From 03a5bff4cc1ff3abd4dd9ed668f6746c3423745f Mon Sep 17 00:00:00 2001
From: Stan Grams <sjg@haxx.space>
Date: Tue, 10 Mar 2026 18:12:27 +0100
Subject: [PATCH] [fix](trx-frontend-http): send decode history as single JSON
 array event

Previously the server emitted N individual SSE events (one per decoded
message) followed by a history_done sentinel. With a 1.3 MB history
this caused thousands of EventSource onmessage callbacks each blocking
the JS main thread, interrupting audio playback and spectrum rendering
for 50+ seconds.

Server: serialize the entire history Vec as a single named "history"
event containing a JSON array, then chain directly into the live
decode stream.  One serde_json::to_string call instead of N.

JS: listen for the "history" event, parse the array once, pass it to
the existing drainDecodeHistory() chunked dispatcher (30 msgs per
setTimeout slice to stay off the main thread), then gate onmessage
dispatching on historyReceived.  Removes the historyBuffer accumulator
and the history_done event entirely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
---
 .../trx-frontend-http/assets/web/app.js       | 29 ++++++++++---------
 .../trx-frontend/trx-frontend-http/src/api.rs | 19 ++++++------
 2 files changed, 25 insertions(+), 23 deletions(-)

diff --git a/src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js b/src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js
index 73cf5ab..7f3e68e 100644
--- a/src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js
+++ b/src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js
@@ -6036,28 +6036,29 @@ function connectDecode() {
   if (window.resetCwHistoryView) window.resetCwHistoryView();
   if (window.resetFt8HistoryView) window.resetFt8HistoryView();
   if (window.resetWsprHistoryView) window.resetWsprHistoryView();
-  const historyBuffer = [];
-  let historyDone = false;
+  // Live messages arrive after the history event; gate on this flag so
+  // onmessage does not dispatch before drainDecodeHistory has started.
+  let historyReceived = false;
   decodeSource = new EventSource("/decode");
   decodeSource.onopen = () => {
     decodeConnected = true;
     updateDecodeStatus("Connected, listening for packets");
   };
-  decodeSource.addEventListener("history_done", () => {
-    historyDone = true;
-    drainDecodeHistory(historyBuffer, 0);
+  // The server sends the entire history as one named "history" event (JSON
+  // array). A single JSON.parse + chunked drain is far cheaper than N
+  // individual EventSource callbacks each blocking the main thread.
+  decodeSource.addEventListener("history", (evt) => {
+    try {
+      const msgs = JSON.parse(evt.data);
+      if (Array.isArray(msgs)) drainDecodeHistory(msgs, 0);
+    } catch (e) { /* ignore parse errors */ }
+    historyReceived = true;
   });
   decodeSource.onmessage = (evt) => {
+    if (!historyReceived) return; // skip anything before history event
     try {
-      const msg = JSON.parse(evt.data);
-      if (!historyDone) {
-        historyBuffer.push(msg);
-      } else {
-        dispatchDecodeMessage(msg);
-      }
-    } catch (e) {
-      // ignore parse errors
-    }
+      dispatchDecodeMessage(JSON.parse(evt.data));
+    } catch (e) { /* ignore parse errors */ }
   };
   decodeSource.onerror = () => {
     // readyState CLOSED (2) = server rejected (404/error), CONNECTING (0) = temporary drop
diff --git a/src/trx-client/trx-frontend/trx-frontend-http/src/api.rs b/src/trx-client/trx-frontend/trx-frontend-http/src/api.rs
index 212bf4e..ffdd63c 100644
--- a/src/trx-client/trx-frontend/trx-frontend-http/src/api.rs
+++ b/src/trx-client/trx-frontend/trx-frontend-http/src/api.rs
@@ -326,14 +326,15 @@ pub async fn decode_events(
         out
     };
 
-    let history_stream = futures_util::stream::iter(history.into_iter().filter_map(|msg| {
-        serde_json::to_string(&msg)
-            .ok()
-            .map(|json| Ok::<Bytes, Error>(Bytes::from(format!("data: {json}\n\n"))))
-    }));
-
-    let history_done =
-        once(async { Ok::<Bytes, Error>(Bytes::from("event: history_done\ndata: {}\n\n")) });
+    // Send the entire history as a single named "history" event (JSON array).
+    // Sending N individual events causes N EventSource callbacks in the browser,
+    // each blocking the main thread — for large histories this interrupts audio
+    // and spectrum rendering for tens of seconds.
+    let history_event = {
+        let json = serde_json::to_string(&history).unwrap_or_else(|_| "[]".to_string());
+        Bytes::from(format!("event: history\ndata: {json}\n\n"))
+    };
+    let history_stream = once(async move { Ok::<Bytes, Error>(history_event) });
 
     let decode_stream = futures_util::stream::unfold(decode_rx, |mut rx| async move {
         loop {
@@ -355,7 +356,7 @@ pub async fn decode_events(
     let pings = IntervalStream::new(time::interval(Duration::from_secs(15)))
         .map(|_| Ok::<Bytes, Error>(Bytes::from(": ping\n\n")));
 
-    let stream = history_stream.chain(history_done).chain(select(pings, decode_stream));
+    let stream = history_stream.chain(select(pings, decode_stream));
 
     Ok(HttpResponse::Ok()
         .insert_header((header::CONTENT_TYPE, "text/event-stream"))