reader: track read_filter "can_skip" with a flag

Analogous to the earlier "can_seek" change. Drop the function pointer
in favour of a flag. Over the years, with over a dozen filters, no
filters actually implemented it.

If at a point in the future that changes, one can reinstate it.

Alternatively one could use a ARCHIVE_FILTER_NONE check.

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

libarchive/archive_read.c

@@ -488,11 +488,11 @@ archive_read_open1(struct archive *_a)
 	filter->data = a->client.dataset[0].data;
 	filter->open = client_open_proxy;
 	filter->read = client_read_proxy;
-	filter->skip = client_skip_proxy;
 	filter->close = client_close_proxy;
 	filter->sswitch = client_switch_proxy;
 	filter->name = "none";
 	filter->code = ARCHIVE_FILTER_NONE;
+	filter->can_skip = 1;
 	filter->can_seek = 1;
 
 	a->client.dataset[0].begin_position = 0;
@@ -1563,8 +1563,8 @@ advance_file_pointer(struct archive_read_filter *filter, int64_t request)
 		return (total_bytes_skipped);
 
 	/* If there's an optimized skip function, use it. */
-	if (filter->skip != NULL) {
-		bytes_skipped = (filter->skip)(filter, request);
+	if (filter->can_skip != 0) {
+		bytes_skipped = client_skip_proxy(filter, request);
 		if (bytes_skipped < 0) {	/* error */
 			filter->fatal = 1;
 			return (bytes_skipped);

libarchive/archive_read_private.h

@@ -91,8 +91,6 @@ struct archive_read_filter {
 	int (*open)(struct archive_read_filter *self);
 	/* Return next block. */
 	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);
 	/* 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 */
@@ -104,6 +102,7 @@ struct archive_read_filter {
 
 	const char	*name;
 	int		 code;
+	int		 can_skip;
 	int		 can_seek;
 
 	/* Used by reblocking logic. */

libarchive/archive_read_support_filter_bzip2.c

@@ -200,7 +200,6 @@ bzip2_reader_init(struct archive_read_filter *self)
 	state->out_block_size = out_block_size;
 	state->out_block = out_block;
 	self->read = bzip2_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = bzip2_filter_close;
 
 	return (ARCHIVE_OK);

libarchive/archive_read_support_filter_compress.c

@@ -227,7 +227,6 @@ compress_bidder_init(struct archive_read_filter *self)
 	state->out_block_size = out_block_size;
 	state->out_block = out_block;
 	self->read = compress_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = compress_filter_close;
 
 	/* XXX MOVE THE FOLLOWING OUT OF INIT() XXX */

libarchive/archive_read_support_filter_gzip.c

@@ -315,7 +315,6 @@ gzip_bidder_init(struct archive_read_filter *self)
 	state->out_block_size = out_block_size;
 	state->out_block = out_block;
 	self->read = gzip_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = gzip_filter_close;
 #ifdef HAVE_ZLIB_H
 	self->read_header = gzip_read_header;

libarchive/archive_read_support_filter_lz4.c

@@ -229,7 +229,6 @@ lz4_reader_init(struct archive_read_filter *self)
 	self->data = state;
 	state->stage = SELECT_STREAM;
 	self->read = lz4_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = lz4_filter_close;
 
 	return (ARCHIVE_OK);

libarchive/archive_read_support_filter_lzop.c

@@ -189,7 +189,6 @@ lzop_bidder_init(struct archive_read_filter *self)
 
 	self->data = state;
 	self->read = lzop_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = lzop_filter_close;
 
 	return (ARCHIVE_OK);

libarchive/archive_read_support_filter_program.c

@@ -429,7 +429,6 @@ __archive_read_program(struct archive_read_filter *self, const char *cmd)
 
 	self->data = state;
 	self->read = program_filter_read;
-	self->skip = NULL;
 	self->close = program_filter_close;
 
 	/* XXX Check that we can read at least one byte? */

libarchive/archive_read_support_filter_rpm.c

@@ -135,7 +135,6 @@ rpm_bidder_init(struct archive_read_filter *self)
 	self->code = ARCHIVE_FILTER_RPM;
 	self->name = "rpm";
 	self->read = rpm_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = rpm_filter_close;
 
 	rpm = (struct rpm *)calloc(sizeof(*rpm), 1);

libarchive/archive_read_support_filter_uu.c

@@ -361,7 +361,6 @@ uudecode_bidder_init(struct archive_read_filter *self)
 	self->code = ARCHIVE_FILTER_UU;
 	self->name = "uu";
 	self->read = uudecode_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = uudecode_filter_close;
 
 	uudecode = (struct uudecode *)calloc(sizeof(*uudecode), 1);

libarchive/archive_read_support_filter_xz.c

@@ -486,7 +486,6 @@ xz_lzma_bidder_init(struct archive_read_filter *self)
 	state->out_block_size = out_block_size;
 	state->out_block = out_block;
 	self->read = xz_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = xz_filter_close;
 
 	state->stream.avail_in = 0;

libarchive/archive_read_support_filter_zstd.c

@@ -190,7 +190,6 @@ zstd_bidder_init(struct archive_read_filter *self)
 	state->out_block = out_block;
 	state->dstream = dstream;
 	self->read = zstd_filter_read;
-	self->skip = NULL; /* not supported */
 	self->close = zstd_filter_close;
 
 	state->eof = 0;