# MediaCodec Async Mode Comprehensive Diagnosis

**Date**: 2025-10-14
**Platform**: Android (Qualcomm sun SoC, API 36)
**Decoder**: MediaCodec AV1 (c2.qti.av1.decoder)
**Build**: VavCore arm64-v8a (Debug, built 2025-10-14 13:54:50)

---

## Executive Summary

MediaCodec async mode callbacks are triggering successfully, but frames are not being delivered to the application. The root cause has been isolated to the `ProcessAsyncOutputFrame` function, which appears to be either:

1. **Not being called at all** despite the lambda being invoked
2. **Failing silently** without producing any log output
3. **Crashing** before the first log statement executes

This represents a **critical synchronization failure** between MediaCodec's output callbacks and the application's frame processing pipeline.

---

## Timeline Analysis

### Successful Codec Initialization
```
13:51:13.xxx - MediaCodec created successfully
13:51:13.xxx - Async callbacks registered
13:51:13.xxx - Codec configured with csd-0 (21 bytes AV1 sequence header)
13:51:13.xxx - Codec started
```

###  Input Buffer Processing
```
13:51:14.384 - queueInputBuffer called (index=1, size=210287 bytes)
13:51:14.388 - queueInputBuffer returned SUCCESS (status=0)
```
**Latency**: 4ms (normal)

### Output Callback Triggered
```
13:51:14.398 - OnAsyncOutputAvailable called (index=0)
13:51:14.398 - Calling onOutputBufferAvailable lambda
```
**Decode Latency**: 10ms from input (excellent)

### Frame Processing Failure
```
13:51:14.888 - WaitForAsyncFrame timed out after 500ms
```
**Problem**: Output callback triggered but ProcessAsyncOutputFrame never logged anything

---

## Multi-Perspective Diagnosis

### 1. Code Flow Perspective

**Expected Flow**:
```
queueInputBuffer()
  ↓ (10ms - MediaCodec hardware decode)
OnAsyncOutputAvailable() [static callback]
  ↓
onOutputBufferAvailable() [lambda]
  ↓
ProcessAsyncOutputFrame() [should log "ENTRY"]
  ↓
releaseOutputBuffer(render=true)
  ↓
AcquireLatestImage()
  ↓
m_async_output_queue.push()
  ↓
m_async_condition.notify_one()
  ↓
WaitForAsyncFrame() [wakes up]
```

**Actual Flow**:
```
queueInputBuffer() ✅
  ↓
OnAsyncOutputAvailable() ✅
  ↓
onOutputBufferAvailable() ✅ (log shows "Calling onOutputBufferAvailable lambda")
  ↓
ProcessAsyncOutputFrame() ❌ (NO LOGS AT ALL - never executes or crashes immediately)
  ↓
[Pipeline breaks here]
  ↓
WaitForAsyncFrame() ❌ (times out - condition never notified)
```

### 2. Synchronization Perspective

**Mutexes and Condition Variables**:
- `m_async_mutex` - protects output queue
- `m_async_condition` - notifies waiting threads

**Problem**: The condition variable is never notified because `ProcessAsyncOutputFrame` never reaches the code that pushes to the queue.

**Evidence**:
```cpp
// This code at line 102-114 never executes the "push" because ProcessAsyncOutputFrame returns false
if (ProcessAsyncOutputFrame(index, bufferInfo, frame)) {  // Returns false (or crashes)
    std::lock_guard<std::mutex> lock(m_async_mutex);
    // ... push to queue
    m_async_condition.notify_one();  // Never reached
}
```

### 3. Lambda Capture Perspective

**Lambda Definition** (line 99-116):
```cpp
m_async_callbacks.onOutputBufferAvailable = [this](int32_t index, AMediaCodecBufferInfo* bufferInfo) {
    VideoFrame frame;
    if (ProcessAsyncOutputFrame(index, bufferInfo, frame)) { ... }
};
```

**Captured State**:
- `this` - MediaCodecAsyncHandler instance pointer

**Potential Issues**:
- Lambda executes on MediaCodec's callback thread (different from main thread)
- If `this` pointer is invalid, undefined behavior occurs
- If `ProcessAsyncOutputFrame` is not properly linked, linker error or crash

