Implement Sync on OwnedHeaders

The rationale here is the same as the rationale for implementing Sync on
BorrowedMessage (2779f77).

Fix #200.

examples/asynchronous_processing.rs

@@ -17,12 +17,18 @@ use crate::example_utils::setup_logger;
 
 mod example_utils;
 
-async fn record_message_receipt(msg: &BorrowedMessage<'_>) {
+async fn record_borrowed_message_receipt(msg: &BorrowedMessage<'_>) {
     // Simulate some work that must be done in the same order as messages are
     // received; i.e., before truly parallel processing can begin.
     info!("Message received: {}", msg.offset());
 }
 
+async fn record_owned_message_receipt(msg: &OwnedMessage) {
+    // Like `record_borrowed_message_receipt`, but takes an `OwnedMessage`
+    // instead, as in a real-world use case  an `OwnedMessage` might be more
+    // convenient than a `BorrowedMessage`.
+}
+
 // Emulates an expensive, synchronous computation.
 fn expensive_computation<'a>(msg: OwnedMessage) -> String {
     info!("Starting expensive computation on message {}", msg.offset());
@@ -79,10 +85,11 @@ async fn run_async_processor(
         let output_topic = output_topic.to_string();
         async move {
             // Process each message
-            record_message_receipt(&borrowed_message).await;
+            record_borrowed_message_receipt(&borrowed_message).await;
             // Borrowed messages can't outlive the consumer they are received from, so they need to
             // be owned in order to be sent to a separate thread.
             let owned_message = borrowed_message.detach();
+            record_owned_message_receipt(&owned_message).await;
             tokio::spawn(async move {
                 // The body of this block will be executed on the main thread pool,
                 // but we perform `expensive_computation` on a separate thread pool

src/message.rs

@@ -380,6 +380,7 @@ pub struct OwnedHeaders {
 }
 
 unsafe impl Send for OwnedHeaders {}
+unsafe impl Sync for OwnedHeaders {}
 
 impl OwnedHeaders {
     /// Create a new `OwnedHeaders` struct with initial capacity 5.