Fix rpc error when encoding default is legacy

crates/spfs/src/graph/object.rs

@@ -113,6 +113,36 @@ impl Object {
         }
     }
 
+    /// Create an object from the encoded bytes.
+    ///
+    /// In memory, objects always use the latest flatbuffer
+    /// format. The given bytes may be discarded or reconstructed
+    /// if a conversion is necessary, but the header is preserved
+    /// in order to ensure that the object does not change it's
+    /// digest unless explicitly marked to do so.
+    ///
+    /// Unlike [`Object::new`] this constructor doesn't respect the header and
+    /// treats the payload as flatbuffers encoded data.
+    pub fn new_from_flatbuffers_payload<B: Into<bytes::Bytes>>(buf: B) -> crate::Result<Self> {
+        let bytes = buf.into();
+        let header = Header::new(&bytes)?;
+        let Some(_kind) = header.object_kind() else {
+            return Err(ObjectError::UnexpectedKind(header.object_kind_number()).into());
+        };
+        let Some(_format) = header.encoding_format() else {
+            return Err(ObjectError::UnknownEncoding(header.encoding_format_number()).into());
+        };
+        // all we need to do with a flatbuffer is validate it, without
+        // any need to change or reallocate the buffer
+        flatbuffers::root::<spfs_proto::AnyObject>(&bytes[Header::SIZE..])
+            .map_err(ObjectError::InvalidFlatbuffer)?;
+        Ok(Object {
+            buf: bytes,
+            offset: 0,
+            _t: PhantomData,
+        })
+    }
+
     /// Constructs a new [`Object`] instance from the provided flatbuffer.
     ///
     /// # Safety

crates/spfs/src/proto/conversions.rs

@@ -180,7 +180,12 @@ impl TryFrom<super::Object> for graph::Object {
                 "Unexpected and unsupported object kind {:?}",
                 source.kind
             ))),
-            Some(Kind::Buffer(buf)) => graph::Object::new(buf),
+            Some(Kind::Buffer(buf)) => {
+                // Object data passed via RPC is always passed in flatbuffers
+                // format, despite the header possibly claiming to be in
+                // legacy format.
+                graph::Object::new_from_flatbuffers_payload(buf)
+            }
             None => Err(Error::String(
                 "Expected non-empty object kind in rpc message".to_string(),
             )),