Client-side chunks 2: introduce TransportChunk (#6439)

A `TransportChunk` is a `Chunk` that is ready for transport and/or
storage.
It is very cheap to go from `Chunk` to a `TransportChunk` and
vice-versa.

A `TransportChunk` maps 1:1 to a native Arrow `RecordBatch`. It has a
stable ABI, and can be cheaply send across process boundaries.
`arrow2` has no `RecordBatch` type; we will get one once we migrate to
`arrow-rs`.

A `TransportChunk` is self-describing: it contains all the data _and_
metadata needed to index it into storage.

We rely heavily on chunk-level and field-level metadata to communicate
Rerun-specific semantics over the wire, e.g. whether some columns are
already properly sorted.

The Arrow metadata system is fairly limited -- it's all untyped strings
--, but for now that seems good enough. It will be trivial to switch to
something else later, if need be.

- Fixes #1760
- Fixes #1692
- Fixes #3360 
- Fixes #1696

---

Part of a PR series to implement our new chunk-based data model on the
client-side (SDKs):
- #6437
- #6438
- #6439
- #6440
- #6441

crates/re_chunk/src/chunk.rs

@@ -35,7 +35,7 @@ pub type ChunkId = re_tuid::Tuid;
 /// Its time columns might or might not be ascendingly sorted, depending on how the data was logged.
 ///
 /// This is the in-memory representation of a chunk, optimized for efficient manipulation of the
-/// data within.
+/// data within. For transport, see [`crate::TransportChunk`] instead.
 #[derive(Debug, Clone)]
 pub struct Chunk {
     pub(crate) id: ChunkId,
@@ -337,7 +337,16 @@ impl Chunk {
     }
 }
 
-// TODO(cmc): display impl
+impl std::fmt::Display for Chunk {
+    #[inline]
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let chunk = self.to_transport().map_err(|err| {
+            re_log::error_once!("couldn't display Chunk: {err}");
+            std::fmt::Error
+        })?;
+        chunk.fmt(f)
+    }
+}
 
 // TODO(cmc): sizebytes impl + sizebytes caching + sizebytes in transport metadata
 

crates/re_chunk/src/lib.rs

@@ -6,9 +6,11 @@
 
 mod chunk;
 mod shuffle;
+mod transport;
 mod util;
 
 pub use self::chunk::{Chunk, ChunkError, ChunkId, ChunkResult, ChunkTimeline};
+pub use self::transport::TransportChunk;
 pub use self::util::arrays_to_list_array;
 
 pub mod external {

crates/re_chunk/src/shuffle.rs

@@ -297,7 +297,7 @@ mod tests {
                 components.clone().into_iter().collect(),
             )?;
 
-            // eprintln!("{chunk_sorted}");
+            eprintln!("{chunk_sorted}");
 
             assert!(chunk_sorted.is_sorted());
             assert!(chunk_sorted.is_sorted_uncached());
@@ -308,7 +308,7 @@ mod tests {
                 chunk_shuffled
             };
 
-            // eprintln!("{chunk_shuffled}");
+            eprintln!("{chunk_shuffled}");
 
             assert!(!chunk_shuffled.is_sorted());
             assert!(!chunk_shuffled.is_sorted_uncached());
@@ -320,7 +320,7 @@ mod tests {
                 chunk_resorted
             };
 
-            // eprintln!("{chunk_resorted}");
+            eprintln!("{chunk_resorted}");
 
             assert!(chunk_resorted.is_sorted());
             assert!(chunk_resorted.is_sorted_uncached());