playbin之Source插件加载流程源码剖析

之前我们有讲解过uridecodebin的setup_source中会创建source插件，关键函数：

/* create and configure an element that can handle the uri */

source = gen_source_element (decoder);

/** Generate and configure a source element.** Returns: (transfer full): a new #GstElement*/
static GstElement *
gen_source_element (GstURIDecodeBin * decoder)
{GObjectClass *source_class;GstElement *source;GParamSpec *pspec;GstQuery *query;GstSchedulingFlags flags;GError *err = NULL;if (!decoder->uri)goto no_uri;GST_LOG_OBJECT (decoder, "finding source for %s", decoder->uri);if (!gst_uri_is_valid (decoder->uri))goto invalid_uri;if (IS_BLACKLISTED_URI (decoder->uri))goto uri_blacklisted;source =gst_element_make_from_uri (GST_URI_SRC, decoder->uri, "source", &err);if (!source)goto no_source;GST_LOG_OBJECT (decoder, "found source type %s", G_OBJECT_TYPE_NAME (source));source_class = G_OBJECT_GET_CLASS (source);pspec = g_object_class_find_property (source_class, "connection-speed");if (pspec != NULL) {guint64 speed = decoder->connection_speed / 1000;gboolean wrong_type = FALSE;if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_UINT) {GParamSpecUInt *pspecuint = G_PARAM_SPEC_UINT (pspec);speed = CLAMP (speed, pspecuint->minimum, pspecuint->maximum);} else if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_INT) {GParamSpecInt *pspecint = G_PARAM_SPEC_INT (pspec);speed = CLAMP (speed, pspecint->minimum, pspecint->maximum);} else if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_UINT64) {GParamSpecUInt64 *pspecuint = G_PARAM_SPEC_UINT64 (pspec);speed = CLAMP (speed, pspecuint->minimum, pspecuint->maximum);} else if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_INT64) {GParamSpecInt64 *pspecint = G_PARAM_SPEC_INT64 (pspec);speed = CLAMP (speed, pspecint->minimum, pspecint->maximum);} else {GST_WARNING_OBJECT (decoder,"The connection speed property %" G_GUINT64_FORMAT" of type %s is not useful not setting it", speed,g_type_name (G_PARAM_SPEC_TYPE (pspec)));wrong_type = TRUE;}if (!wrong_type) {g_object_set (source, "connection-speed", speed, NULL);GST_DEBUG_OBJECT (decoder,"setting connection-speed=%" G_GUINT64_FORMAT " to source element",speed);}}pspec = g_object_class_find_property (source_class, "subtitle-encoding");if (pspec != NULL && G_PARAM_SPEC_VALUE_TYPE (pspec) == G_TYPE_STRING) {GST_DEBUG_OBJECT (decoder,"setting subtitle-encoding=%s to source element", decoder->encoding);g_object_set (source, "subtitle-encoding", decoder->encoding, NULL);}/* Sink reference before passing it to signal handler.* Language binding might otherwise sink it and then unref.* A floating ref is also tricky for a native signal handler in case* of a "transfer floating" call followed by unref* (e.g. some container_add and then container_remove).* Bottom line; best not have a floating ref boldly going into unknown code.*/g_object_ref_sink (source);g_signal_emit (decoder, gst_uri_decode_bin_signals[SIGNAL_SOURCE_SETUP],0, source);decoder->is_stream = IS_STREAM_URI (decoder->uri);query = gst_query_new_scheduling ();if (gst_element_query (source, query)) {gst_query_parse_scheduling (query, &flags, NULL, NULL, NULL);if ((flags & GST_SCHEDULING_FLAG_BANDWIDTH_LIMITED))decoder->is_stream = TRUE;}gst_query_unref (query);GST_LOG_OBJECT (decoder, "source is stream: %d", decoder->is_stream);decoder->need_queue = IS_QUEUE_URI (decoder->uri);GST_LOG_OBJECT (decoder, "source needs queue: %d", decoder->need_queue);return source;/* ERRORS */
no_uri:{GST_ELEMENT_ERROR (decoder, RESOURCE, NOT_FOUND,(_("No URI specified to play from.")), (NULL));return NULL;}
invalid_uri:{GST_ELEMENT_ERROR (decoder, RESOURCE, NOT_FOUND,(_("Invalid URI \"%s\"."), decoder->uri), (NULL));g_clear_error (&err);return NULL;}
uri_blacklisted:{GST_ELEMENT_ERROR (decoder, RESOURCE, FAILED,(_("This stream type cannot be played yet.")), (NULL));return NULL;}
no_source:{/* whoops, could not create the source element, dig a little deeper to* figure out what might be wrong. */if (err != NULL && err->code == GST_URI_ERROR_UNSUPPORTED_PROTOCOL) {gchar *prot;prot = gst_uri_get_protocol (decoder->uri);if (prot == NULL)goto invalid_uri;gst_element_post_message (GST_ELEMENT_CAST (decoder),gst_missing_uri_source_message_new (GST_ELEMENT (decoder), prot));GST_ELEMENT_ERROR (decoder, CORE, MISSING_PLUGIN,(_("No URI handler implemented for \"%s\"."), prot), (NULL));g_free (prot);} else {GST_ELEMENT_ERROR (decoder, RESOURCE, NOT_FOUND,("%s", (err) ? err->message : "URI was not accepted by any element"),("No element accepted URI '%s'", decoder->uri));}g_clear_error (&err);return NULL;}
}

