshotsToRetain);
+    TableMetadata.Builder updatedMetaBuilder = TableMetadata.buildFrom(base);
+    idsToRemove.addAll(base.snapshots().stream().map(Snapshot::snapshotId)
+        .filter(snapshot -> 
!idsToRetain.contains(snapshot)).collect(Collectors.toSet()));
+    base.refs().keySet().stream().filter(ref -> 
!retainedRefs.containsKey(ref)).forEach(updatedMetaBuilder::removeRef);
+    updatedMetaBuilder.removeSnapshots(idsToRemove);

Review Comment:
   This constructs `idsToRemove`, then removes refs, and then uses 
`idsToRemove`. Removing IDs should happen without mixing other tasks in 
between. I also would add some whitespace to separate ideas like removing 
snapshots and removing refs.



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