index: make jj debug reindex actually reindex

I broke `jj debug reindex` in a27da7d8d5. From that commit, we no
longer delete the pointer to the old index, so nothing happens when we
reload the index. This commit fixes that, and also makes the command
error out if run on a repo with a non-default index type.

lib/src/default_index_store.rs

@@ -66,6 +66,11 @@ impl DefaultIndexStore {
         }
     }
 
+    pub fn reinit(&self, op_id: &OperationId) {
+        let op_id_file = self.dir.join("operations").join(op_id.hex());
+        std::fs::remove_file(op_id_file).unwrap();
+    }
+
     fn load_index_at_operation(
         &self,
         commit_id_length: usize,
@@ -165,6 +170,10 @@ impl DefaultIndexStore {
 }
 
 impl IndexStore for DefaultIndexStore {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
     fn name(&self) -> &str {
         "default"
     }

lib/src/index.rs

@@ -32,6 +32,8 @@ pub enum IndexWriteError {
 }
 
 pub trait IndexStore: Send + Sync + Debug {
+    fn as_any(&self) -> &dyn Any;
+
     fn name(&self) -> &str;
 
     fn get_index_at_op(&self, op: &Operation, store: &Arc<Store>) -> Box<dyn ReadonlyIndex>;

src/commands/mod.rs

@@ -30,7 +30,7 @@ use itertools::Itertools;
 use jujutsu_lib::backend::{CommitId, ObjectId, TreeValue};
 use jujutsu_lib::commit::Commit;
 use jujutsu_lib::dag_walk::topo_order_reverse;
-use jujutsu_lib::default_index_store::IndexEntry;
+use jujutsu_lib::default_index_store::{DefaultIndexStore, IndexEntry};
 use jujutsu_lib::matchers::EverythingMatcher;
 use jujutsu_lib::op_store::{RefTarget, WorkspaceId};
 use jujutsu_lib::repo::{ReadonlyRepo, Repo};
@@ -3107,12 +3107,23 @@ fn cmd_debug(
         DebugCommands::ReIndex(_reindex_matches) => {
             let workspace_command = command.workspace_helper(ui)?;
             let repo = workspace_command.repo();
-            let repo = repo.reload_at(repo.operation());
-            writeln!(
-                ui,
-                "Finished indexing {:?} commits.",
-                repo.index().num_commits()
-            )?;
+            let default_index_store: Option<&DefaultIndexStore> =
+                repo.index_store().as_any().downcast_ref();
+            if let Some(default_index_store) = default_index_store {
+                let op = repo.operation();
+                default_index_store.reinit(op.id());
+                let repo = repo.reload_at(op);
+                writeln!(
+                    ui,
+                    "Finished indexing {:?} commits.",
+                    repo.index().num_commits()
+                )?;
+            } else {
+                return Err(user_error(format!(
+                    "Cannot reindex indexes of type '{}'",
+                    repo.index_store().name()
+                )));
+            }
         }
         DebugCommands::Operation(operation_args) => {
             let workspace_command = command.workspace_helper(ui)?;