fix: udp:// slots read the socket on a thread of their own; no slot checks during a push (#251)

CHANGELOG.md

@@ -12,6 +12,15 @@ Versions after 2.17 are `MAJOR.MINOR.PATCH` (2.17 is followed by 2.17.1 or 2.18.
 `docs/testing.md`, "Versions and releases"); the headings below keep the numbers as they
 were released.
 
+## 2.18.3 — 2026-10-21
+
+- `udp://` slots and the U cell: a thread of its own reads the socket into a ring of 1024
+  datagrams, so key-frame bursts no longer overflow the small socket buffer an app gets
+  (`rmem_max`) while the decoder or the UI hold up the receive; the log shows the buffer the
+  kernel gave. While the U cell receives a push, the other slots get no stream checks, which
+  made the box drop datagrams. `/stats.json` adds `udp` to the cell: `rcvbuf`, `received`,
+  `ring_dropped`, `rtp_lost` and, where the app may read it, `socket_drops` (#251).
+
 ## 2.18.2 — 2026-10-20
 
 - Single view: the decoder is set up for the stream's size from its SPS, so streams above

android/app/build.gradle

@@ -23,8 +23,8 @@ android {
         applicationId "com.openipc.decoder"
         minSdk 21
         targetSdk 36
-        versionCode 45
-        versionName "2.18.2"
+        versionCode 46
+        versionName "2.18.3"
 
         buildConfigField "String", "GIT_HASH", "\"${getGitHash()}\""
     }

android/app/src/main/java/com/openipc/decoder/Decoder.java

@@ -1170,6 +1170,8 @@ public void run() {
                 v.setVisibility(showStats ? View.VISIBLE : View.GONE);
                 if (log) Log.i(TAG, "Quad-" + i + ": " + line);
             }
+            // Not only on debug requests: a push that starts ends the checks within a second
+            streamProbes.hold(pushReceived());
             checkDecoderStuck(cells);
             planDecoding(cells, now);
             updateDebugUrls();
@@ -1697,6 +1699,9 @@ private Map<String, Object> debugState() {
                     if (jl != null) c.put("jieli", jl);
                     Map<String, Object> hb = cell.heartbeatStats();
                     if (hb != null) c.put("heartbeat", hb);
+                    // A udp:// push: a full socket buffer or ring, or loss on the way (#251)
+                    Map<String, Object> udp = cell.udpListenStats();
+                    if (udp != null) c.put("udp", udp);
                     if (cell.rotate() != 0) c.put("rotate", cell.rotate());
                     if (cell.mirror()) {
                         c.put("mirror", true);
@@ -1723,8 +1728,20 @@ private void updateProbes() {
             urls[i] = mHosts[i] != null && !mHosts[i].isEmpty() && !mHosts[i].equals(DEFAULT_URL)
                     ? mHosts[i] : null;
         }
+        boolean push = pushReceived();
+        streamProbes.hold(push);
         streamProbes.update(urls, mTransports, udpViewShown ? null : mHost,
-                mResumed && (udpViewShown || !quadEnabled));
+                mResumed && (udpViewShown ? !push : !quadEnabled));
+    }
+
+    /**
+     * The U cell receives a push: no checks of other cameras then, their RTSP sessions made
+     * the box drop datagrams of the push (#251).
+     */
+    private boolean pushReceived() {
+        QuadCell[] cells = quadCells;
+        QuadCell u = udpViewShown && cells != null ? cells[U_CELL] : null;
+        return u != null && u.isConnected();
     }
 
     /**
@@ -1754,7 +1771,9 @@ private String probeText(int i, long nowMs) {
         if (skip != null) return skip;
         StreamProbe.Result r = probeResult(i);
         if (r != null) return r.text(nowMs) + (streamProbes.isRunning(i) ? " \u00b7 checking" : "");
-        return streamProbes.isRunning(i) ? "Checking stream\u2026" : "Waiting for a stream check";
+        if (streamProbes.isRunning(i)) return "Checking stream\u2026";
+        return pushReceived() ? "Not checked while the U cell receives a push"
+                : "Waiting for a stream check";
     }
 
     /**
@@ -1818,8 +1837,9 @@ private List<Object> probeSlots() {
                 StreamProbe.Result r = probeResult(i);
                 Map<String, Object> p = new LinkedHashMap<>();
                 p.put("state", r != null ? (r.ok ? "ok" : "error")
-                        : streamProbes.isRunning(i) ? "checking" : u || StreamProbes.skipReason(
-                        mHosts[i], DEFAULT_URL, udpViewShown ? null : mHost) != null
+                        : streamProbes.isRunning(i) ? "checking" : u || pushReceived()
+                        || StreamProbes.skipReason(mHosts[i], DEFAULT_URL,
+                                udpViewShown ? null : mHost) != null
                         ? "not checked" : "waiting");
                 if (r != null) {
                     p.put("checked_s_ago", (now - r.atMs) / 1000);

android/app/src/main/java/com/openipc/decoder/QuadCell.java

@@ -438,6 +438,7 @@ static boolean isKeyFrameNal(int type, boolean h265) {
 
     /** The cooingdv heartbeat of an RTSP cell once it used one (#187), else null. */
     java.util.Map<String, Object> heartbeatStats() { return session.heartbeatStats(); }
+    java.util.Map<String, Object> udpListenStats() { return session.udpListenStats(); }
 
     /** Degrees counter-clockwise the frames are turned (#200, #201), 0 for none. */
     int rotate() { return session.rotate(); }
@@ -516,6 +517,9 @@ private void checkUdpStall() {
     /** On key frames only, a frame comes once per GOP, which can be several seconds (#85). */
     static final long KEY_FRAMES_LIVE_MS = 10_000;
 
+    /** A stream is coming in: "connected", paused or not. */
+    boolean isConnected() { return "connected".equals(state); }
+
     /**
      * Connected, not paused and a frame shown within the last {@code ms} milliseconds (#98);
      * on key frames within KEY_FRAMES_LIVE_MS, or the cell would blink between key frames.