input_plugin: add method update()

Update the struct attributes, important for facades like the "rewind"
plugin.  To replace buffer().

src/input/rewind_input_plugin.c

@@ -105,6 +105,15 @@ input_rewind_close(struct input_stream *is)
 	g_free(r);
 }
 
+static void
+input_rewind_update(struct input_stream *is)
+{
+	struct input_rewind *r = (struct input_rewind *)is;
+
+	if (!reading_from_buffer(r))
+		copy_attributes(r);
+}
+
 static struct tag *
 input_rewind_tag(struct input_stream *is)
 {
@@ -210,6 +219,7 @@ input_rewind_seek(struct input_stream *is, goffset offset, int whence,
 
 static const struct input_plugin rewind_input_plugin = {
 	.close = input_rewind_close,
+	.update = input_rewind_update,
 	.tag = input_rewind_tag,
 	.buffer = input_rewind_buffer,
 	.read = input_rewind_read,

src/input_plugin.h

@@ -51,6 +51,12 @@ struct input_plugin {
 	struct input_stream *(*open)(const char *uri, GError **error_r);
 	void (*close)(struct input_stream *is);
 
+	/**
+	 * Update the public attributes.  Call before access.  Can be
+	 * NULL if the plugin always keeps its attributes up to date.
+	 */
+	void (*update)(struct input_stream *is);
+
 	struct tag *(*tag)(struct input_stream *is);
 	int (*buffer)(struct input_stream *is, GError **error_r);
 	size_t (*read)(struct input_stream *is, void *ptr, size_t size,

src/input_stream.c

@@ -67,6 +67,16 @@ input_stream_open(const char *url, GError **error_r)
 	return false;
 }
 
+void
+input_stream_update(struct input_stream *is)
+{
+	assert(is != NULL);
+	assert(is->plugin != NULL);
+
+	if (is->plugin->update != NULL)
+		is->plugin->update(is);
+}
+
 bool
 input_stream_seek(struct input_stream *is, goffset offset, int whence,
 		  GError **error_r)

src/input_stream.h

@@ -86,6 +86,13 @@ input_stream_open(const char *uri, GError **error_r);
 void
 input_stream_close(struct input_stream *is);
 
+/**
+ * Update the public attributes.  Call before accessing attributes
+ * such as "ready" or "offset".
+ */
+void
+input_stream_update(struct input_stream *is);
+
 /**
  * Seeks to the specified position in the stream.  This will most
  * likely fail if the "seekable" flag is false.