Wait for change works well if combined with tiny wait

src/stream_writer.rs

@@ -7,22 +7,21 @@ pub struct StreamWriter {
     pending: Arc<RwLock<Vec<u8>>>,
     done: Arc<RwLock<bool>>,
     // A way for the write side to signal new data to the stream side
-    // ETA: WHICH DOESN'T WORK AND I DON'T KNOW WHY
-    // write_index: Arc<RwLock<i64>>,
-    // write_index_sender: Arc<tokio::sync::watch::Sender<i64>>,
-    // write_index_receiver: tokio::sync::watch::Receiver<i64>,
+    write_index: Arc<RwLock<i64>>,
+    write_index_sender: Arc<tokio::sync::watch::Sender<i64>>,
+    write_index_receiver: tokio::sync::watch::Receiver<i64>,
 }
 
 impl StreamWriter {
     pub fn new() -> Self {
-        // let write_index = 0;
-        // let (tx, rx) = tokio::sync::watch::channel(write_index);
+        let write_index = 0;
+        let (tx, rx) = tokio::sync::watch::channel(write_index);
         Self {
             pending: Arc::new(RwLock::new(vec![])),
             done: Arc::new(RwLock::new(false)),
-            // write_index: Arc::new(RwLock::new(write_index)),
-            // write_index_sender: Arc::new(tx),
-            // write_index_receiver: rx,
+            write_index: Arc::new(RwLock::new(write_index)),
+            write_index_sender: Arc::new(tx),
+            write_index_receiver: rx,
         }
     }
 
@@ -35,14 +34,12 @@ impl StreamWriter {
             Err(e) =>
                 Err(anyhow::anyhow!("Internal error: StreamWriter::append can't take lock: {}", e))
         };
-        // This was meant to wake up listener threads when there was new data but it ended up
-        // just stalling until input was complete. TODO: investigate so we can get rid of the
-        // duration-based polling.
-        // {
-        //     let mut write_index = self.write_index.write().unwrap();
-        //     *write_index = *write_index + 1;
-        //     self.write_index_sender.send(*write_index).unwrap();
-        // }
+        {
+            let mut write_index = self.write_index.write().unwrap();
+            *write_index = *write_index + 1;
+            self.write_index_sender.send(*write_index).unwrap();
+            drop(write_index);
+        }
         result
     }
 
@@ -89,28 +86,33 @@ impl StreamWriter {
                             if self.is_done() {
                                 return;
                             } else {
-                                // Not sure how to do this better.  I tried using a signal that data
-                                // had changed (via tokio::sync::watch::channel()), but that effectively
-                                // blocked - we got the first chunk quickly but then it stalled waiting
-                                // for the change notification.  Polling is awful (and this interval is
-                                // probably too aggressive) but I don't know how to get signalling
-                                // to work!
-                                tokio::time::sleep(tokio::time::Duration::from_micros(1)).await;
-
-                                // For the record: this is what I tried:
-                                // match self.write_index_receiver.changed().await {
-                                //     Ok(_) => continue,
-                                //     Err(e) => {
-                                //         // If this ever happens (which it, cough, shouldn't), it means all senders have
-                                //         // closed, which _should_ mean we are done.  Log the error
-                                //         // but don't return it to the stream: the response as streamed so far
-                                //         // _should_ be okay!
-                                //         tracing::error!("StreamWriter::as_stream: error receiving write updates: {}", e);
-                                //         return;
-                                //     }
-                                // }
+                                // This tiny wait seems to help the write-stream pipeline to flow more smmoothly.
+                                // If we go straight to the 'changed().await' then the pipeline seems to stall after
+                                // a few dozen writes, and everything else gets held up until the entire output
+                                // has been written.  There may be better ways of doing this; I haven't found them
+                                // yet.
+                                //
+                                // (By the way, having the timer but not the change notification also worked. But if
+                                // writes came slowly, that would result in very aggressive polling. So hopefully this
+                                // gives us the best of both worlds.)
+                                tokio::time::sleep(tokio::time::Duration::from_nanos(10)).await;
+
+                                match self.write_index_receiver.changed().await {
+                                    Ok(_) => continue,
+                                    Err(e) => {
+                                        // If this ever happens (which it, cough, shouldn't), it means all senders have
+                                        // closed, which _should_ mean we are done.  Log the error
+                                        // but don't return it to the stream: the response as streamed so far
+                                        // _should_ be okay!
+                                        tracing::error!("StreamWriter::as_stream: error receiving write updates: {}", e);
+                                        return;
+                                    }
+                                }
                             }
                         } else {
+                            // This tiny wait seems to help the write-stream pipeline to flow more smmoothly.
+                            // See the comment on the 'empty buffer' case.
+                            tokio::time::sleep(tokio::time::Duration::from_nanos(10)).await;
                             yield Ok(v);
                         }
                     },