pluginfeature: Allow updating initial rank of plugin feature

Introducing "GST_PLUGIN_FEATURE_RANK" environment variable in order for users
to adjust rank of plugin(s) via environment.

A "feature" and "rank" key-value pair should be separable by ":",
and each key-value pair is recognized per "," delimiters. The rank
can be a numerical value or one of pre-defined rank values
such as "NONE", "MARGINAL", "SECONDARY", and "PRIMARY" in case-insensitive manner.

In addition to pre-defined { NONE, MARGINAL, SECONDARY, PRIMARY },
"MAX" can be passed to key value used to ensure having a higher rank
than other plugin features.

Example)
- GST_PLUGIN_FEATURE_RANK=qtdemux:256,h264parse:NONE
Set rank of qtdemux plugin to 256 (primary) and 0 (none) for h264parse.

docs/gst/running.md

@@ -312,3 +312,20 @@ Common values are 'egl', 'glx', 'wgl' or 'cgl'.
 
 Influences the OpenGL API requested by the OpenGL platform. Common
 values are 'opengl' or 'gles2'.
+
+**`GST_PLUGIN_FEATURE_RANK`. (Since: 1.18)**
+
+This environment variable can be used to adjust rank of each plugin feature.
+
+The variable takes a comma-separated list of `plugin_feature:rank`
+pairs to set specific ranks for the individual plugin features.
+The rank can be an arbitrary numerical value or one of pre-defined rank values
+from `NONE`(0) to `PRIMARY`(256) in case-insensitive manner.
+In addition to the pre-defined rank values, `MAX` is an acceptable value to set
+higher rank than the rank of other existing plugin features.
+
+Example: `GST_PLUGIN_FEATURE_RANK=foo:PRIMARY,bar:primary,foobar:128`
+
+As a result of the above example,
+the `foo` and` bar` plugin feature rank values are `PRIMARY`(256)
+and `SECONDARY`(128) rank value will be assigned to `foobar`.

gst/gst.c

@@ -809,6 +809,10 @@ init_post (GOptionContext * context, GOptionGroup * group, gpointer data,
       GLIB_MINOR_VERSION, GLIB_MICRO_VERSION);
   GST_INFO ("initialized GStreamer successfully");
 
+  /* Adjust initial plugin rank based on the GST_PLUGIN_FEATURE_RANK
+   * environment variable */
+  _priv_gst_plugin_feature_rank_initialize ();
+
 #ifndef GST_DISABLE_GST_DEBUG
   _priv_gst_tracing_init ();
 #endif

gst/gst_private.h

@@ -137,6 +137,7 @@ G_GNUC_INTERNAL  void  _priv_gst_debug_init (void);
 G_GNUC_INTERNAL  void  _priv_gst_context_initialize (void);
 G_GNUC_INTERNAL  void  _priv_gst_toc_initialize (void);
 G_GNUC_INTERNAL  void  _priv_gst_date_time_initialize (void);
+G_GNUC_INTERNAL  void  _priv_gst_plugin_feature_rank_initialize (void);
 
 /* cleanup functions called from gst_deinit(). */
 G_GNUC_INTERNAL  void  _priv_gst_allocator_cleanup (void);

gst/gstpluginfeature.c

@@ -405,3 +405,97 @@ gst_plugin_feature_rank_compare_func (gconstpointer p1, gconstpointer p2)
 
   return diff;
 }
+
+static gboolean
+parse_feature_name (gchar * str, const gchar ** feature)
+{
+  if (!str)
+    return FALSE;
+
+  /* works in place */
+  g_strstrip (str);
+
+  if (str[0] != '\0') {
+    *feature = str;
+    return TRUE;
+  }
+
+  return FALSE;
+}
+
+static gboolean
+parse_feature_rank (gchar * str, GstRank * rank)
+{
+  if (!str)
+    return FALSE;
+
+  /* works in place */
+  g_strstrip (str);
+
+  if (g_ascii_isdigit (str[0])) {
+    unsigned long l;
+    char *endptr;
+    l = strtoul (str, &endptr, 10);
+    if (endptr > str && endptr[0] == 0) {
+      *rank = (GstRank) l;
+    } else {
+      return FALSE;
+    }
+  } else if (g_ascii_strcasecmp (str, "NONE") == 0) {
+    *rank = GST_RANK_NONE;
+  } else if (g_ascii_strcasecmp (str, "MARGINAL") == 0) {
+    *rank = GST_RANK_MARGINAL;
+  } else if (g_ascii_strcasecmp (str, "SECONDARY") == 0) {
+    *rank = GST_RANK_SECONDARY;
+  } else if (g_ascii_strcasecmp (str, "PRIMARY") == 0) {
+    *rank = GST_RANK_PRIMARY;
+  } else if (g_ascii_strcasecmp (str, "MAX") == 0) {
+    *rank = (GstRank) G_MAXINT;
+  } else {
+    return FALSE;
+  }
+
+  return TRUE;
+}
+
+void
+_priv_gst_plugin_feature_rank_initialize (void)
+{
+  const gchar *env;
+  gchar **split;
+  gchar **walk;
+
+  env = g_getenv ("GST_PLUGIN_FEATURE_RANK");
+
+  if (!env)
+    return;
+
+  split = g_strsplit (env, ",", 0);
+
+  for (walk = split; *walk; walk++) {
+    if (strchr (*walk, ':')) {
+      gchar **values = g_strsplit (*walk, ":", 2);
+
+      if (values[0] && values[1]) {
+        GstRank rank;
+        const gchar *str;
+
+        if (parse_feature_name (values[0], &str)
+            && parse_feature_rank (values[1], &rank)) {
+          GstPluginFeature *feature;
+
+          feature = gst_registry_find_feature (gst_registry_get (), str,
+              GST_TYPE_ELEMENT_FACTORY);
+          if (feature) {
+            gst_plugin_feature_set_rank (feature, rank);
+            GST_DEBUG ("Update rank of plugin feature \"%s\" to %d", str, rank);
+          }
+        }
+      }
+
+      g_strfreev (values);
+    }
+  }
+
+  g_strfreev (split);
+}