index: move IndexLoadError to "readonly" module, rename accordingly

I thought IndexLoadError and DefaultIndexStoreError would represent "load" and
"store" failures respectively, but they aren't. Actually, DefaultIndexStoreError
is the store-level error, and IndexLoadError should be wrapped in it.

lib/src/default_index/mod.rs

@@ -24,11 +24,11 @@ mod store;
 pub use self::composite::{AsCompositeIndex, CompositeIndex, IndexLevelStats, IndexStats};
 pub use self::entry::{IndexEntry, IndexPosition};
 pub use self::mutable::DefaultMutableIndex;
-pub use self::readonly::DefaultReadonlyIndex;
+pub use self::readonly::{DefaultReadonlyIndex, ReadonlyIndexLoadError};
 pub use self::rev_walk::{
     RevWalk, RevWalkDescendants, RevWalkDescendantsGenerationRange, RevWalkGenerationRange,
 };
-pub use self::store::{DefaultIndexStore, DefaultIndexStoreError, IndexLoadError};
+pub use self::store::{DefaultIndexStore, DefaultIndexStoreError};
 
 #[cfg(test)]
 mod tests {

lib/src/default_index/mutable.rs

@@ -32,8 +32,7 @@ use tempfile::NamedTempFile;
 
 use super::composite::{AsCompositeIndex, CompositeIndex, IndexSegment};
 use super::entry::{IndexEntry, IndexPosition, SmallIndexPositionsVec};
-use super::readonly::{DefaultReadonlyIndex, ReadonlyIndexSegment};
-use super::store::IndexLoadError;
+use super::readonly::{DefaultReadonlyIndex, ReadonlyIndexLoadError, ReadonlyIndexSegment};
 use crate::backend::{ChangeId, CommitId, ObjectId};
 use crate::commit::Commit;
 use crate::file_util::persist_content_addressed_temp_file;
@@ -302,10 +301,10 @@ impl MutableIndexSegment {
             change_id_length,
         )
         .map_err(|err| match err {
-            IndexLoadError::IndexCorrupt(err) => {
+            ReadonlyIndexLoadError::IndexCorrupt(err) => {
                 panic!("Just-created index file is corrupt: {err}")
             }
-            IndexLoadError::IoError(err) => err,
+            ReadonlyIndexLoadError::IoError(err) => err,
         })
     }
 }

lib/src/default_index/readonly.rs

@@ -18,23 +18,32 @@ use std::any::Any;
 use std::cmp::Ordering;
 use std::fmt::{Debug, Formatter};
 use std::fs::File;
+use std::io;
 use std::io::Read;
 use std::path::Path;
 use std::sync::Arc;
 
 use byteorder::{LittleEndian, ReadBytesExt};
 use smallvec::SmallVec;
+use thiserror::Error;
 
 use super::composite::{AsCompositeIndex, CompositeIndex, IndexSegment};
 use super::entry::{IndexEntry, IndexPosition, SmallIndexPositionsVec};
 use super::mutable::DefaultMutableIndex;
-use super::store::IndexLoadError;
 use crate::backend::{ChangeId, CommitId, ObjectId};
 use crate::default_revset_engine;
 use crate::index::{HexPrefix, Index, MutableIndex, PrefixResolution, ReadonlyIndex};
 use crate::revset::{ResolvedExpression, Revset, RevsetEvaluationError};
 use crate::store::Store;
 
+#[derive(Debug, Error)]
+pub enum ReadonlyIndexLoadError {
+    #[error("Index file '{0}' is corrupt.")]
+    IndexCorrupt(String),
+    #[error("I/O error while loading index file: {0}")]
+    IoError(#[from] io::Error),
+}
+
 struct CommitGraphEntry<'a> {
     data: &'a [u8],
     commit_id_length: usize,
@@ -166,7 +175,7 @@ impl ReadonlyIndexSegment {
         name: String,
         commit_id_length: usize,
         change_id_length: usize,
-    ) -> Result<Arc<ReadonlyIndexSegment>, IndexLoadError> {
+    ) -> Result<Arc<ReadonlyIndexSegment>, ReadonlyIndexLoadError> {
         let parent_filename_len = file.read_u32::<LittleEndian>()?;
         let num_parent_commits;
         let maybe_parent_file;
@@ -200,7 +209,7 @@ impl ReadonlyIndexSegment {
         let parent_overflow_size = (num_parent_overflow_entries as usize) * 4;
         let expected_size = graph_size + lookup_size + parent_overflow_size;
         if data.len() != expected_size {
-            return Err(IndexLoadError::IndexCorrupt(name));
+            return Err(ReadonlyIndexLoadError::IndexCorrupt(name));
         }
         Ok(Arc::new(ReadonlyIndexSegment {
             parent_file: maybe_parent_file,

lib/src/default_index/store.rs

@@ -26,7 +26,7 @@ use tempfile::NamedTempFile;
 use thiserror::Error;
 
 use super::mutable::DefaultMutableIndex;
-use super::readonly::{DefaultReadonlyIndex, ReadonlyIndexSegment};
+use super::readonly::{DefaultReadonlyIndex, ReadonlyIndexLoadError, ReadonlyIndexSegment};
 use crate::backend::{CommitId, ObjectId};
 use crate::commit::CommitByCommitterTimestamp;
 use crate::dag_walk;
@@ -38,14 +38,6 @@ use crate::op_store::{OpStoreError, OperationId};
 use crate::operation::Operation;
 use crate::store::Store;
 
-#[derive(Error, Debug)]
-pub enum IndexLoadError {
-    #[error("Index file '{0}' is corrupt.")]
-    IndexCorrupt(String),
-    #[error("I/O error while loading index file: {0}")]
-    IoError(#[from] io::Error),
-}
-
 #[derive(Debug, Error)]
 pub enum DefaultIndexStoreError {
     #[error("Failed to associate commit index file with a operation {op_id:?}: {source}")]
@@ -93,7 +85,7 @@ impl DefaultIndexStore {
         commit_id_length: usize,
         change_id_length: usize,
         op_id: &OperationId,
-    ) -> Result<Arc<ReadonlyIndexSegment>, IndexLoadError> {
+    ) -> Result<Arc<ReadonlyIndexSegment>, ReadonlyIndexLoadError> {
         let op_id_file = self.dir.join("operations").join(op_id.hex());
         let buf = fs::read(op_id_file).unwrap();
         let index_file_id_hex = String::from_utf8(buf).unwrap();
@@ -248,7 +240,7 @@ impl IndexStore for DefaultIndexStore {
                 store.change_id_length(),
                 op.id(),
             ) {
-                Err(IndexLoadError::IndexCorrupt(_)) => {
+                Err(ReadonlyIndexLoadError::IndexCorrupt(_)) => {
                     // If the index was corrupt (maybe it was written in a different format),
                     // we just reindex.
                     // TODO: Move this message to a callback or something.