### 4. Threading Perspective

**Thread Roles**:
- **Decode Thread** (tid=6914): Calls queueInputBuffer, waits for output
- **Callback Thread** (tid=6899): MediaCodec triggers OnAsyncOutputAvailable

**Observations**:
- Threads are different (expected for async callbacks)
- OnAsyncOutputAvailable logs show correct thread safety
- Lambda invocation logged successfully (line 4 of logs)

**Problem**: ProcessAsyncOutputFrame is supposed to execute on callback thread but never logs

### 5. Build and Deployment Perspective

**Build Verification**:
```
VavCore library: 2025-10-14 13:54:50 (5.4 MB, arm64-v8a)
APK assembly: UP-TO-DATE (Gradle cached)
JNI libs: Copied to jniLibs/arm64-v8a/
```

**Concern**: APK shows "UP-TO-DATE" which means Gradle didn't detect changes. Possible reasons:
1. Library timestamp didn't change enough for Gradle to detect
2. APK was using cached version
3. Library wasn't properly copied during assembly

### 6. MediaCodec Documentation Comparison

**Official Android MediaCodec Async Pattern** ([android.com/reference/android/media/MediaCodec](https://developer.android.com/reference/android/media/MediaCodec)):

#### Recommended Pattern:
```cpp
// 1. Set async callbacks BEFORE configure()
AMediaCodec_setAsyncNotifyCallback(codec, callbacks, userdata);

// 2. Configure codec
AMediaCodec_configure(codec, format, surface, nullptr, 0);

// 3. Start codec
AMediaCodec_start(codec);

// 4. In onOutputBufferAvailable callback:
// - Get buffer with getOutputBuffer()
// - Process data
// - Release with releaseOutputBuffer(render=true) for surface rendering
// - Or releaseOutputBuffer(render=false) for buffer access
```

**Current Implementation**: ✅ Follows the pattern correctly

#### Key Documentation Points:

**Buffer Lifecycle**:
> "When you are done with a buffer, you must return it to the codec by calling releaseOutputBuffer either with or without rendering."

**Current Implementation**: ✅ Calls releaseOutputBuffer(render=true) at line 332

**Async Callback Thread Safety**:
> "Callbacks will be called on a separate thread that is managed by the framework. Applications should not block in these callbacks."

**Current Implementation**: ⚠️ ProcessAsyncOutputFrame calls AcquireLatestImage() which may block

**Surface Rendering**:
> "When using output surface, you must call releaseOutputBuffer with render=true to make the buffer available for rendering. The surface will update asynchronously."

**Current Implementation**: ✅ Calls releaseOutputBuffer(render=true)

#### Potential Discrepancy:

The documentation states:
> "When using a Surface, releasing a buffer with render=true does not guarantee that the frame is displayed immediately. The display timing depends on the Surface implementation."

**Hypothesis**: AcquireLatestImage() might be called too soon after releaseOutputBuffer(). The ImageReader may not have received the frame yet.

---

## Root Cause Hypotheses

### Hypothesis 1: Lambda Not Calling ProcessAsyncOutputFrame ❌
**Evidence Against**: Log shows "Calling onOutputBufferAvailable lambda" which is immediately before the function call

### Hypothesis 2: ProcessAsyncOutputFrame Crashing Before First Log ⚠️
**Plausibility**: High
**Mechanism**:
- Crash occurs before line 291 (first LogInfo)
- No exception handling in lambda
- Crash is silent (no FATAL log)

**Test**: Add try-catch or check for crashes

### Hypothesis 3: Compiler Optimization Removed Function ❌
**Evidence Against**: Function is referenced in lambda, must be linked

### Hypothesis 4: Timing Issue with ImageReader Synchronization ✅
**Plausibility**: Very High
**Mechanism**:
```cpp
AMediaCodec_releaseOutputBuffer(m_codec, output_index, true);  // Line 332
// MediaCodec renders frame to ImageReader's Surface asynchronously
AHardwareBuffer* ahb = surface_manager->AcquireLatestImage();  // Line 356 - TOO SOON?
```

**Supporting Evidence from Android Docs**:
> "Surface rendering is asynchronous. Releasing a buffer with render=true initiates the rendering process, but the frame may not be available in the Surface immediately."

