Use 'skip' when ignoring data in tar archives. This dramatically

increases performance when extracting a single entry from a large
uncompressed archive, especially on slow devices such as USB hard
drives.

Requires a number of changes:
   * New archive_read_open2() supports a 'skip' client function
   * Old archive_read_open() is implemented as a wrapper now, to
     continue supporting the old API/ABI.
   * _read_open_fd and _read_open_file sprout new 'skip' functions.
   * compression layer gets a new 'skip' operation.
   * compression_none passes skip requests through to client.
   * compression_{gzip,bzip2,compress} simply ignore skip requests.

Thanks to: Benjamin Lutz, who designed and implemented the whole thing.
   I'm just committing it.  ;-)

TODO: Need to update the documentation a little bit.

1 files changed, 46 insertions, 1 deletions

diff --git a/lib/libarchive/archive_read_support_format_tar.c b/lib/libarchive/archive_read_support_format_tar.c
index 271a41f..cc0b5ed 100644
--- a/lib/libarchive/archive_read_support_format_tar.c
+++ b/lib/libarchive/archive_read_support_format_tar.c
@@ -193,6 +193,7 @@ static int	archive_read_format_tar_bid(struct archive *);
 static int	archive_read_format_tar_cleanup(struct archive *);
 static int	archive_read_format_tar_read_data(struct archive *a,
 		    const void **buff, size_t *size, off_t *offset);
+static int	archive_read_format_tar_skip(struct archive *a);
 static int	archive_read_format_tar_read_header(struct archive *,
 		    struct archive_entry *);
 static int	checksum(struct archive *, const void *);
@@ -260,7 +261,7 @@ archive_read_support_format_tar(struct archive *a)
 	    archive_read_format_tar_bid,
 	    archive_read_format_tar_read_header,
 	    archive_read_format_tar_read_data,
-	    NULL,
+	    archive_read_format_tar_skip,
 	    archive_read_format_tar_cleanup);
 
 	if (r != ARCHIVE_OK)
@@ -522,6 +523,50 @@ archive_read_format_tar_read_data(struct archive *a,
 	}
 }
 
+static int
+archive_read_format_tar_skip(struct archive *a)
+{
+	ssize_t bytes_skipped;
+	struct tar* tar;
+	struct sparse_block *p;
+	int r = ARCHIVE_OK;
+	const void *b;	/* dummy variables */
+	size_t s;
+	off_t o;
+	
+	
+	tar = *(a->pformat_data);
+	if (a->compression_skip == NULL) {
+		while (r == ARCHIVE_OK)
+			r = archive_read_format_tar_read_data(a, &b, &s, &o);
+		return (r);
+	}
+	bytes_skipped = (a->compression_skip)(a, tar->entry_bytes_remaining);
+	if (bytes_skipped < 0)
+		return (ARCHIVE_FATAL);
+	/* same code as above in _tar_read_data() */
+	tar->entry_bytes_remaining -= bytes_skipped;
+	while (tar->sparse_list != NULL &&
+	    tar->sparse_list->remaining == 0) {
+		p = tar->sparse_list;
+		tar->sparse_list = p->next;
+		free(p);
+		if (tar->sparse_list != NULL)
+			tar->entry_offset = tar->sparse_list->offset;
+	}
+	if (tar->sparse_list != NULL) {
+		if (tar->sparse_list->remaining < bytes_skipped)
+			bytes_skipped = tar->sparse_list->remaining;
+		tar->sparse_list->remaining -= bytes_skipped;
+	}
+	tar->entry_offset += bytes_skipped;
+	tar->entry_bytes_remaining -= bytes_skipped;
+	/* Reuse padding code above. */
+	while (r == ARCHIVE_OK)
+		r = archive_read_format_tar_read_data(a, &b, &s, &o);
+	return (r);
+}
+
 /*
  * This function recursively interprets all of the headers associated
  * with a single entry.