pbs video downloads stopped working with "ERROR: m3u8 download detected but ffmpeg or avconv could not be found."

[GrumpyOldTroll]

1. Problem: Fetching pbs videos used to succeed (using a mp4 from alternate_encoding, which is still there), but started failing sometime in the last few months.
2. Fix: Probably a better solution than what I tried would be to discover the downloaders for the available formats and give them a method that allows them to reject it without attempting the download, and walk backwards through the available formats and checking that method before selecting the last entry in available_formats as the preferred encoding.
3. Proposed solution: I have a simpler hack patch, which removes all m3u8 files from the available_formats if ffmpeg is not available. This is not a good way to leave the knowledge inside the downloaders, but after this patch, the same command succeeds:

$ git diff --staged youtube_dl/YoutubeDL.py
diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index 4cf83c5..8633a6f 100755
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -914,6 +914,9 @@ class YoutubeDL(object):
         if not available_formats:
             return None

+        if not FFmpegMergerPP(self).available:
+            available_formats = filter(lambda x: x.get(u'protocol') != u'm3u8', available_formats)
+
         if format_spec in ['best', 'worst', None]:
             format_idx = 0 if format_spec == 'worst' else -1
             audiovideo_formats = [
$

1. Verbose output:

Here's the output of a sample run, demonstrating the problem (presumably must not have ffmpeg installed):

$ youtube-dl/youtube_dl/__main__.py --verbose http://video.pbs.org/video/2365472415/
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'--verbose', u'http://video.pbs.org/video/2365472415/']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2015.05.10
[debug] Git HEAD: c1c924a
[debug] Python version 2.7.6 - Darwin-14.3.0-x86_64-i386-64bit
[debug] exe versions: none
[debug] Proxy map: {}
[PBS] 2365472415: Downloading JSON metadata
[PBS] 2365472415: Downloading recommended_encoding video url info
[PBS] 2365472415: Downloading m3u8 information
[PBS] 2365472415: Downloading alternate_encoding video url info
[debug] Invoking downloader on u'http://ga.video.cdn.pbs.org/videos/nova/c21a548f-9003-4a20-aebf-58865db2084c/181693/hd-mezzanine-16x9/069e992c_nova_4208_test_rez-16x9-hls-2500k.m3u8'
[download] Destination: Invisible Universe Revealed-2365472415.mp4
ERROR: m3u8 download detected but ffmpeg or avconv could not be found. Please install one.
  File "youtube-dl/youtube_dl/__main__.py", line 19, in <module>
    youtube_dl.main()
  File "/private/tmp/youtube-dl/youtube_dl/__init__.py", line 399, in main
    _real_main(argv)
  File "/private/tmp/youtube-dl/youtube_dl/__init__.py", line 389, in _real_main
    retcode = ydl.download(all_urls)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 1483, in download
    res = self.extract_info(url)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 660, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 706, in process_ie_result
    return self.process_video_result(ie_result, download=download)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 1154, in process_video_result
    self.process_info(new_info)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 1416, in process_info
    success = dl(filename, info_dict)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 1358, in dl
    return fd.download(name, info)
  File "/private/tmp/youtube-dl/youtube_dl/downloader/common.py", line 342, in download
    return self.real_download(filename, info_dict)
  File "/private/tmp/youtube-dl/youtube_dl/downloader/hls.py", line 27, in real_download
    self.report_error('m3u8 download detected but ffmpeg or avconv could not be found. Please install one.')
  File "/private/tmp/youtube-dl/youtube_dl/downloader/common.py", line 155, in report_error
    self.ydl.report_error(*args, **kargs)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 527, in report_error
    self.trouble(error_message, tb)
  File "/private/tmp/youtube-dl/youtube_dl/YoutubeDL.py", line 489, in trouble
    tb_data = traceback.format_list(traceback.extract_stack())

I think that covers everything the guidelines asked for. Cheers. (edit: added markdown to get raw text to show up better)