feat(tvix/castore): add composition module

Change-Id: I0868f3278db85ae5fe030089ee9033837bc08748
Signed-off-by: Yureka <tvl@yuka.dev>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11853
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/castore/src/blobservice/memory.rs

@@ -6,6 +6,7 @@ use tonic::async_trait;
 use tracing::instrument;
 
 use super::{BlobReader, BlobService, BlobWriter};
+use crate::composition::{CompositionContext, ServiceBuilder};
 use crate::B3Digest;
 
 #[derive(Clone, Default)]
@@ -37,6 +38,22 @@ impl BlobService for MemoryBlobService {
     }
 }
 
+#[derive(serde::Deserialize, Debug)]
+#[serde(deny_unknown_fields)]
+pub struct MemoryBlobServiceConfig {}
+
+#[async_trait]
+impl ServiceBuilder for MemoryBlobServiceConfig {
+    type Output = dyn BlobService;
+    async fn build<'a>(
+        &'a self,
+        _instance_name: &str,
+        _context: &CompositionContext<dyn BlobService>,
+    ) -> Result<Arc<dyn BlobService>, Box<dyn std::error::Error + Send + Sync + 'static>> {
+        Ok(Arc::new(MemoryBlobService::default()))
+    }
+}
+
 pub struct MemoryBlobWriter {
     db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>,