feat(nix-compat/nar/reader): provide passthrough buffered I/O r/7035

Allow taking advantage of the buffer of the underlying reader to avoid
unnecessary copies of file data.

We can't easily implement the methods of BufRead directly, since we
have some extra I/O to perform in the final consume() invocation.

That could be resolved at the cost of additional bookkeeping, but this
will suffice for now.

Change-Id: I8100cf0abd79e7469670b8596bd989be5db44a91
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10089
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

1 files changed, 5 insertions, 5 deletions

diff --git a/tvix/store/src/nar/import.rs b/tvix/store/src/nar/import.rs
index 000fc05663b2..e9065a670d21 100644
--- a/tvix/store/src/nar/import.rs
+++ b/tvix/store/src/nar/import.rs
@@ -135,13 +135,13 @@ fn process_file_reader(
 
     // write the blob.
     let mut blob_writer = {
-        let mut dest = SyncIoBridge::new(blob_writer);
-        io::copy(&mut file_reader, &mut dest)?;
+        let mut dst = SyncIoBridge::new(blob_writer);
 
-        dest.shutdown()?;
+        file_reader.copy(&mut dst)?;
+        dst.shutdown()?;
 
-        // return back the blob_reader
-        dest.into_inner()
+        // return back the blob_writer
+        dst.into_inner()
     };
 
     // close the blob_writer, retrieve the digest.