fix: U cell / udp:// slots — lossy pushed stream decoded instead of staying black (#133)

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

@@ -1402,6 +1402,10 @@ private Map<String, Object> debugState() {
                     c.put("lost_fps", cell.lostFps());
                     // NAL units dropped because one of their packets was lost (#113)
                     c.put("nal_dropped", cell.nalDropped());
+                    // Slices passed on cut short at a lost piece (some may still be dropped by
+                    // a full queue); absent while that is off (#133)
+                    int salvaged = cell.nalSalvaged();
+                    if (salvaged >= 0) c.put("nal_damaged", salvaged);
                     // Every frame, or key frames only while the decoder has no room (#85)
                     c.put("decode", cell.isKeyFramesOnly() ? "key frames" : "full");
                     if (cell.knowsLoad()) c.put("load", cell.isKeyFramesOnly()
@@ -1715,13 +1719,20 @@ public void run() {
             int frames = videoDecoder.framesOut() + jpegFramesOut;
             long bytes = client != null ? client.getStreamBytes() : 0;
             boolean stalled = !quadEnabled && !udpViewShown && client != null
-                    && client.playsOverUdp() && "connected".equals(singleState)
+                    && (client.playsOverUdp() || client.listens())
+                    && "connected".equals(singleState)
                     && bytes - lastSingleBytes > 10_000 && frames == lastSingleFrames;
             lastSingleFrames = frames;
             lastSingleBytes = bytes;
             singleStallSeconds = stalled ? singleStallSeconds + 1 : 0;
             if (singleStallSeconds >= UDP_STALL_SECONDS) {
                 singleStallSeconds = 0;
+                // Every key frame loses a piece: decode them damaged rather than not at all;
+                // a pushed stream has no TCP to go to (#133)
+                if (!nalAssembler.salvaging()) {
+                    nalAssembler.setSalvage(true);
+                    Log.w(TAG, "No complete key frame over UDP, decoding damaged ones");
+                }
                 client.reportUdpBroken("packets but no decoded frame for " + UDP_STALL_SECONDS
                         + " s");
             }

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

@@ -44,6 +44,13 @@ interface Sink {
     private int dropped;
     /** Of those, dropped because a packet of them was lost (not for size) (#113). */
     private int lostDropped;
+    /**
+     * Pass a NAL unit with a lost piece on cut short at the gap instead of dropping it (#133):
+     * for a stream that loses a piece of every key frame, where dropping them leaves nothing to
+     * decode. The decoder conceals the missing part; a damaged picture beats a black one.
+     */
+    private volatile boolean salvage;
+    private int salvaged;
 
     NalAssembler(int bufferSize, Runnable onCodecSwitch, FramePool framePool) {
         this.nalBuffer = new byte[bufferSize];
@@ -63,6 +70,11 @@ void reset() {
     /** NAL units dropped because one of their packets was lost (#113). */
     int lossCount() { return lostDropped; }
 
+    void setSalvage(boolean on) { salvage = on; }
+    boolean salvaging() { return salvage; }
+    /** Slices passed on to the sink cut short at a lost piece (#133); the queue may drop some. */
+    int salvagedCount() { return salvaged; }
+
     /**
      * Process one RTP packet ({@code length} bytes at {@code offset}, header included).
      * Returns false if the RTP header is malformed.
@@ -89,7 +101,7 @@ boolean assemble(byte[] buf, int offset, int length, boolean codecH265, Sink sin
         lastSeq = seq;
         if (gap && inFragment) {
             lostDropped++;
-            drop("packet loss inside a fragmented NAL unit");
+            dropOrSalvage("packet loss inside a fragmented NAL unit", sink);
             orphan = true; // its remaining pieces belong to the unit just counted
         }
 
@@ -104,7 +116,7 @@ boolean assemble(byte[] buf, int offset, int length, boolean codecH265, Sink sin
         // Any other packet ends a fragmented NAL unit that did not finish; its remaining
         // pieces, if any arrive, belong to the unit counted here
         if (inFragment) {
-            drop("fragmented NAL unit interrupted by another packet");
+            dropOrSalvage("fragmented NAL unit interrupted by another packet", sink);
             orphan = true;
         } else {
             orphan = false;
@@ -131,7 +143,7 @@ private void fragmentUnit(byte[] buf, int start, int end, boolean h265, Sink sin
         if (first) {
             if (inFragment) {
                 lostDropped++;
-                drop("fragmented NAL unit without its end");
+                dropOrSalvage("fragmented NAL unit without its end", sink);
             }
             orphan = false;
             nalSize = 0;
@@ -201,6 +213,32 @@ private void emit(byte[] buf, int pos, int size, Sink sink) {
         sink.onNal(out);
     }
 
