storage controller: don't hold detached tenants in memory

storage_controller/src/persistence.rs

@@ -330,11 +330,20 @@ impl Persistence {
 
     /// At startup, load the high level state for shards, such as their config + policy.  This will
     /// be enriched at runtime with state discovered on pageservers.
+    ///
+    /// We exclude shards configured to be detached.  During startup, if we see any attached locations
+    /// for such shards, they will automatically be detached as 'orphans'.
     pub(crate) async fn list_tenant_shards(&self) -> DatabaseResult<Vec<TenantShardPersistence>> {
+        use crate::schema::tenant_shards::dsl::*;
         self.with_measured_conn(
             DatabaseOperation::ListTenantShards,
             move |conn| -> DatabaseResult<_> {
-                Ok(crate::schema::tenant_shards::table.load::<TenantShardPersistence>(conn)?)
+                let query = tenant_shards.filter(
+                    placement_policy.ne(serde_json::to_string(&PlacementPolicy::Detached).unwrap()),
+                );
+                let result = query.load::<TenantShardPersistence>(conn)?;
+
+                Ok(result)
             },
         )
         .await