关键函数：

（1） gst_uri_is_valid (decoder->uri) 对uri进行校验

static void
gst_uri_protocol_check_internal (const gchar * uri, gchar ** endptr)
{gchar *check = (gchar *) uri;g_assert (uri != NULL);g_assert (endptr != NULL);if (g_ascii_isalpha (*check)) {check++;while (g_ascii_isalnum (*check) || *check == '+'|| *check == '-' || *check == '.')check++;}*endptr = check;
}

（2）IS_BLACKLISTED_URI (decoder->uri)黑名单校验

/* blacklisted URIs, we know they will always fail. */
static const gchar *blacklisted_uris[] = { NULL };

（3）decoder->is_stream = IS_STREAM_URI (decoder->uri);

/* list of URIs that we consider to be streams and that need buffering.* We have no mechanism yet to figure this out with a query. */
static const gchar *stream_uris[] = { "http://", "https://", "mms://","mmsh://", "mmsu://", "mmst://", "fd://", "myth://", "ssh://","ftp://", "sftp://",NULL
};

（4）decoder->need_queue = IS_QUEUE_URI (decoder->uri); 

/* list of URIs that need a queue because they are pretty bursty */
static const gchar *queue_uris[] = { "cdda://", NULL };

最关键函数：

  source = gst_element_make_from_uri (GST_URI_SRC, decoder->uri, "source", &err); 

/*** gst_element_make_from_uri:* @type: Whether to create a source or a sink* @uri: URI to create an element for* @elementname: (allow-none): Name of created element, can be %NULL.* @error: (allow-none): address where to store error information, or %NULL.** Creates an element for handling the given URI.** Returns: (transfer floating): a new element or %NULL if none* could be created*/
GstElement *
gst_element_make_from_uri (const GstURIType type, const gchar * uri,const gchar * elementname, GError ** error)
{GList *possibilities, *walk;gchar *protocol;GstElement *ret = NULL;g_return_val_if_fail (gst_is_initialized (), NULL);g_return_val_if_fail (GST_URI_TYPE_IS_VALID (type), NULL);g_return_val_if_fail (error == NULL || *error == NULL, NULL);if (!gst_uri_is_valid (uri)) {g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_BAD_URI,_("Invalid URI: %s"), uri);return NULL;}GST_DEBUG ("type:%d, uri:%s, elementname:%s", type, uri, elementname);protocol = gst_uri_get_protocol (uri);possibilities = get_element_factories_from_uri_protocol (type, protocol);if (!possibilities) {GST_DEBUG ("No %s for URI '%s'", type == GST_URI_SINK ? "sink" : "source",uri);/* The error message isn't great, but we don't expect applications to* show that error to users, but call the missing plugins functions */g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_UNSUPPORTED_PROTOCOL,_("No URI handler for the %s protocol found"), protocol);g_free (protocol);return NULL;}g_free (protocol);possibilities = g_list_sort (possibilities, (GCompareFunc) sort_by_rank);walk = possibilities;while (walk) {GstElementFactory *factory = walk->data;GError *uri_err = NULL;ret = gst_element_factory_create (factory, elementname);if (ret != NULL) {GstURIHandler *handler = GST_URI_HANDLER (ret);if (gst_uri_handler_set_uri (handler, uri, &uri_err))break;GST_WARNING ("%s didn't accept URI '%s': %s", GST_OBJECT_NAME (ret), uri,uri_err->message);if (error != NULL && *error == NULL)g_propagate_error (error, uri_err);elseg_error_free (uri_err);gst_object_unref (ret);ret = NULL;}walk = walk->next;}gst_plugin_feature_list_free (possibilities);GST_LOG_OBJECT (ret, "created %s for URL '%s'",type == GST_URI_SINK ? "sink" : "source", uri);/* if the first handler didn't work, but we found another one that works */if (ret != NULL)g_clear_error (error);return ret;
}

