demux: add null demuxer

It's useless, but can be used for fancy --lavfi-complex nonsense.

DOCS/man/mpv.rst

@@ -708,7 +708,9 @@ PROTOCOLS
     Stitch together parts of multiple files and play them.
 
 ``null://``
-    Simulate an empty file.
+    Simulate an empty file. If opened for writing, it will discard all data.
+    The ``null`` demuxer will specifically pass autoprobing if this protocol
+    is used (while it's not automatically invoked for empty files).
 
 ``memory://data``
     Use the ``data`` part as source data.

DOCS/man/options.rst

@@ -3603,6 +3603,8 @@ Miscellaneous
         - ``--lavfi-complex='[aid1] asplit [t1] [ao] ; [t1] showvolume [t2] ; [vid1] [t2] overlay [vo]'``
           Play audio track 1, and overlay the measured volume for each speaker
           over video track 1.
+        - ``null:// --lavfi-complex='life [vo]'``
+          Conways' Life Game.
 
     See the FFmpeg libavfilter documentation for details on the filter.
 

demux/demux.c

@@ -53,6 +53,7 @@ extern const demuxer_desc_t demuxer_desc_playlist;
 extern const demuxer_desc_t demuxer_desc_disc;
 extern const demuxer_desc_t demuxer_desc_rar;
 extern const demuxer_desc_t demuxer_desc_libarchive;
+extern const demuxer_desc_t demuxer_desc_null;
 extern const demuxer_desc_t demuxer_desc_timeline;
 
 /* Please do not add any new demuxers here. If you want to implement a new
@@ -76,6 +77,7 @@ const demuxer_desc_t *const demuxer_list[] = {
     &demuxer_desc_lavf,
     &demuxer_desc_mf,
     &demuxer_desc_playlist,
+    &demuxer_desc_null,
     NULL
 };
 

demux/demux_null.c

@@ -0,0 +1,34 @@
+/*
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "misc/bstr.h"
+#include "stream/stream.h"
+#include "demux.h"
+
+static int try_open_file(struct demuxer *demux, enum demux_check check)
+{
+    if (strcmp(demux->stream->info->name, "null") != 0 &&
+        check != DEMUX_CHECK_REQUEST)
+        return -1;
+    return 0;
+}
+
+const struct demuxer_desc demuxer_desc_null = {
+    .name = "null",
+    .desc = "null demuxer",
+    .open = try_open_file,
+};

wscript_build.py

@@ -175,6 +175,7 @@ def build(ctx):
         ( "demux/demux_mf.c" ),
         ( "demux/demux_mkv.c" ),
         ( "demux/demux_mkv_timeline.c" ),
+        ( "demux/demux_null.c" ),
         ( "demux/demux_playlist.c" ),
         ( "demux/demux_raw.c" ),
         ( "demux/demux_rar.c" ),