[external/FFmpeg] Fix and improve --ffmpeg-location handling

* pass YoutubeDL (FileDownloader) to FFmpegPostProcessor constructor
* consolidate path search in FFmpegPostProcessor
* make availability of FFmpegFD depend on existence of FFmpegPostProcessor
* detect ffmpeg executable on instantiation of FFmpegFD
* resolves #32735

2 files changed, 24 insertions, 31 deletions

diff --git a/youtube_dl/downloader/external.py b/youtube_dl/downloader/external.py
index f22fa6013..4fbc0f520 100644
--- a/youtube_dl/downloader/external.py
+++ b/youtube_dl/downloader/external.py
@@ -13,7 +13,12 @@ from ..compat import (
     compat_str,
     compat_subprocess_Popen,
 )
-from ..postprocessor.ffmpeg import FFmpegPostProcessor, EXT_TO_OUT_FORMATS
+
+try:
+    from ..postprocessor.ffmpeg import FFmpegPostProcessor, EXT_TO_OUT_FORMATS
+except ImportError:
+    FFmpegPostProcessor = None
+
 from ..utils import (
     cli_option,
     cli_valueless_option,
@@ -362,13 +367,14 @@ class FFmpegFD(ExternalFD):
 
     @classmethod
     def available(cls):
-        return FFmpegPostProcessor().available
+        # actual availability can only be confirmed for an instance
+        return bool(FFmpegPostProcessor)
 
     def _call_downloader(self, tmpfilename, info_dict):
-        url = info_dict['url']
-        ffpp = FFmpegPostProcessor(downloader=self)
+        # `downloader` means the parent `YoutubeDL`
+        ffpp = FFmpegPostProcessor(downloader=self.ydl)
         if not ffpp.available:
-            self.report_error('m3u8 download detected but ffmpeg or avconv could not be found. Please install one.')
+            self.report_error('ffmpeg required for download but no ffmpeg (nor avconv) executable could be found. Please install one.')
             return False
         ffpp.check_version()
 
@@ -397,6 +403,7 @@ class FFmpegFD(ExternalFD):
         # if end_time:
         #     args += ['-t', compat_str(end_time - start_time)]
 
+        url = info_dict['url']
         cookies = self.ydl.cookiejar.get_cookies_for_url(url)
         if cookies:
             args.extend(['-cookies', ''.join(
diff --git a/youtube_dl/postprocessor/ffmpeg.py b/youtube_dl/postprocessor/ffmpeg.py
index 801160e6c..e5ffdf378 100644
--- a/youtube_dl/postprocessor/ffmpeg.py
+++ b/youtube_dl/postprocessor/ffmpeg.py
@@ -96,6 +96,7 @@ class FFmpegPostProcessor(PostProcessor):
 
         self._paths = None
         self._versions = None
+        location = None
         if self._downloader:
             prefer_ffmpeg = self._downloader.params.get('prefer_ffmpeg', True)
             location = self._downloader.params.get('ffmpeg_location')
@@ -118,32 +119,17 @@ class FFmpegPostProcessor(PostProcessor):
                     location = os.path.dirname(os.path.abspath(location))
                     if basename in ('ffmpeg', 'ffprobe'):
                         prefer_ffmpeg = True
-
-                self._paths = dict(
-                    (p, os.path.join(location, p)) for p in programs)
-                self._versions = dict(
-                    (p, get_ffmpeg_version(self._paths[p])) for p in programs)
-        if self._versions is None:
-            self._versions = dict(
-                (p, get_ffmpeg_version(p)) for p in programs)
-            self._paths = dict((p, p) for p in programs)
-
-        if prefer_ffmpeg is False:
-            prefs = ('avconv', 'ffmpeg')
-        else:
-            prefs = ('ffmpeg', 'avconv')
-        for p in prefs:
-            if self._versions[p]:
-                self.basename = p
-                break
-
-        if prefer_ffmpeg is False:
-            prefs = ('avprobe', 'ffprobe')
-        else:
-            prefs = ('ffprobe', 'avprobe')
-        for p in prefs:
-            if self._versions[p]:
-                self.probe_basename = p
+        self._paths = dict(
+            (p, p if location is None else os.path.join(location, p))
+            for p in programs)
+        self._versions = dict(
+            x for x in (
+                (p, get_ffmpeg_version(self._paths[p])) for p in programs)
+            if x[1] is not None)
+
+        for p in ('ffmpeg', 'avconv')[::-1 if prefer_ffmpeg is False else 1]:
+            if self._versions.get(p):
+                self.basename = self.probe_basename = p
                 break
 
     @property