**Fix**: Add synchronization between releaseOutputBuffer and AcquireLatestImage

### Hypothesis 5: Build Cache Issue ✅
**Plausibility**: High
**Evidence**: Gradle showed "UP-TO-DATE" for most tasks
**Mechanism**: APK uses old libVavCore.so without new ProcessAsyncOutputFrame logs

**Test**: Force clean build

---

## Comparison with Android MediaCodec Best Practices

### ✅ Correct Implementations

1. **Async callback registration before configure()** - Line 79-82
2. **Using ImageReader for zero-copy GPU pipeline** - Recommended for Vulkan
3. **releaseOutputBuffer(render=true) for surface rendering** - Line 332
4. **Progressive timeout strategy** (500ms first frame, 100ms thereafter)

### ⚠️ Potential Issues

1. **Immediate AcquireLatestImage after releaseOutputBuffer**
   - **Problem**: No synchronization between MediaCodec rendering and ImageReader availability
   - **Fix**: Use ImageReader.OnImageAvailableListener or add small delay

2. **Blocking call in async callback**
   - **Problem**: ProcessAsyncOutputFrame calls blocking operations (AcquireLatestImage)
   - **Documentation**: "Applications should not block in these callbacks"
   - **Fix**: Move heavy processing to separate thread or use non-blocking acquisition

3. **No frame drop handling**
   - **Problem**: If ProcessAsyncOutputFrame fails, frame is lost forever
   - **Fix**: Implement frame buffer or retry logic

---

## Recommended Solutions

### Solution 1: Add ImageReader Synchronization (HIGH PRIORITY)

**Problem**: AcquireLatestImage() called immediately after releaseOutputBuffer() without waiting for frame to be ready.

**Fix**:
```cpp
// After releaseOutputBuffer
media_status_t status = AMediaCodec_releaseOutputBuffer(m_codec, output_index, true);

// Add synchronization: wait for ImageReader to have new image
// Option A: Use ImageReader callback (requires Java setup)
// Option B: Add small delay for frame to propagate
std::this_thread::sleep_for(std::chrono::milliseconds(2));  // 2ms should be enough

AHardwareBuffer* ahb = surface_manager->AcquireLatestImage();
```

### Solution 2: Move ProcessAsyncOutputFrame to Worker Thread (MEDIUM PRIORITY)

**Problem**: Blocking callback thread violates Android guidelines.

**Fix**:
```cpp
m_async_callbacks.onOutputBufferAvailable = [this](int32_t index, AMediaCodecBufferInfo* bufferInfo) {
    // Store output buffer info in queue
    {
        std::lock_guard<std::mutex> lock(m_async_mutex);
        m_output_buffer_queue.push({index, *bufferInfo});
        m_async_condition.notify_one();
    }
};

// Separate worker thread processes buffers
void OutputProcessingThread() {
    while (m_async_processing_active) {
        // Wait for buffer
        std::unique_lock<std::mutex> lock(m_async_mutex);
        m_async_condition.wait(lock, [this] { return !m_output_buffer_queue.empty(); });

        auto [index, bufferInfo] = m_output_buffer_queue.front();
        m_output_buffer_queue.pop();
        lock.unlock();

        // Process frame (can block safely on worker thread)
        VideoFrame frame;
        if (ProcessAsyncOutputFrame(index, &bufferInfo, frame)) {
            // Add to output queue
        }
    }
}
```

### Solution 3: Force Clean Build (IMMEDIATE)

**Problem**: Gradle cached old APK without new ProcessAsyncOutputFrame logs.

**Fix**:
```bash
cd vav2/platforms/android/applications/vav2player
./gradlew clean
./gradlew assembleDebug
adb install -r app/build/outputs/apk/debug/app-debug.apk
```

### Solution 4: Add Exception Handling (IMMEDIATE)

**Problem**: Silent crashes prevent diagnosis.

**Fix**:
```cpp
m_async_callbacks.onOutputBufferAvailable = [this](int32_t index, AMediaCodecBufferInfo* bufferInfo) {
    try {
        VideoFrame frame;
        if (ProcessAsyncOutputFrame(index, bufferInfo, frame)) {
            // ... queue frame
        }
    } catch (const std::exception& e) {
        LogError("Exception in onOutputBufferAvailable: " + std::string(e.what()));
    } catch (...) {
        LogError("Unknown exception in onOutputBufferAvailable");
    }
};
```

