cspann: check level of root partition in delete vector fixup

Dangling vectors should only exist in leaf partitions. However, the
root partition's level changes when it's split. If there is a pending
delete vector fixup for a partition that gets split, this results in
a RemoveFromPartition call with the wrong level (e.g. LeafLevel when
the partition's level is now SecondLevel). This causes an assert in
the memstore, which expects the correct level.

The fix is to get the root partition's level when performing a
delete vector fixup.

Epic: CRDB-42943

Release note: None

Author: andy-kimball

pkg/sql/vecindex/cspann/fixup_worker.go

@@ -799,6 +799,19 @@ func (fw *fixupWorker) deleteVector(
 		return nil
 	}
 
+	// If removing from a root partition, check that it's level is LeafLevel. It
+	// might not be if it was recently split.
+	if partitionKey == RootKey {
+		metadata, err := fw.txn.GetPartitionMetadata(
+			ctx, fw.treeKey, partitionKey, false /* forUpdate */)
+		if metadata.Level != LeafLevel {
+			// Root partition's level has been updated, so just abort.
+			return nil
+		} else if err != nil {
+			return errors.Wrapf(err, "getting root partition's level")
+		}
+	}
+
 	err = fw.index.removeFromPartition(ctx, fw.txn, fw.treeKey, partitionKey, LeafLevel, childKey)
 	if errors.Is(err, ErrPartitionNotFound) {
 		log.VEventf(ctx, 2, "partition %d no longer exists, do not delete vector", partitionKey)

pkg/sql/vecindex/cspann/testdata/delete.ddt

@@ -230,6 +230,55 @@ vec1: (1, 2)
     ├───• vec4 (5, 5)
     └───• vec2 (7, 4)
 
+# ----------------------------------------------------------------------
+# Delete vector from root partition just after a split.
+# ----------------------------------------------------------------------
+
+# Load root partition that exceeds the max partition size.
+load-index min-partition-size=1 max-partition-size=3 beam-size=2
+• 1 (0, 0)
+│
+├───• vec1 (1, 2)
+├───• vec2 (7, 4)
+├───• vec3 (4, 3)
+└───• vec4 (5, 5)
+----
+Loaded 4 vectors.
+
+# Delete vector from primary index, but not from secondary index.
+delete not-found
+vec3
+----
+• 1 (0, 0)
+│
+├───• vec1 (1, 2)
+├───• vec2 (7, 4)
+├───• vec3 (MISSING)
+└───• vec4 (5, 5)
+
+# Search triggers a split, followed by an attempt to delete the missing vector.
+# However, the split changes the root partition level, so the fixup should
+# abort.
+search max-results=1
+(4, 3)
+----
+vec4: 5 (centroid=7.07)
+4 leaf vectors, 4 vectors, 4 full vectors, 1 partitions
+
+# Split should detect missing vector and remove it.
+format-tree
+----
+• 1 (3.5, 3.25)
+│
+├───• 2 (6, 4.5)
+│   │
+│   ├───• vec4 (5, 5)
+│   └───• vec2 (7, 4)
+│
+└───• 3 (1, 2)
+    │
+    └───• vec1 (1, 2)
+
 # ----------------------------------------------------------------------
 # Search root partition with only missing vectors.
 # ----------------------------------------------------------------------