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MediaCodec Async Mode

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# 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

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vav2/platforms/windows/vavcore/src/Decoder/MediaCodecAsyncHandler.cpp
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@@ -98,20 +98,26 @@ bool MediaCodecAsyncHandler::InitializeAsyncMode() {
m_async_callbacks.onOutputBufferAvailable = [this](int32_t index, AMediaCodecBufferInfo* bufferInfo) {
// Output buffer available - process in callback
VideoFrame frame;
if (ProcessAsyncOutputFrame(index, bufferInfo, frame)) {
std::lock_guard<std::mutex> lock(m_async_mutex);
try {
VideoFrame frame;
if (ProcessAsyncOutputFrame(index, bufferInfo, frame)) {
std::lock_guard<std::mutex> lock(m_async_mutex);
AsyncFrameData async_data;
async_data.frame = std::make_unique<VideoFrame>(std::move(frame));
async_data.timestamp_us = bufferInfo->presentationTimeUs;
// TODO: NDK 26 does not expose keyframe flag in AMediaCodecBufferInfo
// Keyframe detection needs to be done via other means (e.g., frame analysis)
async_data.is_keyframe = false; // Placeholder - keyframe flag not available in NDK 26
async_data.decode_start_time = std::chrono::steady_clock::now();
AsyncFrameData async_data;
async_data.frame = std::make_unique<VideoFrame>(std::move(frame));
async_data.timestamp_us = bufferInfo->presentationTimeUs;
// TODO: NDK 26 does not expose keyframe flag in AMediaCodecBufferInfo
// Keyframe detection needs to be done via other means (e.g., frame analysis)
async_data.is_keyframe = false; // Placeholder - keyframe flag not available in NDK 26
async_data.decode_start_time = std::chrono::steady_clock::now();
m_async_output_queue.push(std::move(async_data));
m_async_condition.notify_one();
m_async_output_queue.push(std::move(async_data));
m_async_condition.notify_one();
}
} catch (const std::exception& e) {
LogError("Exception in onOutputBufferAvailable: " + std::string(e.what()));
} catch (...) {
LogError("Unknown exception in onOutputBufferAvailable");
}
};
@@ -398,6 +404,14 @@ bool MediaCodecAsyncHandler::ProcessAsyncOutputFrame(int32_t output_index, AMedi
return false;
}
// CRITICAL FIX: Release previous frame's Image BEFORE releaseOutputBuffer()
// This prevents ImageReader buffer pool exhaustion that causes releaseOutputBuffer() to block
MediaCodecSurfaceManager* surface_manager = m_decoder->GetSurfaceManager();
if (surface_manager) {
surface_manager->ReleaseImage();
LogInfo("ProcessAsyncOutputFrame: Released previous frame's Image to prevent buffer deadlock");
}
LogInfo("ProcessAsyncOutputFrame: Getting output buffer...");
// Get output buffer (for validation, not actually used in surface mode)
@@ -432,6 +446,12 @@ bool MediaCodecAsyncHandler::ProcessAsyncOutputFrame(int32_t output_index, AMedi
return false;
}
// CRITICAL FIX: Wait for frame to be rendered to ImageReader's Surface
// releaseOutputBuffer(render=true) is asynchronous - MediaCodec needs time to render
// According to Android docs, we should use OnImageAvailableListener, but this is a simpler fix
LogInfo("ProcessAsyncOutputFrame: Waiting for frame to be rendered to ImageReader (5ms)...");
std::this_thread::sleep_for(std::chrono::milliseconds(5));
// Step 2: Acquire AHardwareBuffer from ImageReader
// Get SurfaceManager from decoder
LogInfo("ProcessAsyncOutputFrame: Getting SurfaceManager from decoder...");
@@ -444,17 +464,26 @@ bool MediaCodecAsyncHandler::ProcessAsyncOutputFrame(int32_t output_index, AMedi
LogInfo("ProcessAsyncOutputFrame: SurfaceManager obtained successfully");
// Acquire latest image from ImageReader
// Acquire latest image from ImageReader (with retry for timing issues)
LogInfo("ProcessAsyncOutputFrame: Calling AcquireLatestImage...");
AHardwareBuffer* ahb = surface_manager->AcquireLatestImage();
AHardwareBuffer* ahb = nullptr;
const int max_retries = 3;
for (int retry = 0; retry < max_retries && !ahb; ++retry) {
ahb = surface_manager->AcquireLatestImage();
if (!ahb && retry < max_retries - 1) {
LogWarning("ProcessAsyncOutputFrame: AcquireLatestImage returned null, retrying in 2ms (attempt " +
std::to_string(retry + 1) + "/" + std::to_string(max_retries) + ")");
std::this_thread::sleep_for(std::chrono::milliseconds(2));
}
}
LogInfo("ProcessAsyncOutputFrame: AcquireLatestImage returned: ahb=" +
std::to_string(reinterpret_cast<uintptr_t>(ahb)));
if (!ahb) {
// This is normal during initial buffering - no image ready yet
LogWarning("ProcessAsyncOutputFrame: No image available from ImageReader (buffering) - THIS IS THE FAILURE POINT!");
LogError("ProcessAsyncOutputFrame: Failed to acquire image from ImageReader after " +
std::to_string(max_retries) + " attempts");
return false;
}