（1）protocol = gst_uri_get_protocol (uri) 获取协议，例如http/https/ftp等

（2）possibilities = get_element_factories_from_uri_protocol (type, protocol)，从注册表中搜索可匹配上的source的lists

类型：GST_URI_SRC 协议：protocols

static GList *
get_element_factories_from_uri_protocol (const GstURIType type,const gchar * protocol)
{GList *possibilities;SearchEntry entry;g_return_val_if_fail (protocol, NULL);entry.type = type;entry.protocol = protocol;possibilities = gst_registry_feature_filter (gst_registry_get (),search_by_entry, FALSE, &entry);return possibilities;
}

（3）对possibilities排序，从中选择最终匹配的source

  possibilities = g_list_sort (possibilities, (GCompareFunc) sort_by_rank);walk = possibilities;while (walk) {GstElementFactory *factory = walk->data;GError *uri_err = NULL;ret = gst_element_factory_create (factory, elementname);if (ret != NULL) {GstURIHandler *handler = GST_URI_HANDLER (ret);if (gst_uri_handler_set_uri (handler, uri, &uri_err))break;GST_WARNING ("%s didn't accept URI '%s': %s", GST_OBJECT_NAME (ret), uri,uri_err->message);if (error != NULL && *error == NULL)g_propagate_error (error, uri_err);elseg_error_free (uri_err);gst_object_unref (ret);ret = NULL;}walk = walk->next;}

a）根据rank排序

b） 对每一个source尝试gst_uri_handler_set_uri ，返回成功则选择该source插件     

    ret = gst_element_factory_create (factory, elementname);if (ret != NULL) {GstURIHandler *handler = GST_URI_HANDLER (ret);if (gst_uri_handler_set_uri (handler, uri, &uri_err))break;

c) gst_uri_handler_set_uri主要是从，soruce中获取接口iface = GST_URI_HANDLER_GET_INTERFACE (handler)，对比source支持的get_protocols是否与当前uri匹配，匹配则成功。

/*** gst_uri_handler_set_uri:* @handler: A #GstURIHandler* @uri: URI to set* @error: (allow-none): address where to store a #GError in case of*    an error, or %NULL** Tries to set the URI of the given handler.** Returns: %TRUE if the URI was set successfully, else %FALSE.*/
gboolean
gst_uri_handler_set_uri (GstURIHandler * handler, const gchar * uri,GError ** error)
{GstURIHandlerInterface *iface;gboolean ret;gchar *protocol;g_return_val_if_fail (GST_IS_URI_HANDLER (handler), FALSE);g_return_val_if_fail (error == NULL || *error == NULL, FALSE);iface = GST_URI_HANDLER_GET_INTERFACE (handler);g_return_val_if_fail (iface != NULL, FALSE);g_return_val_if_fail (iface->set_uri != NULL, FALSE);if (!gst_uri_is_valid (uri)) {g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_BAD_URI,_("Invalid URI: %s"), uri);return FALSE;}protocol = gst_uri_get_protocol (uri);if (iface->get_protocols) {const gchar *const *protocols;const gchar *const *p;gboolean found_protocol = FALSE;protocols = iface->get_protocols (G_OBJECT_TYPE (handler));if (protocols != NULL) {for (p = protocols; *p != NULL; ++p) {if (g_ascii_strcasecmp (protocol, *p) == 0) {found_protocol = TRUE;break;}}if (!found_protocol) {g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_UNSUPPORTED_PROTOCOL,_("URI scheme '%s' not supported"), protocol);g_free (protocol);return FALSE;}}}ret = iface->set_uri (handler, uri, error);g_free (protocol);return ret;
}

备注：

Source插件需要实现接口GstURIHandlerInterface

/*** GstURIHandlerInterface:* @parent: The parent interface type* @get_type: Method to tell whether the element handles source or sink URI.* @get_protocols: Method to return the list of protocols handled by the element.* @get_uri: Method to return the URI currently handled by the element.* @set_uri: Method to set a new URI.** Any #GstElement using this interface should implement these methods.*/
struct _GstURIHandlerInterface {GTypeInterface parent;/* vtable *//*< public >*//* querying capabilities */GstURIType             (* get_type)           (GType type);const gchar * const *  (* get_protocols)      (GType type);/* using the interface */gchar *                (* get_uri)            (GstURIHandler * handler);gboolean               (* set_uri)            (GstURIHandler * handler,const gchar   * uri,GError       ** error);
};

souphttpsrc的例子：

 

