feat(tvix/store): do not buffer blob data

Use the FastCDC::cut function to ask fastcd for cutting points as we
receive the data. Make sure to keep the last chunk in the temporary
buffer, as we might not actually cut at the end.

Also, use rayon to calculate the blake3 hash if the input data is
> 128KiB.

Change-Id: I6195f3b74eac5516965cb12d8d026aa720c8b891
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8135
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/store/src/proto/tests/grpc_blobservice.rs

@@ -166,6 +166,39 @@ async fn put_read_stat_large() {
     }
     assert_eq!(BLOB_B.len() as u32, size_in_stat);
 
+    // Chunks are chunked up the same way we would do locally, when initializing the chunker with the same values.
+    // TODO: make the chunker config better accessible, so we don't need to synchronize this.
+    {
+        let chunker_avg_size = 64 * 1024;
+        let chunker_min_size = chunker_avg_size / 4;
+        let chunker_max_size = chunker_avg_size * 4;
+
+        // initialize a chunker with the current buffer
+        let blob_b = BLOB_B.to_vec();
+        let chunker = fastcdc::v2020::FastCDC::new(
+            &blob_b,
+            chunker_min_size,
+            chunker_avg_size,
+            chunker_max_size,
+        );
+
+        let mut num_chunks = 0;
+        for (i, chunk) in chunker.enumerate() {
+            assert_eq!(
+                resp.chunks[i].size, chunk.length as u32,
+                "expected locally-chunked chunk length to match stat response"
+            );
+
+            num_chunks += 1;
+        }
+
+        assert_eq!(
+            resp.chunks.len(),
+            num_chunks,
+            "expected number of chunks to match"
+        );
+    }
+
     // Reading the whole blob by its digest via the read() interface should succeed.
     {
         let resp = service