matroska: add temporary webm typefinder

Add webm typefinder just for the release, so webm works for
people whose distros don't patch gst-plugins-base as well.
We'll remove this again after the release.

gst/matroska/matroska.c

@@ -28,9 +28,62 @@
 #include "matroska-ids.h"
 #include "webm-mux.h"
 
+#include <string.h>
+
+/*** video/webm typefinder (temporary) ***/
+static GstStaticCaps webm_caps = GST_STATIC_CAPS ("video/webm");
+
+#define WEBM_CAPS (gst_static_caps_get(&webm_caps))
+static void
+webm_type_find (GstTypeFind * tf, gpointer ununsed)
+{
+  static const guint8 webm_doctype[] = { 'w', 'e', 'b', 'm' };
+  guint8 *data;
+  gint len_mask = 0x80, size = 1, n = 1, total;
+
+  /* 4 bytes for EBML ID, 1 byte for header length identifier */
+  data = gst_type_find_peek (tf, 0, 4 + 1);
+  if (!data)
+    return;
+
+  /* ebml header? */
+  if (data[0] != 0x1A || data[1] != 0x45 || data[2] != 0xDF || data[3] != 0xA3)
+    return;
+
+  /* length of header */
+  total = data[4];
+  while (size <= 8 && !(total & len_mask)) {
+    size++;
+    len_mask >>= 1;
+  }
+  if (size > 8)
+    return;
+  total &= (len_mask - 1);
+  while (n < size)
+    total = (total << 8) | data[4 + n++];
+
+  /* get new data for full header, 4 bytes for EBML ID,
+   * EBML length tag and the actual header */
+  data = gst_type_find_peek (tf, 0, 4 + size + total);
+  if (!data)
+    return;
+
+  /* the header must contain the document type 'webm'. For now,
+   * we don't parse the whole header but simply check for the
+   * availability of that array of characters inside the header.
+   * Not fully fool-proof, but good enough. */
+  for (n = 4 + size; n <= 4 + size + total - sizeof (webm_doctype); n++) {
+    if (!memcmp (&data[n], webm_doctype, sizeof (webm_doctype))) {
+      gst_type_find_suggest (tf, GST_TYPE_FIND_MAXIMUM, WEBM_CAPS);
+      return;
+    }
+  }
+}
+
 static gboolean
 plugin_init (GstPlugin * plugin)
 {
+  static const gchar *webm_exts[] = { "webm", "weba", "webv", NULL };
   gboolean ret;
 
   gst_matroska_register_tags ();
@@ -40,6 +93,8 @@ plugin_init (GstPlugin * plugin)
       GST_TYPE_MATROSKA_MUX);
   ret &= gst_element_register (plugin, "webmmux", GST_RANK_PRIMARY,
       GST_TYPE_WEBM_MUX);
+  ret &= gst_type_find_register (plugin, "good-webm", GST_RANK_MARGINAL,
+      webm_type_find, (gchar **) webm_exts, WEBM_CAPS, NULL, NULL);
 
   return ret;
 }