refactor(tvix/store/composition): rename 'default' to 'root'

This becomes the root of the composition. `default` implies we can
directly access anything else, which we cannot. `root` makes this more
understandable, and it's all internal only anyways.

Change-Id: I297511bc05a7c32c59510b9d192b40d1bd937b5f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12746
Reviewed-by: yuka <yuka@yuka.dev>
Tested-by: BuildkiteCI

tvix/castore/src/blobservice/grpc.rs

@@ -385,7 +385,7 @@ mod tests {
                     .await
                     .expect("must succeed"),
             );
-            GRPCBlobService::from_client("default".into(), client)
+            GRPCBlobService::from_client("root".into(), client)
         };
 
         let has = grpc_client

tvix/castore/src/blobservice/tests/utils.rs

@@ -30,7 +30,7 @@ pub async fn make_grpc_blob_service_client() -> Box<dyn BlobService> {
     let mut maybe_right = Some(right);
 
     Box::new(GRPCBlobService::from_client(
-        "default".into(),
+        "root".into(),
         BlobServiceClient::new(
             Endpoint::try_from("http://[::]:50051")
                 .unwrap()

tvix/castore/src/composition.rs

@@ -62,7 +62,7 @@
 //!   "blobstore2": {
 //!     "type": "memory"
 //!   },
-//!   "default": {
+//!   "root": {
 //!     "type": "combined",
 //!     "local": "blobstore1",
 //!     "remote": "blobstore2"
@@ -72,7 +72,7 @@
 //! let blob_services_configs = with_registry(&REG, || serde_json::from_value(blob_services_configs_json))?;
 //! let mut blob_service_composition = Composition::new(&REG);
 //! blob_service_composition.extend_with_configs::<dyn BlobService>(blob_services_configs);
-//! let blob_service: Arc<dyn BlobService> = blob_service_composition.build("default").await?;
+//! let blob_service: Arc<dyn BlobService> = blob_service_composition.build("root").await?;
 //! # Ok(())
 //! # })
 //! # }
@@ -281,7 +281,7 @@ pub fn add_default_services(reg: &mut Registry) {
 pub struct CompositionContext<'a> {
     // The stack used to detect recursive instantiations and prevent deadlocks
     // The TypeId of the trait object is included to distinguish e.g. the
-    // BlobService "default" and the DirectoryService "default".
+    // BlobService "root" and the DirectoryService "root".
     stack: Vec<(TypeId, String)>,
     registry: &'static Registry,
     composition: Option<&'a Composition>,
@@ -529,7 +529,7 @@ mod test {
     #[tokio::test]
     async fn concurrent() {
         let blob_services_configs_json = serde_json::json!({
-            "default": {
+            "root": {
                 "type": "memory",
             }
         });
@@ -539,8 +539,8 @@ mod test {
         let mut blob_service_composition = Composition::new(&REG);
         blob_service_composition.extend_with_configs::<dyn BlobService>(blob_services_configs);
         let (blob_service1, blob_service2) = tokio::join!(
-            blob_service_composition.build::<dyn BlobService>("default"),
-            blob_service_composition.build::<dyn BlobService>("default")
+            blob_service_composition.build::<dyn BlobService>("root"),
+            blob_service_composition.build::<dyn BlobService>("root")
         );
         assert!(Arc::ptr_eq(
             &blob_service1.unwrap(),
@@ -552,15 +552,15 @@ mod test {
     #[tokio::test]
     async fn reject_recursion() {
         let blob_services_configs_json = serde_json::json!({
-            "default": {
+            "root": {
                 "type": "combined",
                 "local": "other",
                 "remote": "other"
             },
             "other": {
                 "type": "combined",
-                "local": "default",
-                "remote": "default"
+                "local": "root",
+                "remote": "root"
             }
         });
 
@@ -569,11 +569,11 @@ mod test {
         let mut blob_service_composition = Composition::new(&REG);
         blob_service_composition.extend_with_configs::<dyn BlobService>(blob_services_configs);
         match blob_service_composition
-            .build::<dyn BlobService>("default")
+            .build::<dyn BlobService>("root")
             .await
         {
             Err(CompositionError::Recursion(stack)) => {
-                assert_eq!(stack, vec!["default".to_string(), "other".to_string()])
+                assert_eq!(stack, vec!["root".to_string(), "other".to_string()])
             }
             other => panic!("should have returned an error, returned: {:?}", other.err()),
         }

tvix/castore/src/directoryservice/object_store.rs

@@ -64,7 +64,7 @@ impl ObjectStoreDirectoryService {
         let (object_store, path) = object_store::parse_url_opts(url, options)?;
 
         Ok(Self {
-            instance_name: "default".into(),
+            instance_name: "root".into(),
             object_store: Arc::new(object_store),
             base_path: path,
         })

tvix/castore/src/directoryservice/redb.rs

@@ -56,7 +56,7 @@ impl RedbDirectoryService {
         create_schema(&db)?;
 
         Ok(Self {
-            instance_name: "default".into(),
+            instance_name: "root".into(),
             db: Arc::new(db),
         })
     }

tvix/castore/src/directoryservice/tests/utils.rs

@@ -33,7 +33,7 @@ pub async fn make_grpc_directory_service_client() -> Box<dyn DirectoryService> {
     // Create a client, connecting to the right side. The URI is unused.
     let mut maybe_right = Some(right);
     Box::new(GRPCDirectoryService::from_client(
-        "default".into(),
+        "root".into(),
         DirectoryServiceClient::new(
             Endpoint::try_from("http://[::]:50051")
                 .unwrap()