reader: track read_filter "can_seek" with a flag

Currently we the ::seek callback can be either NULL or client_seek_proxy.

Since we already explicitly use the latter, instead of ::seek just drop
the function pointer all together, in favour of a simple flag.

Alternatively one could use a ARCHIVE_FILTER_NONE check.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

libarchive/archive_read.c

@@ -489,11 +489,11 @@ archive_read_open1(struct archive *_a)
 	filter->open = client_open_proxy;
 	filter->read = client_read_proxy;
 	filter->skip = client_skip_proxy;
-	filter->seek = client_seek_proxy;
 	filter->close = client_close_proxy;
 	filter->sswitch = client_switch_proxy;
 	filter->name = "none";
 	filter->code = ARCHIVE_FILTER_NONE;
+	filter->can_seek = 1;
 
 	a->client.dataset[0].begin_position = 0;
 	if (!a->filter || !a->bypass_filter_bidding)
@@ -1633,7 +1633,7 @@ __archive_read_filter_seek(struct archive_read_filter *filter, int64_t offset,
 
 	if (filter->closed || filter->fatal)
 		return (ARCHIVE_FATAL);
-	if (filter->seek == NULL)
+	if (filter->can_seek == 0)
 		return (ARCHIVE_FAILED);
 
 	client = &(filter->archive->client);

libarchive/archive_read_private.h

@@ -93,8 +93,6 @@ struct archive_read_filter {
 	ssize_t (*read)(struct archive_read_filter *, const void **);
 	/* Skip forward this many bytes. */
 	int64_t (*skip)(struct archive_read_filter *self, int64_t request);
-	/* Seek to an absolute location. */
-	int64_t (*seek)(struct archive_read_filter *self, int64_t offset, int whence);
 	/* Close (just this filter) and free(self). */
 	int (*close)(struct archive_read_filter *self);
 	/* Function that handles switching from reading one block to the next/prev */
@@ -106,6 +104,7 @@ struct archive_read_filter {
 
 	const char	*name;
 	int		 code;
+	int		 can_seek;
 
 	/* Used by reblocking logic. */
 	char		*buffer;