index: add explicit reindexing method that can propagate error

cli/src/commands/debug.rs

@@ -20,6 +20,7 @@ use clap::Subcommand;
 use jj_lib::backend::ObjectId;
 use jj_lib::default_index::{AsCompositeIndex as _, DefaultIndexStore, DefaultReadonlyIndex};
 use jj_lib::local_working_copy::LocalWorkingCopy;
+use jj_lib::repo::Repo;
 use jj_lib::revset;
 use jj_lib::working_copy::WorkingCopy;
 
@@ -240,12 +241,9 @@ fn cmd_debug_reindex(
         default_index_store
             .reinit()
             .map_err(|err| CommandError::InternalError(err.to_string()))?;
-        let repo = repo.reload_at(repo.operation())?;
+        let default_index = default_index_store
-        let default_index: &DefaultReadonlyIndex = repo
+            .build_index_at_operation(repo.operation(), repo.store())
-            .readonly_index()
+            .map_err(|err| CommandError::InternalError(err.to_string()))?;
-            .as_any()
-            .downcast_ref()
-            .expect("Default index should be a DefaultReadonlyIndex");
         writeln!(
             ui.stderr(),
             "Finished indexing {:?} commits.",

lib/src/default_index/store.rs

@@ -119,6 +119,19 @@ impl DefaultIndexStore {
         .map_err(DefaultIndexStoreError::LoadIndex)
     }
 
+    /// Rebuilds index for the given `operation`.
+    ///
+    /// The index to be built will be calculated from one of the ancestor
+    /// operations if exists. Use `reinit()` to rebuild index from scratch.
+    pub fn build_index_at_operation(
+        &self,
+        operation: &Operation,
+        store: &Arc<Store>,
+    ) -> Result<DefaultReadonlyIndex, DefaultIndexStoreError> {
+        let index_segment = self.build_index_segments_at_operation(operation, store)?;
+        Ok(DefaultReadonlyIndex::from_segment(index_segment))
+    }
+
     #[tracing::instrument(skip(self, store))]
     fn build_index_segments_at_operation(
         &self,