[Qemu-devel] [PATCH 3/4] block/parallels: Don't update header until the first actual write

[Jeff Cody]

The on disk image format 'inuse' header field is updated blindly if the
image is opened RDWR.  This can cause problems if the QEMU runstate is
set to INMIGRATE, at which point the underlying file is set to INACTIVE.
This causes an assert in bdrv_co_pwritev().

Do something similar to what is done in VHDX; latch the first write, and
update the header the first time we modify the file.

Signed-off-by: Jeff Cody <address@hidden>
---
 block/parallels.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/block/parallels.c b/block/parallels.c
index fed199eccd..c560e2fcf2 100644
--- a/block/parallels.c
+++ b/block/parallels.c
@@ -100,6 +100,8 @@ typedef struct BDRVParallelsState {
     unsigned int tracks;
 
     unsigned int off_multiplier;
+
+    bool first_write_latch;
 } BDRVParallelsState;
 
 
@@ -317,6 +319,16 @@ static coroutine_fn int 
parallels_co_writev(BlockDriverState *bs,
     QEMUIOVector hd_qiov;
     int ret = 0;
 
+    if (s->first_write_latch) {
+        s->first_write_latch = false;
+        qemu_co_mutex_lock(&s->lock);
+        ret = parallels_update_header(bs);
+        qemu_co_mutex_unlock(&s->lock);
+    }
+    if (ret < 0) {
+        return ret;
+    }
+
     qemu_iovec_init(&hd_qiov, qiov->niov);
 
     while (nb_sectors > 0) {
@@ -416,6 +428,9 @@ static int parallels_check(BlockDriverState *bs, 
BdrvCheckResult *res,
             /* parallels_close will do the job right */
             res->corruptions_fixed++;
             s->header_unclean = false;
+            /* set that a write has occurred, so that parallels_close() will
+             * update the inuse field in the header */
+            s->first_write_latch = false;
         }
     }
 
@@ -597,6 +612,8 @@ static int parallels_open(BlockDriverState *bs, QDict 
*options, int flags,
     Error *local_err = NULL;
     char *buf;
 
+    s->first_write_latch = true;
+
     bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
                                false, errp);
     if (!bs->file) {
@@ -710,10 +727,6 @@ static int parallels_open(BlockDriverState *bs, QDict 
*options, int flags,
 
     if (flags & BDRV_O_RDWR) {
         s->header->inuse = cpu_to_le32(HEADER_INUSE_MAGIC);
-        ret = parallels_update_header(bs);
-        if (ret < 0) {
-            goto fail;
-        }
     }
 
     s->bat_dirty_block = 4 * getpagesize();
@@ -741,7 +754,9 @@ static void parallels_close(BlockDriverState *bs)
 {
     BDRVParallelsState *s = bs->opaque;
 
-    if (bs->open_flags & BDRV_O_RDWR) {
+    /* Only need to update the header, if we ever actually wrote to the
+     * image at all */
+    if ((bs->open_flags & BDRV_O_RDWR) && !s->first_write_latch) {
         s->header->inuse = 0;
         parallels_update_header(bs);
     }
-- 
2.13.6