input_stream: make seek(), buffer() optional

Make those two methods optional to implement, and let input_stream.c
provide fallbacks.  The buffer() method will be removed one day, and
there is now only one implementation left (input_curl.c).

src/archive/bz2_plugin.c

@@ -256,19 +256,6 @@ bz2_is_eof(struct input_stream *is)
 	return false;
 }
 
-static bool
-bz2_is_seek(G_GNUC_UNUSED struct input_stream *is,
-	G_GNUC_UNUSED off_t offset, G_GNUC_UNUSED int whence)
-{
-	return false;
-}
-
-static int
-bz2_is_buffer(G_GNUC_UNUSED struct input_stream *is)
-{
-	return 0;
-}
-
 /* exported structures */
 
 static const char *const bz2_extensions[] = {
@@ -280,8 +267,6 @@ static const struct input_plugin bz2_inputplugin = {
 	.close = bz2_is_close,
 	.read = bz2_is_read,
 	.eof = bz2_is_eof,
-	.seek = bz2_is_seek,
-	.buffer = bz2_is_buffer
 };
 
 const struct archive_plugin bz2_plugin = {

src/archive/iso_plugin.c

@@ -211,19 +211,6 @@ iso_is_eof(struct input_stream *is)
 	return (context->cur_ofs == context->statbuf->size);
 }
 
-static bool
-iso_is_seek(G_GNUC_UNUSED struct input_stream *is,
-	G_GNUC_UNUSED off_t offset, G_GNUC_UNUSED int whence)
-{
-	return false;
-}
-
-static int
-iso_is_buffer(G_GNUC_UNUSED struct input_stream *is)
-{
-	return 0;
-}
-
 /* exported structures */
 
 static const char *const iso_extensions[] = {
@@ -235,8 +222,6 @@ static const struct input_plugin iso_inputplugin = {
 	.close = iso_is_close,
 	.read = iso_is_read,
 	.eof = iso_is_eof,
-	.seek = iso_is_seek,
-	.buffer = iso_is_buffer
 };
 
 const struct archive_plugin iso_plugin = {

src/archive/zip_plugin.c

@@ -167,12 +167,6 @@ zip_is_seek(G_GNUC_UNUSED struct input_stream *is,
 	return false;
 }
 
-static int
-zip_is_buffer(G_GNUC_UNUSED struct input_stream *is)
-{
-	return 0;
-}
-
 /* exported structures */
 
 static const char *const zip_extensions[] = {
@@ -185,7 +179,6 @@ static const struct input_plugin zip_inputplugin = {
 	.read = zip_is_read,
 	.eof = zip_is_eof,
 	.seek = zip_is_seek,
-	.buffer = zip_is_buffer
 };
 
 const struct archive_plugin zip_plugin = {

src/input_file.c

@@ -116,16 +116,9 @@ input_file_eof(struct input_stream *is)
 	return is->offset >= is->size;
 }
 
-static int
-input_file_buffer(G_GNUC_UNUSED struct input_stream *is)
-{
-	return 0;
-}
-
 const struct input_plugin input_plugin_file = {
 	.open = input_file_open,
 	.close = input_file_close,
-	.buffer = input_file_buffer,
 	.read = input_file_read,
 	.eof = input_file_eof,
 	.seek = input_file_seek,

src/input_stream.c

@@ -86,6 +86,7 @@ input_stream_open(struct input_stream *is, const char *url)
 			assert(is->plugin->close != NULL);
 			assert(is->plugin->read != NULL);
 			assert(is->plugin->eof != NULL);
+			assert(!is->seekable || is->plugin->seek != NULL);
 
 			return true;
 		}
@@ -97,6 +98,9 @@ input_stream_open(struct input_stream *is, const char *url)
 bool
 input_stream_seek(struct input_stream *is, off_t offset, int whence)
 {
+	if (is->plugin->seek == NULL)
+		return false;
+
 	return is->plugin->seek(is, offset, whence);
 }
 
@@ -133,5 +137,8 @@ bool input_stream_eof(struct input_stream *is)
 
 int input_stream_buffer(struct input_stream *is)
 {
+	if (is->plugin->buffer == NULL)
+		return 0;
+
 	return is->plugin->buffer(is);
 }