mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-30 13:41:48 +00:00
262 lines
11 KiB
Text
262 lines
11 KiB
Text
What to do when a plugin is missing
|
|
-----------------------------------
|
|
|
|
The mechanism and API described in this document requires GStreamer core and
|
|
gst-plugins-base versions >= 0.10.12. Further information on some aspects of
|
|
this document can be found in the libgstbaseutils API reference.
|
|
|
|
We only discuss playback pipelines for now.
|
|
|
|
A three step process:
|
|
|
|
1) GStreamer level
|
|
|
|
- elements will use a "missing-plugin" element message to report missing
|
|
plugins, with the following fields set:
|
|
|
|
- type: (string) { "urisource", "urisink", "decoder", "encoder", "element" }
|
|
(we do not distinguish between demuxer/decoders/parsers etc.)
|
|
- detail: (string) or (caps) depending on the type { ANY }
|
|
ex: "mms, "mmsh", "audio/x-mp3,rate=48000,..."
|
|
- name: (string) { ANY }
|
|
ex: "MMS protocol handler",..
|
|
|
|
- missing uri handler
|
|
|
|
ex. mms://foo.bar/file.asf
|
|
|
|
When no protocol handler is installed for mms://, the application will not be
|
|
able to instantiate an element for that uri (gst_element_make_from_uri()
|
|
returns NULL).
|
|
|
|
Playbin will post a "missing-plugin" element message with the type set to
|
|
"urisource", detail set to "mms". Optionally the friendly name can be filled
|
|
in as well.
|
|
|
|
- missing typefind function
|
|
|
|
We don't recognize the type of the file, this should normally not happen
|
|
because all the typefinders are in the basic GStreamer installation.
|
|
There is not much useful information we can give about how to resolve this
|
|
issue. It is possible to use the first N bytes of the data to determine the
|
|
type (and needed plugin) on the server. We don't explore this option in this
|
|
document yet, but the proposal is flexible enough to accommodate this in the
|
|
future should the need arise.
|
|
|
|
- missing demuxer
|
|
|
|
Typically after running typefind on the data we determine the type of the
|
|
file. If there is no plugin found for the type, a "missing-plugin" element
|
|
message is posted by decodebin with the following fields: Type set to
|
|
"decoder", detail set to the caps for witch no plugin was found. Optionally
|
|
the friendly name can be filled in as well.
|
|
|
|
- missing decoder
|
|
|
|
The demuxer will dynamically create new pads with specific caps while it
|
|
figures out the contents of the container format. Decodebin tries to find the
|
|
decoders for these formats in the registry. If there is no decoder found, a
|
|
"missing-plugin" element message is posted by decodebin with the following
|
|
fields: Type set to "decoder", detail set to the caps for which no plugin
|
|
was found. Optionally the friendly name can be filled in as well. There is
|
|
no distinction made between the missing demuxer and decoder at the
|
|
application level.
|
|
|
|
- missing element
|
|
|
|
Decodebin and playbin will create a set of helper elements when they set up
|
|
their decoding pipeline. These elements are typically colorspace, sample rate,
|
|
audio sinks,... Their presence on the system is required for the functionality
|
|
of decodebin. It is typically a package dependency error if they are not
|
|
present but in case of a corrupted system the following "missing-plugin"
|
|
element message will be emitted: type set to "element", detail set to the
|
|
element factory name and the friendly name optionally set to a description
|
|
of the element's functionality in the decoding pipeline.
|
|
|
|
Except for reporting the missing plugins, no further policy is enforced at the
|
|
GStreamer level. It is up to the application to decide whether a missing
|
|
plugin constitutes a problem or not.
|
|
|
|
|
|
2) application level
|
|
|
|
The application's job is to listen for the "missing-plugin" element messages
|
|
and to decide on a policy to handle them. Following cases exist:
|
|
|
|
- partially missing plugins
|
|
|
|
The application will be able to complete a state change to PAUSED but there
|
|
will be a "missing-plugin" element message on the GstBus.
|
|
|
|
This means that it will be possible to play back part of the media file but not
|
|
all of it.
|
|
|
|
For example: suppose we have an .avi file with mp3 audio and divx video. If we
|
|
have the mp3 audio decoder but not the divx video decoder, it will be possible
|
|
to play only the audio part but not the video part. For an audio playback
|
|
application, this is not a problem but a video player might want to decide on:
|
|
|
|
- require the use to install the additionally required plugins.
|
|
- inform the user that only the audio will be played back
|
|
- ask the user if it should download the additional codec or only play the
|
|
audio part.
|
|
- ...
|
|
|
|
- completely unplayable stream
|
|
|
|
The application will receive an ERROR message from GStreamer informing it that
|
|
playback stopped (before it could reach PAUSED). This happens because none of
|
|
the streams is connected to a decoder. The error code and domain should be one
|
|
of the following in this case:
|
|
- GST_CORE_ERROR_MISSING_PLUGIN (domain: GST_CORE_ERROR)
|
|
- GST_STREAM_ERROR_CODEC_NOT_FOUND (domain: GST_STREAM_ERROR)
|
|
|
|
The application can then see that there are a set of "missing-plugin" element
|
|
messages on the GstBus and can decide to trigger the download procedure. It
|
|
does that as described in the following section.
|
|
|
|
"missing-plugin" element messages can be identified using the function
|
|
gst_is_missing_plugin_message().
|
|
|
|
|
|
3) Plugin download stage
|
|
|
|
At this point the application has
|
|
- collected one or more "missing-plugin" element messages
|
|
- made a decision that additional plugins should be installed
|
|
|
|
It will call a GStreamer utility function to convert each "missing-plugin"
|
|
message into an identifier string describing the missing capability. This is
|
|
done using the function gst_missing_plugin_message_get_installer_detail().
