gnu: tar: Fix sparse file detection on Btrfs file systems.

* gnu/packages/patches/tar-remove-wholesparse-check.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/base.scm (tar)[source]: Add patch.

gnu/local.mk

@@ -1091,6 +1091,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/t1lib-CVE-2010-2642.patch		\
   %D%/packages/patches/t1lib-CVE-2011-0764.patch		\
   %D%/packages/patches/t1lib-CVE-2011-1552+.patch		\
+  %D%/packages/patches/tar-remove-wholesparse-check.patch	\
   %D%/packages/patches/tar-skip-unreliable-tests.patch		\
   %D%/packages/patches/tclxml-3.2-install.patch			\
   %D%/packages/patches/tcsh-fix-autotest.patch			\

gnu/packages/base.scm

@@ -177,7 +177,8 @@ implementation offers several extensions over the standard utility.")
             (sha256
              (base32
               "1lyjyk8z8hdddsxw0ikchrsfg3i0x3fsh7l63a8jgaz1n7dr5gzi"))
-            (patches (search-patches "tar-skip-unreliable-tests.patch"))))
+            (patches (search-patches "tar-skip-unreliable-tests.patch"
+                                     "tar-remove-wholesparse-check.patch"))))
    (build-system gnu-build-system)
    ;; Note: test suite requires ~1GiB of disk space.
    (arguments

gnu/packages/patches/tar-remove-wholesparse-check.patch

@@ -0,0 +1,68 @@
+This patch is needed in order to build Guix on a Btrfs file system.
+
+
+From da922703282b0d3b8837a99a9c7fdd32f1d20d49 Mon Sep 17 00:00:00 2001
+From: Mark H Weaver <mhw@netris.org>
+Date: Tue, 9 Jan 2018 20:16:14 -0500
+Subject: [PATCH] Remove nonportable check for files containing only zeroes.
+
+This check benefitted only one unlikely case (large files containing
+only zeroes, on systems that do not support SEEK_HOLE) and was based
+on an assumption about file system behavior that is not mandated by
+POSIX and no longer holds in practice, namely that for sufficiently
+large files, (st_blocks == 0) implies that the file contains only
+zeroes.  Examples of file systems that violate this assumption include
+Linux's /proc file system and Btrfs.
+
+* src/sparse.c (sparse_scan_file_wholesparse): Remove this function.
+(sparse_scan_file_seek): Remove the initial check for files containing
+only zeroes.
+---
+ src/sparse.c | 24 ------------------------
+ 1 file changed, 24 deletions(-)
+
+diff --git a/src/sparse.c b/src/sparse.c
+index d41c0ea..3de6560 100644
+--- a/src/sparse.c
++++ b/src/sparse.c
+@@ -261,26 +261,6 @@ sparse_scan_file_raw (struct tar_sparse_file *file)
+   return tar_sparse_scan (file, scan_end, NULL);
+ }
+ 
+-static bool
+-sparse_scan_file_wholesparse (struct tar_sparse_file *file)
+-{
+-  struct tar_stat_info *st = file->stat_info;
+-  struct sp_array sp = {0, 0};
+-
+-  /* Note that this function is called only for truly sparse files of size >= 1
+-     block size (checked via ST_IS_SPARSE before).  See the thread
+-     http://www.mail-archive.com/bug-tar@gnu.org/msg04209.html for more info */
+-  if (ST_NBLOCKS (st->stat) == 0)
+-    {
+-      st->archive_file_size = 0;
+-      sp.offset = st->stat.st_size;
+-      sparse_add_map (st, &sp);
+-      return true;
+-    }
+-
+-  return false;
+-}
+-
+ #ifdef SEEK_HOLE
+ /* Try to engage SEEK_HOLE/SEEK_DATA feature. */
+ static bool
+@@ -343,10 +323,6 @@ sparse_scan_file_seek (struct tar_sparse_file *file)
+ static bool
+ sparse_scan_file (struct tar_sparse_file *file)
+ {
+-  /* always check for completely sparse files */
+-  if (sparse_scan_file_wholesparse (file))
+-    return true;
+-
+   switch (hole_detection)
+     {
+     case HOLE_DETECTION_DEFAULT:
+-- 
+2.15.1
+