+    /** The unit collected so far, cut short, when salvaging (#133); dropped otherwise. */
+    private void dropOrSalvage(String reason, Sink sink) {
+        // Start code and NAL header plus some slice data, or there is nothing to decode; only
+        // slices: a cut parameter set would be taken for a valid configuration (Qodo on #146)
+        if (!salvage || nalSize < 16 || !isSlice(nalBuffer[4], lastCodec == 1)) {
+            drop(reason);
+            return;
+        }
+        Frame out = framePool.obtain(nalSize);
+        System.arraycopy(nalBuffer, 0, out.data(), 0, nalSize);
+        out.setLength(nalSize);
+        nalSize = 0;
+        inFragment = false;
+        if (salvaged++ % 100 == 0) {
+            Log.w(TAG, "NAL unit passed on cut short (" + salvaged + "): " + reason);
+        }
+        sink.onNal(out);
+    }
+
+    /** A coded slice (VCL NAL unit): H.264 types 1–5, H.265 types 0–31. */
+    static boolean isSlice(byte nalHeader, boolean h265) {
+        if (h265) return ((nalHeader >> 1) & 0x3F) <= 31;
+        int type = nalHeader & 0x1F;
+        return type >= 1 && type <= 5;
+    }
+
     private void drop(String reason) {
         nalSize = 0;
         inFragment = false;

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

@@ -274,6 +274,9 @@ void stop() {
     public void onStatus(String status) {
         // A new connection (or a lost one) starts the UDP stall count afresh (Qodo on #125)
         if (!status.equals(state)) udpStallSamples = 0;
+        // A pushed stream that went quiet may come back whole: no damaged decoding until it
+        // stalls again (Qodo on #146)
+        if ("disconnected".equals(status)) nalAssembler.setSalvage(false);
         state = status;
         // A connection that works again clears the last connection error
         if ("connected".equals(status) && problem != null && !problem.startsWith("Decoder")) {
@@ -346,9 +349,15 @@ String sample(long nowMs) {
             // MJPEG has no key frames: its cheap mode shows one frame per second (#132)
             return size + codec + (jpeg ? " reduced" : " key frames") + " · " + rate + " fps"
                     + ((jpeg ? jpegRenderer.isSoftware() : decoder.isSoftware())
-                            ? " · software" : "");
+                            ? " · software" : "") + damaged();
         }
-        return size + codec + " " + Math.round(outFps) + "/" + Math.round(inFps) + " fps";
+        return size + codec + " " + Math.round(outFps) + "/" + Math.round(inFps) + " fps"
+                + damaged();
+    }
+
+    /** Corner note while damaged key frames are decoded (#133). */
+    private String damaged() {
+        return !jpeg && nalAssembler.salvaging() ? " · damaged" : "";
     }
 
     boolean isKeyFramesOnly() { return keyFramesOnly; }
@@ -421,7 +430,9 @@ private void checkUdpStall() {
         lastVideoPackets = packets;
         lastLossCount = loss;
         Surface s = surface;
-        boolean stalled = session.playsOverUdp() && "connected".equals(state)
+        // A pushed stream (udp:// slot, the U cell) counts too: no TCP to fall back to (#133)
+        boolean stalled = (session.playsOverUdp() || session.listens())
+                && "connected".equals(state)
                 && pausedReason == null && problem == null && s != null && s.isValid()
                 && arriving && outFps == 0;
         if (!stalled) {
@@ -433,6 +444,12 @@ private void checkUdpStall() {
         if (losing || udpStallSamples > 0) udpStallSamples++;
         if (udpStallSamples >= UDP_STALL_SAMPLES) {
             udpStallSamples = 0;
+            // Every key frame loses a piece: decode them damaged rather than not at all. A
+            // pushed stream has no TCP to go to; over RTSP, Auto goes to TCP anyway (#133)
+            if (!jpeg && !nalAssembler.salvaging()) {
+                nalAssembler.setSalvage(true);
+                Log.w(TAG, tag + ": no complete key frame over UDP, decoding damaged ones");
+            }
             session.reportUdpBroken("packets but no decoded frame for " + UDP_STALL_SAMPLES
                     + " s (" + loss + (jpeg ? " JPEG frames" : " NAL units") + " lost)");
         }
@@ -441,6 +458,9 @@ private void checkUdpStall() {
     /** NAL units dropped so far because a packet of them was lost (#113). */
     int nalDropped() { return nalAssembler.lossCount(); }
 
+    /** Slices passed on cut short at a lost piece, or -1 while that is off (#133). */
+    int nalSalvaged() { return nalAssembler.salvaging() ? nalAssembler.salvagedCount() : -1; }
+
     float inFps() { return inFps; }
     float outFps() { return outFps; }
     float lostFps() { return lostFps; }
@@ -517,6 +537,8 @@ public void onSdp(Sdp sdp) {
         lastLossCount = sdp.videoJpeg ? jpegAssembler.droppedCount() : nalAssembler.lossCount();
         udpStallSamples = 0;
         nalAssembler.reset();
+        // A new stream gets a chance to be complete again (#133)
+        nalAssembler.setSalvage(false);
         jpegAssembler.reset();
         lastUnknownPayload = -1;
         videoPt = sdp.videoPt;

