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[Libguestfs] [PATCH libnbd 2/2] copy: Fix sparsifying if start is not aligned

When using --sparse=, we may have a case when command start is not
aligned to sparse_size and command length is smaller than sparse_size.
In this case we advance i after the end of the command, and abort when
we try to write after end.

To reproduce this issue (with temporary debug logs):

$ ./nbdcopy --sparse=$((64*1024)) --threads=1 --requests=1 nbd+unix:///?socket=/tmp/src.sock null:
sparsifying range start=0 end=4096 len=4096
write unaligned tail offset=0 end=4096 len=4096
sparsifying range start=1048576 end=1114112 len=65536
write last data start=1048576 end=1114112 len=65536
sparsifying range start=2097152 end=2121728 len=24576
write unaligned tail offset=2097152 end=2121728 len=24576
sparsifying range start=2138112 end=2146304 len=8192
write last data start=2138112 end=2162688 len=24576
lt-nbdcopy: multi-thread-copying.c:351: copy_subcommand: Assertion `offset + len <= end' failed.
Aborted (core dumped)

Fixed by clipping initial value of i to end of the image.
 copy/multi-thread-copying.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/copy/multi-thread-copying.c b/copy/multi-thread-copying.c
index a1f4b19..ac917e9 100644
--- a/copy/multi-thread-copying.c
+++ b/copy/multi-thread-copying.c
@@ -367,6 +367,12 @@ copy_subcommand (struct command *command, uint64_t offset, size_t len,
   return newcommand;
+static inline uint64_t
+min(uint64_t a, uint64_t b)
+    return a < b ? a : b;
 /* Callback called when src has finished one read command.  This
  * initiates a write.
@@ -399,7 +405,7 @@ finished_read (void *vp, int *error)
     /* Iterate over whole blocks in the command, starting on a block
      * boundary.
-    for (i = ROUND_UP (start, sparse_size);
+    for (i = min (ROUND_UP (start, sparse_size), end);
          i + sparse_size <= end;
          i += sparse_size) {

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