|
|
|
|
The application will then pass these strings to gst_install_plugins_async()
|
|
or gst_install_plugins_sync() to initiate the download. See the API
|
|
documentation there (libgstbaseutils, part of gst-plugins-base) for more
|
|
details.
|
|
|
|
When new plugins have been installed, the application will have to initiate
|
|
a re-scan of the GStreamer plugin registry using gst_update_registry().
|
|
|
|
|
|
4) Format of the (UTF-8) string ID passed to the external installer system
|
|
|
|
The string is made up of several fields, separated by '|' characters.
|
|
The fields are:
|
|
|
|
- plugin system identifier, ie. "gstreamer"
|
|
This identifier determines the format of the rest of the detail string.
|
|
Automatic plugin installers should not process detail strings with
|
|
unknown identifiers. This allows other plugin-based libraries to use
|
|
the same mechanism for their automatic plugin installation needs, or
|
|
for the format to be changed should it turn out to be insufficient.
|
|
|
|
- plugin system version, e.g. "0.10"
|
|
This is required so that when there is a GStreamer-0.12 or GStreamer-1.0
|
|
at somem point in future, the different major versions can still co-exist
|
|
and use the same plugin install mechanism in the same way.
|
|
|
|
- application identifier, e.g. "totem"
|
|
This may also be in the form of "pid/12345" if the program name can't
|
|
be obtained for some reason.
|
|
|
|
- human-readable localised description of the required component,
|
|
e.g. "Vorbis audio decoder"
|
|
|
|
- identifier string for the required component, e.g.
|
|
|
|
- urisource-$(PROTOCOL_REQUIRED)
|
|
e.g. urisource-http or urisource-mms
|
|
|
|
- element-$(ELEMENT_REQUIRED),
|
|
e.g. element-videoconvert
|
|
|
|
- decoder-$(CAPS_REQUIRED)
|
|
e.g. decoder-audio/x-vorbis or
|
|
decoder-application/ogg or
|
|
decoder-audio/mpeg, mpegversion=(int)4 or
|
|
decoder-video/mpeg, systemstream=(boolean)true, mpegversion=(int)2
|
|
|
|
- encoder-$(CAPS_REQUIRED)
|
|
e.g. encoder-audio/x-vorbis
|
|
|
|
- optional further fields not yet specified
|
|
|
|
|
|
An entire ID string might then look like this, for example:
|
|
|
|
gstreamer|0.10|totem|Vorbis audio decoder|decoder-audio/x-vorbis
|
|
|
|
Plugin installers parsing this ID string should expect further fields also
|
|
separated by '|' symbols and either ignore them, warn the user, or error
|
|
out when encountering them.
|
|
|
|
The human-readable description string is provided by the libgstbaseutils
|
|
library that can be found in gst-plugins-base versions >= 0.10.12 and can
|
|
also be used by demuxers to find out the codec names for taglists from given
|
|
caps in a unified and consistent way.
|
|
|
|
Applications can create these detail strings using the function
|
|
gst_missing_plugin_message_get_installer_detail() on a given missing-plugin
|
|
message.
|
|
|
|
|
|
5) Using missing-plugin messages for error reporting:
|
|
|
|
Missing-plugin messages are also useful for error reporting purposes, either
|
|
in the case where the application does not support libgimme-codec, or the
|
|
external installer is not available or not able to install the required
|
|
plugins.
|
|
|
|
When creating error messages, applications may use the function
|
|
gst_missing_plugin_message_get_description() to obtain a possibly translated
|
|
description from each missing-plugin message (e.g. "Matroska demuxer" or
|
|
"Theora video depayloader"). This can be used to report to the user exactly
|
|
what it is that is missing.
|
|
|
|
|
|
6) Notes for packagers
|
|
|
|
- An easy way to introspect plugin .so files is:
|
|
|
|
$ gst-inspect --print-plugin-auto-install-info /path/to/libgstfoo.so
|
|
|
|
The output will be something like:
|
|
|
|
decoder-audio/x-vorbis
|
|
element-vorbisdec
|
|
element-vorbisenc
|
|
element-vorbisparse
|
|
element-vorbistag
|
|
encoder-audio/x-vorbis
|
|
|
|
BUT could also be like this (from the faad element in this case):
|
|
|
|
decoder-audio/mpeg, mpegversion=(int){ 2, 4 }
|
|
|
|
NOTE that this does not exactly match the caps string that the installer
|
|
will get from the application. The application will always ever ask for
|
|
one of
|
|
|
|
decoder-audio/mpeg, mpegversion=(int)2
|
|
decoder-audio/mpeg, mpegversion=(int)4
|
|
|
|
|
|
- when introspecting, keep in mind that there are GStreamer plugins that
|
|
in turn load external plugins. Examples of these are pitfdll, ladspa, or
|
|
the GStreamer libvisual plugin. Those plugins will only announce elements
|
|
for the currently installed external plugins at the time of introspection!
|
|
With the exception of pitfdll, this is not really relevant to the playback
|
|
case, but may become an issue in future when applications like buzztard,
|
|
jokosher or pitivi start requestion elements by name, for example ladspa
|
|
effect elements or so.
|
|
|
|
This case could be handled if those wrapper plugins would also provide a
|
|
gst-install-xxx-plugins-helper, where xxx={ladspa|visual|...}. Thus if the
|
|
distro specific gst-install-plugins-helper can't resolve a request for e.g.
|
|
element-bml-sonicverb it can forward the request to
|
|
gst-install-bml-plugins-helper (bml is the buzz machine loader).
|
|
|
|
|
|
7) Further references:
|
|
|
|
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gstreamer-base-utils.html
|