sql: drop view cascade does not handle deps being dropped earlier

Fixes: cockroachdb#68600

Previously, drop view cascade would only check if a dependency
was dropped when initially going over the list. It did not check
before executing the drop logic, so an earlier dependency could
cause the current object to move to a dropped state. This was
inadequate because cascaded drops of views could fail due to this
reason. To address this, this patch skips dropping dependencies
if at a later stage they are found to be dropped.

Release note (bug fix): Cascaded drop of views could run into
'table ...is already being dropped' errors incorrectly.

pkg/sql/drop_view.go

@@ -236,13 +236,16 @@ func (p *planner) dropViewImpl(
  if err != nil {
  return cascadeDroppedViews, err
  }
-
- cascadedViews, err := p.dropViewImpl(ctx, dependentDesc, queueJob, "dropping dependent view", behavior)
- if err != nil {
- return cascadeDroppedViews, err
+ // Check if the dependency was already marked as dropped,
+ // while dealing with any earlier dependent views.
+ if !dependentDesc.Dropped() {
+ cascadedViews, err := p.dropViewImpl(ctx, dependentDesc, queueJob, "dropping dependent view", behavior)
+ if err != nil {
+ return cascadeDroppedViews, err
+ }
+ cascadeDroppedViews = append(cascadeDroppedViews, cascadedViews...)
+ cascadeDroppedViews = append(cascadeDroppedViews, qualifiedView.FQString())
  }
- cascadeDroppedViews = append(cascadeDroppedViews, cascadedViews...)
- cascadeDroppedViews = append(cascadeDroppedViews, qualifiedView.FQString())
  }
  }
 

pkg/sql/logictest/testdata/logic_test/views

@@ -1202,3 +1202,30 @@ query T
 SELECT * FROM v15
 ----
 b
+
+subtest view-cascade-nesting
+
+statement ok
+CREATE TABLE t1nest (id INT PRIMARY KEY, name varchar(256))
+
+statement ok
+CREATE VIEW v1nest AS (SELECT name FROM t1nest)
+
+statement ok
+CREATE VIEW v2nest AS (SELECT name AS n1, name AS n2 FROM v1nest)
+
+statement ok
+CREATE VIEW v3nest AS (SELECT name, n1 FROM v1nest, v2nest);
+
+statement ok
+DROP table t1nest CASCADE
+
+# Validate the objects being dropped
+query IT
+SELECT "reportingID", info::JSONB - 'Timestamp' - 'DescriptorID'
+FROM system.eventlog
+WHERE "eventType" in ('drop_view', 'drop_table')
+ORDER BY "timestamp" DESC, info
+LIMIT 1
+----
+ 1 {"CascadeDroppedViews": ["db2.public.v3nest", "db2.public.v2nest", "db2.public.v1nest"], "EventType": "drop_table", "Statement": "DROP TABLE db2.public.t1nest CASCADE", "TableName": "db2.public.t1nest", "Tag": "DROP TABLE", "User": "root"}