demux_lavf: add support for libavdevice

libavdevice supports various "special" video and audio inputs, such
as screen-capture or libavfilter filter graphs.

libavdevice inputs are implemented as demuxers. They don't use the
custom stream callbacks (in AVFormatContext.pb). Instead, input
parameters are passed as filename. This means the mpv stream layer has
to be disabled. Do this by adding the pseudo stream handler avdevice://,
whose only purpose is passing the filename to demux_lavf, without
actually doing anything.

Change the logic how the filename is passed to libavformat. Remove
handling of the filename from demux_open_lavf() and move it to
lavf_check_file(). (This also fixes a possible bug when skipping the
"lavf://" prefix.)

libavdevice now can be invoked by specifying demuxer and args as in:

    mpv avdevice://demuxer:args

The args are passed as filename to libavformat. When using libavdevice
demuxers, their actual meaning is highly implementation specific. They
don't refer to actual filenames.

Note:

libavdevice is disabled by default. There is one problem: libavdevice
pulls in libavfilter, which in turn causes symbol clashes with mpv
internals. The problem is that libavfilter includes a mplayer filter
bridge, which is used to interface with a set of nearly unmodified
mplayer filters copied into libavfilter. This filter bridge uses the
same symbol names as mplayer/mpv's filter chain, which results in symbol
clashes at link-time.

This can be prevented by building ffmpeg with --disable-filter=mp, but
unfortunately this is not the default.

This means linking to libavdevice (which in turn forces linking with
libavfilter by default) must be disabled. We try doing this by compiling
a test file that defines one of the clashing symbols (vf_mpi_clear).

To enable libavdevice input, ffmpeg should be built with the options:

    --disable-filter=mp

and mpv with:

    --enable-libavdevice

Originally, I tried to auto-detect it. But the resulting complications
in configure did't seem worth the trouble.

1 files changed, 42 insertions, 0 deletions

diff --git a/configure b/configure
index 7d8c68dec3..9784addbad 100755
--- a/configure
+++ b/configure
@@ -333,6 +333,8 @@ Optional features:
   --disable-libass-osd   disable OSD rendering with libass [autodetect]
   --enable-rpath         enable runtime linker path for extra libs [disabled]
   --disable-libpostproc  disable postprocess filter (vf_pp) [autodetect]
+  --enable-libavdevice   enable libavdevice demuxers [disabled]
+  --enable-libavfilter   enable libavfilter [disabled] [unused]
 
 Codecs:
   --enable-gif              enable GIF support [autodetect]
@@ -490,6 +492,8 @@ _ass=auto
 _libass_osd=auto
 _rpath=no
 libpostproc=auto
+libavfilter=no
+libavdevice=no
 _asmalign_pot=auto
 _stream_cache=yes
 _priority=no
@@ -707,6 +711,10 @@ for ac_option do
   --disable-rpath)      _rpath=no       ;;
   --enable-libpostproc) libpostproc=yes ;;
   --disable-libpostproc) libpostproc=no ;;
+  --enable-libavdevice) libavdevice=auto ;;
+  --disable-libavdevice) libavdevice=no ;;
+  --enable-libavfilter) libavfilter=auto ;;
+  --disable-libavfilter) libavfilter=no ;;
 
   --enable-enca)        _enca=yes       ;;
   --disable-enca)       _enca=no        ;;
@@ -2824,6 +2832,36 @@ else
 fi
 
 
+echocheck "libavfilter >= 3.17.0"
+if test "$libavfilter" = auto ; then
+    libavfilter=no
+    if pkg_config_add "libavfilter >= 3.17.0" ; then
+        libavfilter=yes
+    fi
+fi
+if test "$libavfilter" = yes ; then
+    def_libavfilter='#define CONFIG_LIBAVFILTER 1'
+else
+    def_libavfilter='#undef CONFIG_LIBAVFILTER'
+fi
+echores "$libavfilter"
+
+
+echocheck "libavdevice >= 54.0.0"
+if test "$libavdevice" = auto ; then
+    libavdevice=no
+    if pkg_config_add "libavdevice >= 54.0.0" ; then
+        libavdevice=yes
+    fi
+fi
+if test "$libavdevice" = yes ; then
+    def_libavdevice='#define CONFIG_LIBAVDEVICE 1'
+else
+    def_libavdevice='#undef CONFIG_LIBAVDEVICE'
+fi
+echores "$libavdevice"
+
+
 echocheck "libpostproc >= 52.0.0"
 if test "$libpostproc" = auto ; then
     libpostproc=no
@@ -3245,6 +3283,8 @@ LIBBLURAY = $_bluray
 LIBBS2B = $_libbs2b
 LCMS2 = $_lcms2
 LIBPOSTPROC = $libpostproc
+LIBAVDEVICE = $libavdevice
+LIBAVFILTER = $libavfilter
 LIBSMBCLIENT = $_smb
 LIBQUVI = $_libquvi
 LIBTHEORA = $_theora
@@ -3403,6 +3443,8 @@ $def_mpg123
 $def_zlib
 
 $def_libpostproc
+$def_libavdevice
+$def_libavfilter
 
 
 /* Audio output drivers */