xz compression detection: avoid the need to seek

function                                             old     new   delta
unpack_unxz                                           12      67     +55
unpack_xz_stream                                    2357    2373     +16
xmalloc_read                                         197     199      +2
setup_unzip_on_fd                                    118      99     -19
rpm2cpio_main                                        222     203     -19
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 73/-38)             Total: 35 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/archival/libunarchive/decompress_unxz.c b/archival/libunarchive/decompress_unxz.c
index 5d7513a2e..721acd907 100644
--- a/archival/libunarchive/decompress_unxz.c
+++ b/archival/libunarchive/decompress_unxz.c
@@ -56,9 +56,11 @@ unpack_xz_stream(int src_fd, int dst_fd)
         if (!crc32_table)
                 crc32_table = crc32_filltable(NULL, /*endian:*/ 0);
 
-        membuf = xmalloc(2 * BUFSIZ);
         memset(&iobuf, 0, sizeof(iobuf));
+        /* Preload XZ file signature */
+        membuf = (void*) strcpy(xmalloc(2 * BUFSIZ), HEADER_MAGIC);
         iobuf.in = membuf;
+        iobuf.in_size = HEADER_MAGIC_SIZE;
         iobuf.out = membuf + BUFSIZ;
         iobuf.out_size = BUFSIZ;