utils: rm_offset_get_from_fd: Don't combine separate extents

There can be a hole between two physically adjacent extents. To avoid
breaking any code that calls this function, don't merge their
representation, since it completely hides the hole and makes the
physical length of the on-disk extent look larger than it really is.

Fixes sahib#530

lib/utilities.c

@@ -1189,11 +1189,11 @@ RmOff rm_offset_get_from_fd(int fd, RmOff file_offset, RmOff *file_offset_next,
 #if _RM_OFFSET_DEBUG
     int errnum = 0;
 #endif
+    struct fiemap_extent fm_last;
     rm_util_set_nullable_bool(is_last, FALSE);
     rm_util_set_nullable_bool(is_inline, FALSE);
 
-    /* used for detecting contiguous extents */
-    unsigned long expected = 0;
+    memset(&fm_last, 0, sizeof(fm_last));
 
     fsync(fd);
 
@@ -1227,7 +1227,9 @@ RmOff rm_offset_get_from_fd(int fd, RmOff file_offset, RmOff *file_offset_next,
                 first = FALSE;
             } else {
                 /* check if subsequent extents are contiguous */
-                if(fm_ext.fe_physical != expected) {
+                unsigned long expected_dense = fm_last.fe_physical + fm_last.fe_length;
+                unsigned long expected = fm_last.fe_physical + fm_ext.fe_logical - fm_last.fe_logical;
+                if(fm_ext.fe_physical != expected || fm_ext.fe_physical != expected_dense) {
                     /* current extent is not contiguous with previous, so we can stop */
                     g_free(fm);
                     break;
@@ -1251,7 +1253,7 @@ RmOff rm_offset_get_from_fd(int fd, RmOff file_offset, RmOff *file_offset_next,
 
             /* move offsets in preparation for reading next extent */
             file_offset = fm_ext.fe_logical + fm_ext.fe_length;
-            expected = fm_ext.fe_physical + fm_ext.fe_length;
+            fm_last = fm_ext;
         }
 
         g_free(fm);