Fix block migration bug

v2:
- rebase to master
- fix typos in description

Because of wrong return value of .save_live_pending() in block-migration, migration finishes before the whole disk is transferred. Such situation occurs when the migration process is fast enough, for example when source and dest are on the same host.

It's easy to test this with the following:

bug.sh
=====================================================================
#!/bin/sh

size=$1
addr=$2

rm /tmp/fifo-mig /tmp/a /tmp/b /tmp/sock-mig

./qemu-img create -f qcow2 /tmp/a $size
./qemu-img create -f qcow2 /tmp/b $size

./qemu-io -c "write -P 0x22 $addr 512" /tmp/a

mkfifo /tmp/fifo-mig

./x86_64-softmmu/qemu-system-x86_64 -drive file=/tmp/b,id=disk\
-qmp unix:/tmp/sock-mig,server,nowait\
-incoming "exec: cat /tmp/fifo-mig" &

echo 'migrate -b exec:cat>/tmp/fifo-mig\nquit\n' |\
./x86_64-softmmu/qemu-system-x86_64 -drive file=/tmp/a,id=disk\
-monitor stdio

./scripts/qmp/qmp --path=/tmp/sock-mig quit
sleep 3

echo checking
./qemu-io -c "read -P 0x22 $addr 512" /tmp/b
=====================================================================

For './bug.sh 1G 1M' qemu-io check finishes successfully, but for './bug.sh 1G 1022M' it finishes with 'Pattern verification failed' status.

The following patch fixes this bug.

Vladimir Sementsov-Ogievskiy (1):
migration/block: fix pending() return value

 migration/block.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

1.9.1

Because of wrong return value of .save_live_pending() in migration/block.c, migration finishes before the whole disk is transferred. Such situation occurs when the migration process is fast enough, for example when source and dest are on the same host.

If in the bulk phase we return something < max_size, we will skip transferring the tail of the device. Currently we have "set pending to BLOCK_SIZE if it is zero" for bulk phase, but there no guarantee, that it will be < max_size.

True approach is to return, for example, max_size+1 when we are in the bulk phase.

---
 migration/block.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 74d9eb1..2e92605 100644
@@ -765,8 +765,8 @@ static uint64_t block_save_pending(QEMUFile *f, void *opaque, uint64_t max_size)
                        block_mig_state.read_done * BLOCK_SIZE;
 
     /* Report at least one block pending during bulk phase */
-    if (pending == 0 && !block_mig_state.bulk_completed) {
-        pending = BLOCK_SIZE;
+    if (pending <= max_size && !block_mig_state.bulk_completed) {
+        pending = max_size + BLOCK_SIZE;
     }
     blk_mig_unlock();
     qemu_mutex_unlock_iothread();

1.9.1

Yikes, this is a nasty bug. CCing qemu-stable.

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan