TL;DR - don't just AVAssetWriter.finishWriting {} because then the last written frame is T_End. Instead, use AVAssetWriter.endSession(atSourceTime:) to set T_End to be the time of the last written video frame.
AVCaptureVideoDataOutputSampleBufferDelegate TO THE RESCUE!!
Use AVCapture(Video|Audio)DataOutputSampleBufferDelegate to write buffers to the AVAssetWriter (attach delegates to AVCaptureVideoDataOutput and AVCaptureAudioDataOutput)
Once the session is started and your outputs are going they're going to constantly be spitting out data onto this delegate

1. canWrite is a flag that tracks whether you should be recording (writing sampleBuffers to the AVAssetWriter) or not
2. In order to prevent leading black frames we need to make sure the first frame is a video frame. Until we get a video frame even if we're recording we ignore the frames. startSession(atSourceTime:) sets T0 for the asset, which we're setting to be the time of the first video frame
3. Every time a video frame is written, record that time on a separate queue. This frees up the delegateQueue to only do frame processing//writing as well as guaranteeing that stopping recording (which will be triggered from the main queue) will not have collisions or memory issues when reading the lastVideoFrameWrite
   Now for the fun part!
4. In order to prevent trailing black frames, we have the AVAssetWriter end its session at T_lastVideoFrameTime. This discards all frames (audio and video) that were written after T_lastVideoFrameTime ensuring that both assetTracks inside the AVAssetWriter are as synced up as possible.

RESULTS

BUFFER | asset duration: 1.8683333333333334
BUFFER | video track duration: 1.8683333333333334
BUFFER | Audio track duration: 1.868
BUFFER | Asset Delta: 0.0003333333333332966

BUFFER | asset duration: 1.435
BUFFER | video track duration: 1.435
BUFFER | Audio track duration: 1.4343333333333332
BUFFER | Asset Delta: 0.0006666666666668153

BUFFER | asset duration: 1.8683333333333334
BUFFER | video track duration: 1.8683333333333334
BUFFER | Audio track duration: 1.8682291666666666
BUFFER | Asset Delta: 0.00010416666666679397

BUFFER | asset duration: 1.435
BUFFER | video track duration: 1.435
BUFFER | Audio track duration: 1.4343541666666666
BUFFER | Asset Delta: 0.0006458333333334565

LOOK AT THOSE DELTAS!!!!! all sub millisecond. Very nice.

CODE
To Record

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
    guard CMSampleBufferDataIsReady(sampleBuffer) else {
        return
    }
    if output == audioDataOutput {
        // PROCESS AUDIO BUFFER
    }
    if output == videoDataOutput {
        // PROCESS VIDEO BUFFER
    }
    
    // 1
    let writable = canWrite
    let time = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
    if writable && sessionAtSourceTime == nil {
        // 2
        if output == videoDataOutput {
            sessionAtSourceTime = time
            videoWriter.startSession(atSourceTime: sessionAtSourceTime!)
        } else {
            return
        }
    }
    
    if output == videoDataOutput && writable {
        if videoWriterInput != nil {
            if videoWriterInput.isReadyForMoreMediaData {
                //Write video buffer
                videoWriterInput.append(sampleBuffer)
                // 3
                WBufferCameraSessionController.finishRecordQueue.async {
                    self.lastVideoFrameWrite = time
                }
            }
        }
    } else if writable,
              output == audioDataOutput,
              audioWriterInput != nil,
              audioWriterInput.isReadyForMoreMediaData {
        //Write audio buffer
        audioWriterInput.append(sampleBuffer)
    }
    if output == videoDataOutput {
        bufferDelegate?.didOuputVideoBuffer(buffer: sampleBuffer)
    }
}

Stop Recording

func stopRecording() {
    guard isRecording else {
        return
    }
    guard isStoppingRecording == false else {
        return
    }
    isStoppingRecording = true
    WBufferCameraSessionController.finishRecordQueue.async {
        // 4
        if self.lastVideoFrameWrite != nil {
            self.videoWriter.endSession(atSourceTime: self.lastVideoFrameWrite)
        }
        self.videoWriter.finishWriting { 
             // cleanup, do stuff with finished file if writing was successful
             ...
        }
    ...
    }
}

要删除最后一帧，请参阅nickneedsaname的答案
删除第一帧。我们需要在获得第一个图像帧后开始会话。

// if sessionAtSourceTime == nil {
//     sessionAtSourceTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
//     videoWriter.startSession(atSourceTime: sessionAtSourceTime!)
// }

if output == videoDataOutput, videoWriterInput.isReadyForMoreMediaData {
    // ---> MOVE TO HERE
    if sessionAtSourceTime == nil {
        sessionAtSourceTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
        videoWriter.startSession(atSourceTime: sessionAtSourceTime!)
    }
    
    // Write video buffer
    videoWriterInput.append(sampleBuffer)
    self.lastVideoFrameWriteTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
} else if sessionAtSourceTime != nil && output == audioDataOutput, audioWriterInput.isReadyForMoreMediaData {
    // Write audio buffer
    self.audioWriterInput.append(sampleBuffer)
}