Add opt-in join-the-flight ByteStream write dedup

.github/styles/config/vocabularies/TraceMachina/accept.txt

@@ -89,6 +89,7 @@ API
 APIs
 Backpressure
 Dedup
+Deduplicate
 XCode
 CMake
 Gradle

nativelink-config/src/backcompat.rs

@@ -102,6 +102,7 @@ where
                         max_bytes_per_stream: old_config.max_bytes_per_stream,
                         persist_stream_on_disconnect_timeout_s: old_config
                             .persist_stream_on_disconnect_timeout_s,
+                        experimental_write_dedup: false,
                     },
                 })
                 .collect();

nativelink-config/src/cas_server.rs

@@ -349,6 +349,27 @@ pub struct ByteStreamConfig {
         alias = "persist_stream_on_disconnect_timeout"
     )]
     pub persist_stream_on_disconnect_timeout_s: usize,
+
+    /// Deduplicate concurrent uploads of the same digest ("join the flight").
+    /// When multiple clients upload a blob with the same digest at the same
+    /// time, only the first upload is streamed to the store; the others wait
+    /// for it to durably commit and then complete early without transferring
+    /// their payload, as permitted by the REAPI specification. Uploads of
+    /// blobs that already exist in the store also complete early after a
+    /// single existence check.
+    ///
+    /// If the leading upload fails, waiting uploads receive a retryable
+    /// ABORTED error and one of the retrying clients becomes the new leader.
+    ///
+    /// This saves upload bandwidth and store work when many actions produce
+    /// identical outputs, at the cost of one existence check per new upload.
+    /// The check uses the digest function in the upload resource name, and
+    /// proxy-backed `GrpcStore` instances participate in the same local
+    /// single-flight behavior.
+    ///
+    /// Default: false (disabled)
+    #[serde(default, skip_serializing_if = "is_default")]
+    pub experimental_write_dedup: bool,
 }
 
 // Older bytestream config. All fields are as per the newer docs, but this requires