G_DEFINE_TYPE_WITH_CODE (GstSoupHTTPSrc, gst_soup_http_src, GST_TYPE_PUSH_SRC,G_IMPLEMENT_INTERFACE (GST_TYPE_URI_HANDLER,gst_soup_http_src_uri_handler_init));static void
gst_soup_http_src_uri_handler_init (gpointer g_iface, gpointer iface_data)
{GstURIHandlerInterface *iface = (GstURIHandlerInterface *) g_iface;iface->get_type = gst_soup_http_src_uri_get_type;iface->get_protocols = gst_soup_http_src_uri_get_protocols;iface->get_uri = gst_soup_http_src_uri_get_uri;iface->set_uri = gst_soup_http_src_uri_set_uri;
}

 （6）那我们继续看下gst_element_make_from_uri中的关键步骤 possibilities = get_element_factories_from_uri_protocol (type, protocol)->gst_registry_feature_filter

  possibilities = gst_registry_feature_filter (gst_registry_get (),search_by_entry, FALSE, &entry);

主要逻辑就是从注册表中匹配对应的source:

static gboolean
search_by_entry (GstPluginFeature * feature, gpointer search_entry)
{const gchar *const *protocols;GstElementFactory *factory;SearchEntry *entry = (SearchEntry *) search_entry;if (!GST_IS_ELEMENT_FACTORY (feature))return FALSE;factory = GST_ELEMENT_FACTORY_CAST (feature);if (factory->uri_type != entry->type)return FALSE;protocols = gst_element_factory_get_uri_protocols (factory);if (protocols == NULL) {g_warning ("Factory '%s' implements GstUriHandler interface but returned ""no supported protocols!", gst_plugin_feature_get_name (feature));return FALSE;}while (*protocols != NULL) {if (g_ascii_strcasecmp (*protocols, entry->protocol) == 0)return TRUE;protocols++;}return FALSE;
}

/*** gst_element_factory_get_uri_protocols:* @factory: a #GstElementFactory** Gets a %NULL-terminated array of protocols this element supports or %NULL if* no protocols are supported. You may not change the contents of the returned* array, as it is still owned by the element factory. Use g_strdupv() to* make a copy of the protocol string array if you need to.** Returns: (transfer none) (array zero-terminated=1): the supported protocols*     or %NULL*/
const gchar *const *
gst_element_factory_get_uri_protocols (GstElementFactory * factory)
{g_return_val_if_fail (GST_IS_ELEMENT_FACTORY (factory), NULL);return (const gchar * const *) factory->uri_protocols;
}

