input_plugin: add method check()

To check for errors without reading.  The decoder thread wants to do
that, before it passes the input stream to the plugin.

src/input/curl_input_plugin.c

@@ -774,6 +774,23 @@ input_curl_free(struct input_curl *c)
 	g_free(c);
 }
 
+static bool
+input_curl_check(struct input_stream *is, GError **error_r)
+{
+	struct input_curl *c = (struct input_curl *)is;
+
+	g_mutex_lock(c->mutex);
+
+	bool success = c->postponed_error == NULL;
+	if (!success) {
+		g_propagate_error(error_r, c->postponed_error);
+		c->postponed_error = NULL;
+	}
+
+	g_mutex_unlock(c->mutex);
+	return success;
+}
+
 static struct tag *
 input_curl_tag(struct input_stream *is)
 {
@@ -1318,6 +1335,7 @@ const struct input_plugin input_plugin_curl = {
 
 	.open = input_curl_open,
 	.close = input_curl_close,
+	.check = input_curl_check,
 	.tag = input_curl_tag,
 	.buffer = input_curl_buffer,
 	.read = input_curl_read,

src/input/rewind_input_plugin.c

@@ -107,6 +107,14 @@ input_rewind_close(struct input_stream *is)
 	g_free(r);
 }
 
+static bool
+input_rewind_check(struct input_stream *is, GError **error_r)
+{
+	struct input_rewind *r = (struct input_rewind *)is;
+
+	return input_stream_check(r->input, error_r);
+}
+
 static void
 input_rewind_update(struct input_stream *is)
 {
@@ -221,6 +229,7 @@ input_rewind_seek(struct input_stream *is, goffset offset, int whence,
 
 static const struct input_plugin rewind_input_plugin = {
 	.close = input_rewind_close,
+	.check = input_rewind_check,
 	.update = input_rewind_update,
 	.tag = input_rewind_tag,
 	.buffer = input_rewind_buffer,

src/input/soup_input_plugin.c

@@ -320,6 +320,23 @@ input_soup_close(struct input_stream *is)
 	g_free(s);
 }
 
+static bool
+input_soup_check(struct input_stream *is, GError **error_r)
+{
+	struct input_soup *s = (struct input_soup *)is;
+
+	g_mutex_lock(s->mutex);
+
+	bool success = s->postponed_error == NULL;
+	if (!success) {
+		g_propagate_error(error_r, s->postponed_error);
+		s->postponed_error = NULL;
+	}
+
+	g_mutex_unlock(s->mutex);
+	return success;
+}
+
 static int
 input_soup_buffer(struct input_stream *is, GError **error_r)
 {
@@ -444,6 +461,7 @@ const struct input_plugin input_plugin_soup = {
 
 	.open = input_soup_open,
 	.close = input_soup_close,
+	.check = input_soup_check,
 	.buffer = input_soup_buffer,
 	.read = input_soup_read,
 	.eof = input_soup_eof,