[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

3 files changed, 39 insertions, 32 deletions

diff --git a/test/test_downloader_external.py b/test/test_downloader_external.py
index 029f9b05f..4491bd9de 100644
--- a/test/test_downloader_external.py
+++ b/test/test_downloader_external.py
@@ -18,6 +18,7 @@ from test.helper import (
 )
 from youtube_dl import YoutubeDL
 from youtube_dl.compat import (
+    compat_contextlib_suppress,
     compat_http_cookiejar_Cookie,
     compat_http_server,
     compat_kwargs,
@@ -35,6 +36,9 @@ from youtube_dl.downloader.external import (
     HttpieFD,
     WgetFD,
 )
+from youtube_dl.postprocessor import (
+    FFmpegPostProcessor,
+)
 import threading
 
 TEST_SIZE = 10 * 1024
@@ -227,7 +231,17 @@ class TestAria2cFD(unittest.TestCase):
             self.assertIn('--load-cookies=%s' % downloader._cookies_tempfile, cmd)
 
 
-@ifExternalFDAvailable(FFmpegFD)
+# Handle delegated availability
+def ifFFmpegFDAvailable(externalFD):
+    # raise SkipTest, or set False!
+    avail = ifExternalFDAvailable(externalFD) and False
+    with compat_contextlib_suppress(Exception):
+        avail = FFmpegPostProcessor(downloader=None).available
+    return unittest.skipUnless(
+        avail, externalFD.get_basename() + ' not found')
+
+
+@ifFFmpegFDAvailable(FFmpegFD)
 class TestFFmpegFD(unittest.TestCase):
     _args = []
 
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