Install-plugins

Overview

Using this API, applications can request the installation of missing GStreamer plugins. These may be missing decoders/demuxers or encoders/muxers for a certain format, sources or sinks for a certain URI protocol (e.g. 'http'), or certain elements known by their element factory name ('audioresample').

Whether plugin installation is supported or not depends on the operating system and/or distribution in question. The vendor of the operating system needs to make sure the necessary hooks and mechanisms are in place for plugin installation to work. See below for more detailed information.

From the application perspective, plugin installation is usually triggered either

• when the application itself has found that it wants or needs to install a certain element
• when the application has been notified by an element (such as playbin or decodebin) that one or more plugins are missing and the application has decided that it wants to install one or more of those missing plugins

The install functions in this section all take one or more 'detail strings'. These detail strings contain information about the type of plugin that needs to be installed (decoder, encoder, source, sink, or named element), and some additional information such GStreamer version used and a human-readable description of the component to install for user dialogs.

Applications should not concern themselves with the composition of the string itself. They should regard the string as if it was a shared secret between GStreamer and the plugin installer application.

Detail strings can be obtained using the function gst_missing_plugin_message_get_installer_detail on a missing-plugin message. Such a message will either have been found by the application on a pipeline's GstBus, or the application will have created it itself using gst_missing_element_message_new, gst_missing_decoder_message_new, gst_missing_encoder_message_new, gst_missing_uri_sink_message_new, or gst_missing_uri_source_message_new.

For each GStreamer element/plugin/component that should be installed, the application needs one of those 'installer detail' string mentioned in the previous section. This string can be obtained, as already mentioned above, from a missing-plugin message using the function gst_missing_plugin_message_get_installer_detail. The missing-plugin message is either posted by another element and then found on the bus by the application, or the application has created it itself as described above.

The application will then call gst_install_plugins_async, passing a NULL-terminated array of installer detail strings, and a function that should be called when the installation of the plugins has finished (successfully or not). Optionally, a GstInstallPluginsContext created with gst_install_plugins_context_new may be passed as well. This way additional optional arguments like the application window's XID can be passed to the external installer application.

gst_install_plugins_async will return almost immediately, with the return code indicating whether plugin installation was started or not. If the necessary hooks for plugin installation are in place and an external installer application has in fact been called, the passed in function will be called with a result code as soon as the external installer has finished. If the result code indicates that new plugins have been installed, the application will want to call gst_update_registry so the run-time plugin registry is updated and the new plugins are made available to the application.

A Gtk/GLib main loop must be running in order for the result function to be called when the external installer has finished. If this is not the case, make sure to regularly call in your code:

g_main_context_iteration (NULL,FALSE);

1. Installer hook

When GStreamer applications initiate plugin installation via gst_install_plugins_async or gst_install_plugins_sync, a pre-defined helper application will be called.

The exact path of the helper application to be called is set at compile time, usually by the build system based on the install prefix. For a normal package build into the /usr prefix, this will usually default to /usr/libexec/gst-install-plugins-helper or /usr/lib/gst-install-plugins-helper.

Vendors/distros who want to support GStreamer plugin installation should either provide such a helper script/application or use the meson option -Dinstall_plugins_helper'=/path/to/installer to make GStreamer call an installer of their own directly.

It is strongly recommended that vendors provide a small helper application as interlocutor to the real installer though, even more so if command line argument munging is required to transform the command line arguments passed by GStreamer to the helper application into arguments that are understood by the real installer.

The helper application path defined at compile time can be overridden at runtime by setting the GST_INSTALL_PLUGINS_HELPER environment variable. This can be useful for testing/debugging purposes.

2. Arguments passed to the install helper

GStreamer will pass the following arguments to the install helper (this is in addition to the path of the executable itself, which is by convention argv[0]):

• none to many optional arguments in the form of --foo-bar=val. Example: --transient-for=XID where XID is the X Window ID of the main window of the calling application (so the installer can make itself transient to that window). Unknown optional arguments should be ignored by the installer.

• one 'installer detail string' argument for each plugin to be installed; these strings will have a gstreamer prefix; the exact format of the detail string is explained below

3. Detail string describing the missing plugin

The string is in UTF-8 encoding and is made up of several fields, separated by '|' characters (but neither the first nor the last character is a '|'). 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. "1.0" This is required so that when there is GStreamer-2.0 at some 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 (see below for details about how to map this to the package/plugin that needs installing), e.g.
  • urisource-$(PROTOCOL_REQUIRED), e.g. urisource-http or urisource-mms
  • element-$(ELEMENT_REQUIRED), e.g. element-videoconvert
  • decoder-$(CAPS_REQUIRED), e.g. (do read below for more details!):
    • decoder-audio/x-vorbis
    • decoder-application/ogg
    • decoder-audio/mpeg, mpegversion=(int)4
    • decoder-video/mpeg, systemstream=(boolean)true, mpegversion=(int)2
  • encoder-$(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|1.0|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.

Those unfamiliar with the GStreamer 'caps' system should note a few things about the caps string used in the above decoder/encoder case:

• the first part ("video/mpeg") of the caps string is a GStreamer media type and not a MIME type. Wherever possible, the GStreamer media type will be the same as the corresponding MIME type, but often it is not.
• a caps string may or may not have additional comma-separated fields of various types (as seen in the examples above)
• the caps string of a 'required' component (as above) will always have fields with fixed values, whereas an introspected string (see below) may have fields with non-fixed values. Compare for example:
  • audio/mpeg, mpegversion=(int)4 vs. audio/mpeg, mpegversion=(int){2, 4}
  • video/mpeg, mpegversion=(int)2 vs. video/mpeg, systemstream=(boolean){ true, false}, mpegversion=(int)[1, 2]

4. Exit codes the installer should return

The installer should return one of the following exit codes when it exits:

• 0 if all of the requested plugins could be installed (#GST_INSTALL_PLUGINS_SUCCESS)
• 1 if no appropriate installation candidate for any of the requested plugins could be found. Only return this if nothing has been installed (#GST_INSTALL_PLUGINS_NOT_FOUND)
• 2 if an error occurred during the installation. The application will assume that the user will already have seen an error message by the installer in this case and will usually not show another one (#GST_INSTALL_PLUGINS_ERROR)
• 3 if some of the requested plugins could be installed, but not all (#GST_INSTALL_PLUGINS_PARTIAL_SUCCESS)
• 4 if the user aborted the installation (#GST_INSTALL_PLUGINS_USER_ABORT)

5. How to map the required detail string to packages

It is up to the vendor to find mechanism to map required components from the detail string to the actual packages/plugins to install. This could be a hardcoded list of mappings, for example, or be part of the packaging system metadata.

GStreamer plugin files can be introspected for this information. The gst-inspect utility has a special command line option that will output information similar to what is required. For example $ gst-inspect-1.0 --print-plugin-auto-install-info /path/to/libgstvorbis.so should output something along the lines ofdecoder-audio/x-vorbis,element-vorbisdecelement-vorbisencelement-vorbisparse,element-vorbistag,encoder-audio/x-vorbis`

Note that in the encoder and decoder case the introspected caps can be more complex with additional fields, e.g. audio/mpeg,mpegversion=(int){2,4}, so they will not always exactly match the caps wanted by the application. It is up to the installer to deal with this (either by doing proper caps intersection using the GStreamer GstCaps API, or by only taking into account the media type).

Another potential source of problems are plugins such as ladspa or libvisual where the list of elements depends on the installed ladspa/libvisual plugins at the time. This is also up to the distribution to handle (but usually not relevant for playback applications).