[FFmpeg] Python 2.7 怎样合理的包裹外部指令安定调用第三方工具并安全结束?

背景

• 工程中原先是用 py 基于 opencv 从摄像设备连续获得视频帧处理后再输出到硬盘为视频 /图片
• 但是, 发现对系统负荷过大
• 所以, 尝试用 FFmpeg 代为处理
• 但是, 通过摸索, 找到合理的指令, 并尝试用 py 进行调用时失败

env.

• macOS 10.12.6 i7 3.1Ghz 4 核, 16Gb DDR3 内存
• Python 2.7.10 (default, Oct 13 2016, 22:16:45)
  • [GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.38)] on darwin
• ffmpeg version N-92539-g1035206102 Copyright (c) 2000-2018 the FFmpeg developers
  • built with Apple LLVM version 10.0.0 (clang-1000.11.45.5)
• GNU bash，版本 4.4.12(1)-release (x86_64-apple-darwin16.3.0)

现象

类似指令:

ffmpeg -f avfoundation -s 1920x1080 -framerate 25 -i 0: -vsync 0 -f image2 /path/2/.../wframe/%06d.jpg

直接运行:

ffmpeg version N-92539-g1035206102 Copyright (c) 2000-2018 the FFmpeg developers
  built with Apple LLVM version 10.0.0 (clang-1000.11.45.5)

...

[avfoundation @ 0x7fa94c800000] Selected pixel format (yuv420p) is not supported by the input device.
[avfoundation @ 0x7fa94c800000] Supported pixel formats:
[avfoundation @ 0x7fa94c800000]   uyvy422
[avfoundation @ 0x7fa94c800000]   yuyv422
[avfoundation @ 0x7fa94c800000]   nv12
[avfoundation @ 0x7fa94c800000]   0rgb
[avfoundation @ 0x7fa94c800000]   bgr0
[avfoundation @ 0x7fa94c800000] Overriding selected pixel format to use uyvy422 instead.
[avfoundation @ 0x7fa94c800000] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, avfoundation, from '0:':
  Duration: N/A, start: 11907.278033, bitrate: N/A
    Stream #0:0: Video: rawvideo (UYVY / 0x59565955), uyvy422, 1920x1080, 1000k tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[swscaler @ 0x7fa94b871600] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to '/path/2/.../wframe/%06d.jpg':
  Metadata:
    encoder         : Lavf58.23.100
    Stream #0:0: Video: mjpeg, yuvj422p(pc), 1920x1080, q=2-31, 200 kb/s, 1000k fps, 1000k tbn, 1000k tbc
    Metadata:
      encoder         : Lavc58.40.100 mjpeg
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=   55 fps= 16 q=24.8 Lsize=N/A time=00:00:03.56 bitrate=N/A speed=1.01x
...

没有问题...

核心代码:

import subprocess, time

cmd = ['ffmpeg'
        , '-f', 'avfoundation'
        , '-s', '1920x1080' 
        , '-framerate', '25'    # frames per second
        , '-i', '{}:'.format(drivers['w'])
        , '-vsync', '0'
        , '-f', 'image2'
        , '{}/wframe/%06d.jpg'.format(_expath)
        ]


p = subprocess.Popen(cmd
    , shell=False
    , stdin=subprocess.PIPE
    #, stdout=subprocess.PIPE
    # merge err>out
    #, stderr=subprocess.STDOUT
)

time.sleep(4)
p.terminate()

用 subprocess.Popen 包裹后, 在合理时机用 terminate() 来安全结束, 可是在 mac 中运行时报错:

[avfoundation @ 0x7f9f01000000] Failed to create AV capture input device: Cannot Use UNIQUESKY_CAR_CAMERA
0:: Input/output error

修订 shell=True ,或是打开其它 PIPE 都不能解决;

相同硬件, 到 win10 环境中, 用相同版本 FFmpeg 来调用, 调整好的指令类似:

ffmpeg -f dshow -s:v 1920x1080 -framerate 25 video="Integrated Webcam" -f image2 /path/2/.../wframe/%06d.jpg

一样直接在 cmd 中运行正常, 但是, 用 python 包装后, 一样出现不可用报错:

: Input/output error

[avfoundation @ 0x7fa5f7802c00] Failed to create AV capture input device: Cannot Use UNIQUESKY_CAR_CAMERA #2
0:: Input/output error

: 设备争用

[dshow @ 0000014269bab740] Could not run graph (sometimes caused by a device already in use by other application)
video=@device_pnp_\\?\usb#vid_05a3&pid_9230&mi_00#6&1d3a4c24&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global: I/O error

此时检查:

λ taskkill /IM ffmpeg.exe
错误: 没有找到进程 "ffmpeg.exe"。

并没有其它进程在使用硬件

PS:

用 FFmpeg 在 win10 环境中检验设备时是这样的

λ ffmpeg -list_devices true -f dshow -i dummy
ffmpeg version N-92539-g1035206102 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 8.2.1 (GCC) 20181017

...

[dshow @ 0000021f90a2a840] DirectShow video devices (some may be both video and audio devices)
[dshow @ 0000021f90a2a840]  "Integrated Webcam"
[dshow @ 0000021f90a2a840]     Alternative name "@device_pnp_\\?\usb#vid_1bcf&pid_0b09&mi_00#7&1bf8ceab&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global"
[dshow @ 0000021f90a2a840] DirectShow audio devices
[dshow @ 0000021f90a2a840]  "楹﹀厠椋?(4- USB PnP Sound Device)"
[dshow @ 0000021f90a2a840]     Alternative name "@device_cm_{33D9A762-90C8-11D0-BD43-00A0C911CE86}\wave_{25AB1848-CB77-42E5-9985-767755AA0C9C}"
dummy: Immediate exit requested

即, 所有可用设备有两种指代:

• USB ID, 类似 "Integrated Webcam"
• 内部别名: 类似 "@device_pnp_\?\usb#vid_1bcf&pid_0b09&mi_00#7&1bf8ceab&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global"
• 之前为防止多设备 /重名设备, 用别名来引用的
• 效果和用 ID 名称没区别.

suggest

有老司机称:

其实是这样的，windows 那个 cmd，如果你给它双引号，好像也是会被 shell 吃掉的，然后程序的 argv 没有引号，跟 bash 有点类似;

• 比如你给 python a=1 b="2" "c=3"
• 然后 python 里的 sys.argv == {blah, 'a=1', 'b=2', 'c=3'} 引号就被 cmd 吃掉了
• 单引号不会被 cmd 吃掉，双引号都没了

另外，

• subprocess 运行的时候比较推荐的
• 可能还是第一个参数是个 list，
• 然后 shell=False，
• 这样可以避免若干头疼的引号问题和命令行注入的问题

但是, 按照以上建议无论怎么折腾都是无法正常通过 Python 完成 FFMpeg 的调用.

大家有什么其它思路?

参考:

• subprocess – Work with additional processes - Python Module of the Week
• Read and write video frames in Python using FFMPEG - __del__( self )
• Processing Camera stream in Opencv, pushing it over RTMP (NGINX RTMP Module) using FFMPEG - Stack Overflow
• Hacking FFmpeg With Python - Part One
• python - Getting realtime output from ffmpeg to be used in progress bar (PyQt4, stdout) - Stack Overflow
• ...