看下几个常见的source插件的信息：

gst-inspect-1.0 soupsrc

Factory Details:Rank                     primary (256)Long-name                HTTP client sourceKlass                    Source/NetworkDescription              Receive data as a client over the network via HTTP using SOUPAuthor                   Wouter Cloetens <wouter@mind.be>Plugin Details:Name                     soupDescription              libsoup HTTP client src/sinkFilename                 /libgstsoup.soVersion                  1.20License                  LGPLSource module            gst-plugins-goodBinary package           GStreamer Good Plug-ins gitOrigin URL               Unknown package originGObject+----GInitiallyUnowned+----GstObject+----GstElement+----GstBaseSrc+----GstPushSrc+----GstSoupHTTPSrcImplemented Interfaces:GstURIHandlerPad Templates:SRC template: 'src'Availability: AlwaysCapabilities:ANYElement has no clocking capabilities.URI handling capabilities:Element can act as source.Supported URI protocols:httphttpsicyicyxPads:SRC: 'src'Pad Template: 'src'Element Properties:automatic-redirect  : Automatically follow HTTP redirects (HTTP Status Code 3xx)flags: readable, writableBoolean. Default: trueblocksize           : Size in bytes to read per buffer (-1 = default)flags: readable, writableUnsigned Integer. Range: 0 - 4294967295 Default: 4096compress            : Allow compressed content encodingsflags: readable, writableBoolean. Default: falsecookies             : HTTP request cookiesflags: readable, writableBoxed pointer of type "GStrv"do-timestamp        : Apply current stream time to buffersflags: readable, writableBoolean. Default: falseextra-headers       : Extra headers to append to the HTTP requestflags: readable, writableBoxed pointer of type "GstStructure"http-log-level      : Set log level for soup's HTTP session logflags: readable, writableEnum "SoupLoggerLogLevel" Default: 2, "headers"(0): none             - SOUP_LOGGER_LOG_NONE(1): minimal          - SOUP_LOGGER_LOG_MINIMAL(2): headers          - SOUP_LOGGER_LOG_HEADERS(3): body             - SOUP_LOGGER_LOG_BODYiradio-mode         : Enable internet radio mode (ask server to send shoutcast/icecast metadata interleaved with the actual stream data)flags: readable, writableBoolean. Default: trueis-live             : Act like a live sourceflags: readable, writableBoolean. Default: falsekeep-alive          : Use HTTP persistent connectionsflags: readable, writableBoolean. Default: truelocation            : Location to read fromflags: readable, writableString. Default: nullmethod              : The HTTP method to use (GET, HEAD, OPTIONS, etc)flags: readable, writableString. Default: nullname                : The name of the objectflags: readable, writable, 0x2000String. Default: "souphttpsrc0"num-buffers         : Number of buffers to output before sending EOS (-1 = unlimited)flags: readable, writableInteger. Range: -1 - 2147483647 Default: -1parent              : The parent of the objectflags: readable, writable, 0x2000Object of type "GstObject"proxy               : HTTP proxy server URIflags: readable, writableString. Default: ""proxy-id            : HTTP proxy URI user id for authenticationflags: readable, writableString. Default: nullproxy-pw            : HTTP proxy URI user password for authenticationflags: readable, writableString. Default: nullretries             : Maximum number of retries until giving up (-1=infinite)flags: readable, writableInteger. Range: -1 - 2147483647 Default: 3ssl-ca-file         : Location of a SSL anchor CA file to useflags: readable, writableString. Default: nullssl-strict          : Strict SSL certificate checkingflags: readable, writableBoolean. Default: truessl-use-system-ca-file: Use system CA fileflags: readable, writableBoolean. Default: truetimeout             : Value in seconds to timeout a blocking I/O (0 = No timeout).flags: readable, writableUnsigned Integer. Range: 0 - 3600 Default: 15tls-database        : TLS database with anchor certificate authorities used to validate the server certificateflags: readable, writableObject of type "GTlsDatabase"tls-interaction     : A GTlsInteraction object to be used when the connection or certificate database need to interact with the user.flags: readable, writableObject of type "GTlsInteraction"typefind            : Run typefind before negotiating (deprecated, non-functional)flags: readable, writable, deprecatedBoolean. Default: falseuser-agent          : Value of the User-Agent HTTP request header fieldflags: readable, writableString. Default: "GStreamer souphttpsrc 1.20.6.1 "user-id             : HTTP location URI user id for authenticationflags: readable, writableString. Default: nulluser-pw             : HTTP location URI user password for authenticationflags: readable, writableString. Default: null

 gst-inspect-1.0 filesrc

Factory Details:Rank                     primary (256)Long-name                File SourceKlass                    Source/FileDescription              Read from arbitrary point in a fileAuthor                   Erik Walthinsen <omega@cse.ogi.edu>Plugin Details:Name                     coreelementsDescription              GStreamer core elementsFilename                 /libgstcoreelements.soVersion                  1.20License                  LGPLSource module            gstreamerBinary package           GStreamer gitOrigin URL               Unknown package originGObject+----GInitiallyUnowned+----GstObject+----GstElement+----GstBaseSrc+----GstFileSrcImplemented Interfaces:GstURIHandlerPad Templates:SRC template: 'src'Availability: AlwaysCapabilities:ANYElement has no clocking capabilities.URI handling capabilities:Element can act as source.Supported URI protocols:filePads:SRC: 'src'Pad Template: 'src'Element Properties:blocksize           : Size in bytes to read per buffer (-1 = default)flags: readable, writableUnsigned Integer. Range: 0 - 4294967295 Default: 4096do-timestamp        : Apply current stream time to buffersflags: readable, writablelocation            : Location of the file to readflags: readable, writable, changeable only in NULL or READY stateString. Default: nullname                : The name of the objectflags: readable, writable, 0x2000String. Default: "filesrc0"num-buffers         : Number of buffers to output before sending EOS (-1 = unlimited)flags: readable, writableInteger. Range: -1 - 2147483647 Default: -1parent              : The parent of the objectflags: readable, writable, 0x2000Object of type "GstObject"typefind            : Run typefind before negotiating (deprecated, non-functional)flags: readable, writable, deprecatedBoolean. Default: false