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

@@ -75,6 +75,8 @@ void configure(String host, String userAgent, Transport.Mode transport) {
     boolean isActive() { return session.isActive(); }
     /** RTP over UDP or multicast at the moment (#128). */
     boolean playsOverUdp() { return session.playsOverUdp(); }
+    /** A udp:// slot listening for a pushed stream (#133). */
+    boolean listens() { return session.listens(); }
     /** Packets arrive but nothing decodes: with Auto the session goes over to TCP. */
     void reportUdpBroken(String why) { session.reportUdpBroken(why); }
     /** Transport the stream really uses now, or null (#115). */
@@ -101,13 +103,17 @@ void stop() {
 
     @Override
     public void onStatus(String status) {
+        // A pushed stream that went quiet may come back whole (Qodo on #146)
+        if ("disconnected".equals(status)) nalAssembler.setSalvage(false);
         Listener l = listenerCb;
         if (l != null) l.onStatusChanged(status);
     }
 
     @Override
     public void onSdp(Sdp sdp) {
         nalAssembler.reset();
+        // A new stream gets a chance to be complete again (#133)
+        nalAssembler.setSalvage(false);
         lastUnknownPayload = -1;
         lastRtpTimestamp = -1;
         lastRtpArrivalNs = -1;

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

@@ -140,6 +140,9 @@ private void markUdpFailed(Transport.Mode mode) {
     /** RTSP is playing with RTP over UDP or multicast (not TCP, not a udp:// URL). */
     boolean playsOverUdp() { return udpPlaying; }
 
+    /** A udp:// slot listening for a pushed stream (#133): UDP too, without RTSP. */
+    boolean listens() { return listening; }
+
     /**
      * Packets arrive over UDP but the stream cannot be decoded (#113: key frames keep losing
      * a piece). With Auto or Multicast the session goes over to TCP, as when no video comes

android/app/src/test/java/com/openipc/decoder/NalAssemblerTest.java

@@ -138,6 +138,52 @@ public void lostMiddleFragmentDropsTheNal() {
         assertArrayEquals(b(0, 0, 0, 1, 0x65, 0xD1, 0xD2), out.get(0));
     }
 
+    @Test
+    public void salvagePassesTheNalOnCutShortAtTheGap() {
+        nal.setSalvage(true);
+        // 16 slice bytes before the gap: enough to be worth decoding (#133)
+        feed(rtp(0x7C, 0x85, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12), false);
+        feed(rtp(0x7C, 0x05, 13, 14, 15, 16), false);
+        seq++; // one packet lost
+        feed(rtp(0x7C, 0x05, 0xB1), false);
+        assertEquals(1, out.size());
+        assertArrayEquals(b(0, 0, 0, 1, 0x65, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
+                16), out.get(0));
+        assertEquals(1, nal.salvagedCount());
+        assertEquals(1, nal.lossCount());
+        // The rest of that unit is skipped; the next one is complete again
+        feed(rtp(0x7C, 0x45, 0xC1), false);
+        feed(rtp(0x7C, 0x85, 0xD1), false);
+        feed(rtp(0x7C, 0x45, 0xD2), false);
+        assertEquals(2, out.size());
+        assertArrayEquals(b(0, 0, 0, 1, 0x65, 0xD1, 0xD2), out.get(1));
+    }
+
+    @Test
+    public void salvageNeverPassesACutParameterSet() {
+        nal.setSalvage(true);
+        // FU-A of an SPS (type 7), then a lost packet (Qodo on #146)
+        feed(rtp(0x7C, 0x87, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16), false);
+        seq++;
+        feed(rtp(0x7C, 0x47, 0xC1), false);
+        assertEquals(0, out.size());
+        assertEquals(0, nal.salvagedCount());
+        assertTrue(NalAssembler.isSlice((byte) 0x65, false));
+        assertFalse(NalAssembler.isSlice((byte) 0x68, false));
+        // H.265: IDR_W_RADL (19) is a slice, VPS (32) is not
+        assertTrue(NalAssembler.isSlice((byte) (19 << 1), true));
+        assertFalse(NalAssembler.isSlice((byte) (32 << 1), true));
+    }
+
+    @Test
+    public void salvageOffDropsAsBefore() {
+        feed(rtp(0x7C, 0x85, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16), false);
+        seq++;
+        feed(rtp(0x7C, 0x45, 0xC1), false);
+        assertEquals(0, out.size());
+        assertEquals(0, nal.salvagedCount());
+    }
+
     @Test
     public void continuationWithoutStartIsIgnoredAndCountedOnce() {
         feed(rtp(0x7C, 0x05, 0xB1), false);