tar: optimize -c --sparse when file is entirely sparse

* src/sparse.c (sparse_scan_file): If the file is entirely sparse,
that is, if ST_NBLOCKS is zero, don't bother scanning for nonzero
blocks.  Idea by Kit Westneat, communicated by Bernd Schubert in
<http://lists.gnu.org/archive/html/bug-tar/2010-08/msg00038.html>.
Also, omit unnecessary lseek at start of file.

diff --git a/src/sparse.c b/src/sparse.c
index 8af4b47b8da1920cbabe6f0792339c5acdb49409..9680b6052244cbb0904d5fedf9ba671995cbd1da 100644 (file)
--- a/src/sparse.c
+++ b/src/sparse.c
@@ -217,43 +217,45 @@ sparse_scan_file (struct tar_sparse_file *file)
   struct tar_stat_info *st = file->stat_info;
   int fd = file->fd;
   char buffer[BLOCKSIZE];
-  size_t count;
+  size_t count = 0;
   off_t offset = 0;
   struct sp_array sp = {0, 0};
 
-  if (!lseek_or_error (file, 0))
-    return false;
-
   st->archive_file_size = 0;
 
-  if (!tar_sparse_scan (file, scan_begin, NULL))
-    return false;
-
-  while ((count = safe_read (fd, buffer, sizeof buffer)) != 0
-        && count != SAFE_READ_ERROR)
+  if (ST_NBLOCKS (st->stat) == 0)
+    offset = st->stat.st_size;
+  else
     {
-      /* Analyze the block.  */
-      if (zero_block_p (buffer, count))
+      if (!tar_sparse_scan (file, scan_begin, NULL))
+       return false;
+
+      while ((count = safe_read (fd, buffer, sizeof buffer)) != 0
+            && count != SAFE_READ_ERROR)
        {
-         if (sp.numbytes)
+         /* Analyze the block.  */
+         if (zero_block_p (buffer, count))
            {
-             sparse_add_map (st, &sp);
-             sp.numbytes = 0;
-             if (!tar_sparse_scan (file, scan_block, NULL))
+             if (sp.numbytes)
+               {
+                 sparse_add_map (st, &sp);
+                 sp.numbytes = 0;
+                 if (!tar_sparse_scan (file, scan_block, NULL))
+                   return false;
+               }
+           }
+         else
+           {
+             if (sp.numbytes == 0)
+               sp.offset = offset;
+             sp.numbytes += count;
+             st->archive_file_size += count;
+             if (!tar_sparse_scan (file, scan_block, buffer))
                return false;
            }
-       }
-      else
-       {
-         if (sp.numbytes == 0)
-           sp.offset = offset;
-         sp.numbytes += count;
-         st->archive_file_size += count;
-         if (!tar_sparse_scan (file, scan_block, buffer))
-           return false;
-       }
 
-      offset += count;
+         offset += count;
+       }
     }
 
   if (sp.numbytes == 0)