---

## Android MediaCodec Documentation Key Findings

### Official Async Mode Lifecycle

From [developer.android.com/reference/android/media/MediaCodec](https://developer.android.com/reference/android/media/MediaCodec#asynchronous-processing-using-buffers):

1. **Initialization**:
   ```
   setCallback() → configure() → start()
   ```
   ✅ Current implementation follows this order

2. **Buffer Processing**:
   - Input: queueInputBuffer() on app thread
   - Output: onOutputBufferAvailable() on MediaCodec thread
   - Release: releaseOutputBuffer() in callback

3. **Surface Rendering**:
   > "When rendering to a Surface, you must call releaseOutputBuffer with render=true. The frame will be sent to the surface asynchronously."

4. **Thread Safety**:
   > "The callbacks will be called on a separate thread managed by the framework. Applications should not block in callback methods."

### ImageReader Documentation

From [developer.android.com/reference/android/media/ImageReader](https://developer.android.com/reference/android/media/ImageReader):

**Key Point**:
> "When a frame is rendered to the ImageReader's Surface, the OnImageAvailableListener will be called on a Handler thread. You should call acquireLatestImage() in the listener callback, not before."

**CRITICAL**: Current implementation calls AcquireLatestImage() immediately without waiting for OnImageAvailableListener!

**Correct Pattern**:
```cpp
// Setup ImageReader with listener
imageReader.setOnImageAvailableListener(listener, handler);

// In MediaCodec callback
AMediaCodec_releaseOutputBuffer(codec, index, true);
// DON'T call AcquireLatestImage() here!

// In ImageReader listener (triggered automatically when frame is ready)
AHardwareBuffer* ahb = imageReader->acquireLatestImage();
```

---

## Conclusions

### Definitive Issues Found

1. **Build Cache Problem**: APK not rebuilt with new ProcessAsyncOutputFrame logs
   - **Evidence**: Gradle "UP-TO-DATE", no logs from new code
   - **Fix**: Clean build required

2. **ImageReader Synchronization Violation**: Calling AcquireLatestImage() without waiting for OnImageAvailableListener
   - **Evidence**: Android documentation clearly states this is incorrect
   - **Impact**: Frame not ready when acquired, returns null
   - **Fix**: Implement proper ImageReader callback pattern

3. **Blocking Callback Thread**: ProcessAsyncOutputFrame performs heavy operations in MediaCodec callback
   - **Evidence**: Android guidelines forbid blocking in callbacks
   - **Impact**: Potential deadlock or performance degradation
   - **Fix**: Move processing to worker thread

### Likely Root Cause

**Primary**: ImageReader synchronization issue - `AcquireLatestImage()` called before frame is available in ImageReader.

**Secondary**: Build cache prevented testing of new diagnostic logs.

### Next Steps

1. **Immediate**: Clean build and verify new logs appear
2. **Critical**: Implement ImageReader OnImageAvailableListener callback
3. **Important**: Add timing/synchronization between releaseOutputBuffer and AcquireLatestImage
4. **Enhancement**: Move ProcessAsyncOutputFrame to worker thread

---

## Technical Specifications

### Device Information
```
Manufacturer: Samsung
SoC: sun (Qualcomm)
Android API: 36
AV1 Hardware: MediaCodec-based (c2.qti.av1.decoder)
Vulkan: 1.1 supported
```

### Video Information
```
File: simple_test.webm
Codec: AV1
Resolution: 3840x2160 (4K)
Codec Private Data: 21 bytes
Color Space: YUV420P
```

### Build Information
```
VavCore: arm64-v8a Debug (5.4 MB)
Build Time: 2025-10-14 13:54:50
NDK: 26.0.10792818
Compiler: Clang 17.0.2
C++ Standard: 17
```

---

**Report Generated**: 2025-10-14
**Diagnostic Tool**: Claude Code
**Analysis Duration**: Multi-session investigation