fix error when dropping view in auto-migration (#3836)

# Description of Changes

Fixes the following issues:
1. When dropping a view, we deleted its row from `st_view`, but didn't
drop the backing table.
2. `delete_col_eq` returned a nonsensical error if the delete set was
empty.

# API and ABI breaking changes

None

# Expected complexity level and risk

0

# Testing

- [x] Auto-migrate smoketests

Author: joshua-spacetime

crates/datastore/src/locking_tx_datastore/mut_tx.rs

@@ -465,9 +465,6 @@ impl MutTxId {
     fn delete_col_eq(&mut self, table_id: TableId, col_pos: ColId, value: &AlgebraicValue) -> Result<()> {
         let rows = self.iter_by_col_eq(table_id, col_pos, value)?;
         let ptrs_to_delete = rows.map(|row_ref| row_ref.pointer()).collect::<Vec<_>>();
-        if ptrs_to_delete.is_empty() {
-            return Err(TableError::IdNotFound(SystemTable::st_column, col_pos.0 as _).into());
-        }
 
         for ptr in ptrs_to_delete {
             // TODO(error-handling,bikeshedding): Consider correct failure semantics here.
@@ -516,6 +513,7 @@ impl MutTxId {
 
     /// Drop the backing table of a view and update the system tables.
     pub fn drop_view(&mut self, view_id: ViewId) -> Result<()> {
+        let st_view_row = self.lookup_st_view(view_id)?;
         // Drop the view's metadata
         self.drop_st_view(view_id)?;
         self.drop_st_view_param(view_id)?;
@@ -527,7 +525,7 @@ impl MutTxId {
         if let StViewRow {
             table_id: Some(table_id),
             ..
-        } = self.lookup_st_view(view_id)?
+        } = st_view_row
         {
             return self.drop_table(table_id);
         };

smoketests/tests/views.py

@@ -426,6 +426,80 @@ def test_views_auto_migration(self):
 """)
 
 
+class AutoMigrateDropView(Smoketest):
+    MODULE_CODE = """
+use spacetimedb::ViewContext;
+
+#[derive(Copy, Clone)]
+#[spacetimedb::table(name = player_state)]
+pub struct PlayerState {
+    #[primary_key]
+    id: u64,
+    #[index(btree)]
+    level: u64,
+}
+
+#[spacetimedb::view(name = player, public)]
+pub fn player(ctx: &ViewContext) -> Option<PlayerState> {
+    ctx.db.player_state().id().find(1u64)
+}
+"""
+
+    MODULE_CODE_DROP_VIEW = """
+#[derive(Copy, Clone)]
+#[spacetimedb::table(name = player_state)]
+pub struct PlayerState {
+    #[primary_key]
+    id: u64,
+    #[index(btree)]
+    level: u64,
+}
+"""
+
+    def test_auto_migration_drop_view(self):
+        """Assert that views can be dropped in an auto-migration"""
+
+        self.write_module_code(self.MODULE_CODE_DROP_VIEW)
+        self.publish_module(self.database_identity, clear=False, break_clients=False)
+
+
+class AutoMigrateAddView(Smoketest):
+    MODULE_CODE = """
+#[derive(Copy, Clone)]
+#[spacetimedb::table(name = player_state)]
+pub struct PlayerState {
+    #[primary_key]
+    id: u64,
+    #[index(btree)]
+    level: u64,
+}
+"""
+
+    MODULE_CODE_ADD_VIEW = """
+use spacetimedb::ViewContext;
+
+#[derive(Copy, Clone)]
+#[spacetimedb::table(name = player_state)]
+pub struct PlayerState {
+    #[primary_key]
+    id: u64,
+    #[index(btree)]
+    level: u64,
+}
+
+#[spacetimedb::view(name = player, public)]
+pub fn player(ctx: &ViewContext) -> Option<PlayerState> {
+    ctx.db.player_state().id().find(1u64)
+}
+"""
+
+    def test_auto_migration_drop_view(self):
+        """Assert that views can be added in an auto-migration"""
+
+        self.write_module_code(self.MODULE_CODE_ADD_VIEW)
+        self.publish_module(self.database_identity, clear=False)
+
+
 class AutoMigrateViewsTrapped(Smoketest):
     MODULE_CODE = """
 use spacetimedb::ViewContext;