index: add stub IndexReadError type

This is needed to remove .unwrap()s from DefaultIndexStore.

lib/src/default_index/store.rs

@@ -31,7 +31,9 @@ use crate::backend::{CommitId, ObjectId};
 use crate::commit::CommitByCommitterTimestamp;
 use crate::dag_walk;
 use crate::file_util::persist_content_addressed_temp_file;
-use crate::index::{Index, IndexStore, IndexWriteError, MutableIndex, ReadonlyIndex};
+use crate::index::{
+    Index, IndexReadError, IndexStore, IndexWriteError, MutableIndex, ReadonlyIndex,
+};
 use crate::op_store::{OpStoreError, OperationId};
 use crate::operation::Operation;
 use crate::store::Store;
@@ -233,7 +235,11 @@ impl IndexStore for DefaultIndexStore {
         Self::name()
     }
 
-    fn get_index_at_op(&self, op: &Operation, store: &Arc<Store>) -> Box<dyn ReadonlyIndex> {
+    fn get_index_at_op(
+        &self,
+        op: &Operation,
+        store: &Arc<Store>,
+    ) -> Result<Box<dyn ReadonlyIndex>, IndexReadError> {
         let op_id_hex = op.id().hex();
         let op_id_file = self.dir.join("operations").join(op_id_hex);
         let index_segment = if op_id_file.exists() {
@@ -256,7 +262,7 @@ impl IndexStore for DefaultIndexStore {
         } else {
             self.index_at_operation(store, op).unwrap()
         };
-        Box::new(DefaultReadonlyIndex::from_segment(index_segment))
+        Ok(Box::new(DefaultReadonlyIndex::from_segment(index_segment)))
     }
 
     fn write_index(

lib/src/index.rs

@@ -27,6 +27,11 @@ use crate::operation::Operation;
 use crate::revset::{ResolvedExpression, Revset, RevsetEvaluationError};
 use crate::store::Store;
 
+/// Error while reading index from the `IndexStore`.
+#[derive(Debug, Error)]
+#[error(transparent)]
+pub struct IndexReadError(pub Box<dyn std::error::Error + Send + Sync>);
+
 /// Error while writing index to the `IndexStore`.
 #[derive(Debug, Error)]
 #[error(transparent)]
@@ -37,7 +42,11 @@ pub trait IndexStore: Send + Sync + Debug {
 
     fn name(&self) -> &str;
 
-    fn get_index_at_op(&self, op: &Operation, store: &Arc<Store>) -> Box<dyn ReadonlyIndex>;
+    fn get_index_at_op(
+        &self,
+        op: &Operation,
+        store: &Arc<Store>,
+    ) -> Result<Box<dyn ReadonlyIndex>, IndexReadError>;
 
     fn write_index(
         &self,

lib/src/repo.rs

@@ -253,8 +253,11 @@ impl ReadonlyRepo {
     pub fn readonly_index(&self) -> &dyn ReadonlyIndex {
         self.index
             .get_or_init(|| {
+                // TODO: somehow propagate error, but it's weird if all callers
+                // had Result<T, IndexReadError> signature.
                 self.index_store
                     .get_index_at_op(&self.operation, &self.store)
+                    .unwrap()
             })
             .deref()
     }