app: guard VideoSink.submit against torn-down codec

reconfigure() tears the codec down outside the lock, so an in-flight
MediaCodec callback can reach submit() while `codec` is null. The NPE
lands on the callback handler thread, which has no catch and kills
the process. Drop the frame instead, like any back-pressure casualty.

Also rewrite the back-pressure comment: it described a keyframe-aware
drop policy this class cannot implement (the keyframe flag never
reaches it). The real policy protects config frames only.

1 files changed, 9 insertions, 5 deletions

diff --git a/app/src/main/java/invalid/lena/scrcpy/VideoSink.java b/app/src/main/java/invalid/lena/scrcpy/VideoSink.java
index fa8248b..2d69339 100644
--- a/app/src/main/java/invalid/lena/scrcpy/VideoSink.java
+++ b/app/src/main/java/invalid/lena/scrcpy/VideoSink.java
@@ -19,10 +19,11 @@ import java.util.Iterator;
 // meet through a small queue of pending frames waiting for input buffers,
 // plus a corresponding pool of free input buffer indices.
 //
-// Back-pressure policy: when both queues fill, drop the *next* incoming
-// frame unless it is a config or keyframe (we have no way to know its
-// type from outside). For v1 we simply drop oldest pending non-keyframes
-// if the pending queue grows past a small bound.
+// Back-pressure policy: when no input buffer is free and the pending
+// queue is full, the oldest pending non-config frame is dropped. Config
+// frames (CSD) always survive - the decoder cannot start without them.
+// Keyframes are not distinguishable here (the flag stays in VideoStream)
+// so they drop like any delta frame; the picture heals at the next one.
 //
 // Output timing: releaseOutputBuffer is called with an absolute nano
 // timestamp on the System.nanoTime clock, derived from the wire PTS so
@@ -203,8 +204,11 @@ public final class VideoSink implements VideoFrames {
         }
     }
 
-    // Must be called with `lock` held.
+    // Must be called with `lock` held. codec can be null mid-reconfigure
+    // (teardownCodec runs unlocked); the frame is dropped like any other
+    // back-pressure casualty.
     private void submit(Frame f, int idx) {
+        if (codec == null) return;
         try {
             ByteBuffer buf = codec.getInputBuffer(idx);
             if (buf == null) return;