[downloader] Allow streaming unmerged formats to stdout using ffmpeg

For this to work:
1. The downloader must be ffmpeg
2. The selected formats must have the same protocol
3. The formats must be downloadable by ffmpeg to stdout

Partial solution for: https://github.com/ytdl-org/youtube-dl/issues/28146, https://github.com/ytdl-org/youtube-dl/issues/27265

yt_dlp/downloader/__init__.py

@@ -7,11 +7,12 @@ from ..utils import (
 )
 
 
-def get_suitable_downloader(info_dict, params={}, default=NO_DEFAULT, protocol=None):
+def get_suitable_downloader(info_dict, params={}, default=NO_DEFAULT, protocol=None, to_stdout=False):
     info_dict['protocol'] = determine_protocol(info_dict)
     info_copy = info_dict.copy()
     if protocol:
         info_copy['protocol'] = protocol
+    info_copy['to_stdout'] = to_stdout
     return _get_suitable_downloader(info_copy, params, default)
 
 
@@ -84,10 +85,11 @@ def _get_suitable_downloader(info_dict, params, default):
     external_downloader = (
         downloaders if isinstance(downloaders, compat_str) or downloaders is None
         else downloaders.get(shorten_protocol_name(protocol, True), downloaders.get('default')))
-    if external_downloader and external_downloader.lower() == 'native':
-        external_downloader = 'native'
 
-    if external_downloader not in (None, 'native'):
+    if external_downloader is None:
+        if info_dict['to_stdout'] and FFmpegFD.can_merge_formats(info_dict, params):
+            return FFmpegFD
+    elif external_downloader.lower() != 'native':
         ed = get_external_downloader(external_downloader)
         if ed.can_download(info_dict, external_downloader):
             return ed
@@ -95,9 +97,10 @@ def _get_suitable_downloader(info_dict, params, default):
     if protocol in ('m3u8', 'm3u8_native'):
         if info_dict.get('is_live'):
             return FFmpegFD
-        elif external_downloader == 'native':
+        elif (external_downloader or '').lower() == 'native':
             return HlsFD
-        elif get_suitable_downloader(info_dict, params, None, protocol='m3u8_frag_urls'):
+        elif get_suitable_downloader(
+                info_dict, params, None, protocol='m3u8_frag_urls', to_stdout=info_dict['to_stdout']):
             return HlsFD
         elif params.get('hls_prefer_native') is True:
             return HlsFD

yt_dlp/downloader/dash.py

@@ -22,7 +22,8 @@ class DashSegmentsFD(FragmentFD):
         fragments = info_dict['fragments'][:1] if self.params.get(
             'test', False) else info_dict['fragments']
 
-        real_downloader = get_suitable_downloader(info_dict, self.params, None, protocol='dash_frag_urls')
+        real_downloader = get_suitable_downloader(
+            info_dict, self.params, None, protocol='dash_frag_urls', to_stdout=(filename== '-'))
 
         ctx = {
             'filename': filename,

yt_dlp/downloader/external.py

@@ -36,6 +36,7 @@ from ..utils import (
 
 class ExternalFD(FileDownloader):
     SUPPORTED_PROTOCOLS = ('http', 'https', 'ftp', 'ftps')
+    can_download_to_stdout = False
 
     def real_download(self, filename, info_dict):
         self.report_destination(filename)
@@ -93,7 +94,9 @@ class ExternalFD(FileDownloader):
 
     @classmethod
     def supports(cls, info_dict):
-        return info_dict['protocol'] in cls.SUPPORTED_PROTOCOLS
+        return (
+            (cls.can_download_to_stdout or not info_dict.get('to_stdout'))
+            and info_dict['protocol'] in cls.SUPPORTED_PROTOCOLS)
 
     @classmethod
     def can_download(cls, info_dict, path=None):
@@ -341,6 +344,7 @@ class HttpieFD(ExternalFD):
 
 class FFmpegFD(ExternalFD):
     SUPPORTED_PROTOCOLS = ('http', 'https', 'ftp', 'ftps', 'm3u8', 'm3u8_native', 'rtsp', 'rtmp', 'rtmp_ffmpeg', 'mms')
+    can_download_to_stdout = True
 
     @classmethod
     def available(cls, path=None):

yt_dlp/downloader/hls.py

@@ -86,7 +86,8 @@ class HlsFD(FragmentFD):
         if is_webvtt:
             real_downloader = None  # Packing the fragments is not currently supported for external downloader
         else:
-            real_downloader = get_suitable_downloader(info_dict, self.params, None, protocol='m3u8_frag_urls')
+            real_downloader = get_suitable_downloader(
+                info_dict, self.params, None, protocol='m3u8_frag_urls', to_stdout=(filename== '-'))
         if real_downloader and not real_downloader.supports_manifest(s):
             real_downloader = None
         if real_downloader: