sql/catalog/descs: don't hydrate dropped tables

The invariant that types referenced by tables only exists for non-dropped
tables. We were not checking the state of the table when choosing to hydrate.
This lead to pretty catastropic failures when the invariant was violated.

Fixes #54343.

Release note (bug fix): Fixed bug from earlier alphas where dropping a database
which contained tables using user-defined types could result in panics.

pkg/sql/catalog/descs/collection.go

@@ -1083,13 +1083,17 @@ func (tc *Collection) resolveSchemaByID(
 // hydrateTypesInTableDesc installs user defined type metadata in all types.T
 // present in the input TableDescriptor. It always returns the same type of
 // TableDescriptor that was passed in. It ensures that ImmutableTableDescriptors
-// are not modified during the process of metadata installation.
+// are not modified during the process of metadata installation. Dropped tables
+// do not get hydrated.
 //
 // TODO(ajwerner): This should accept flags to indicate whether we can resolve
 // offline descriptors.
 func (tc *Collection) hydrateTypesInTableDesc(
 	ctx context.Context, txn *kv.Txn, desc catalog.TableDescriptor,
 ) (catalog.TableDescriptor, error) {
+	if desc.Dropped() {
+		return desc, nil
+	}
 	switch t := desc.(type) {
 	case *tabledesc.Mutable:
 		// It is safe to hydrate directly into Mutable since it is
@@ -1602,6 +1606,10 @@ func HydrateGivenDescriptors(ctx context.Context, descs []catalog.Descriptor) er
 		// Now hydrate all table descriptors.
 		for i := range descs {
 			desc := descs[i]
+			// Never hydrate dropped descriptors.
+			if desc.Dropped() {
+				continue
+			}
 			if tblDesc, ok := desc.(*tabledesc.Immutable); ok {
 				if err := typedesc.HydrateTypesInTableDescriptor(
 					ctx,

pkg/sql/logictest/testdata/logic_test/enums

@@ -1202,3 +1202,21 @@ INSERT INTO table_with_not_null_enum_no_vals VALUES (1);
 statement ok
 ROLLBACK; DROP TABLE table_with_not_null_enum_no_vals; DROP TYPE enum_with_no_vals;
 
+# Regression test that hydrating descriptors does not happen on dropped
+# descriptors. See #54343.
+
+subtest dropped_database_with_enum
+
+statement ok
+CREATE DATABASE to_drop;
+USE to_drop;
+CREATE TYPE greeting AS ENUM ('hi');
+CREATE TABLE t(a greeting);
+USE defaultdb;
+DROP DATABASE to_drop CASCADE;
+
+# Before the bug-fix which introduced this test, this call would load all
+# descriptors, including dropped ones, and hydrate them, causing a panic as
+# the referenced type no longer exists.
+statement ok
+SELECT * FROM crdb_internal.tables;