[RFC PATCH 5/6] block: Allow bdrv_get_allocated_file_size to run in bdrv context

[Fabiano Rosas]

We're about to move calls to 'fstat' into the thread-pool to avoid
blocking VCPU threads should the system call take too long.

To achieve that we first need to make sure none of its callers is
holding the aio_context lock, otherwise yielding before scheduling the
aiocb handler would result in a deadlock when the qemu_global_mutex is
released and another thread tries to acquire the aio_context.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 block/qapi.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/block/qapi.c b/block/qapi.c
index ae6cd1c2ff..cd197abf1f 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -222,6 +222,26 @@ int bdrv_query_snapshot_info_list(BlockDriverState *bs,
     return 0;
 }
 
+static int64_t bdrv_get_actual_size(BlockDriverState *bs)
+{
+    int64_t size;
+    AioContext *old_ctx = NULL;
+
+    if (qemu_in_coroutine()) {
+        aio_context_release(bdrv_get_aio_context(bs));
+        old_ctx = bdrv_co_enter(bs);
+    }
+
+    size = bdrv_get_allocated_file_size(bs);
+
+    if (qemu_in_coroutine() && old_ctx) {
+        bdrv_co_leave(bs, old_ctx);
+        aio_context_acquire(bdrv_get_aio_context(bs));
+    }
+
+    return size;
+}
+
 /**
  * Helper function for other query info functions.  Store information about @bs
  * in @info, setting @errp on error.
@@ -250,7 +270,7 @@ static void bdrv_do_query_node_info(BlockDriverState *bs,
     info->filename        = g_strdup(bs->filename);
     info->format          = g_strdup(bdrv_get_format_name(bs));
     info->virtual_size    = size;
-    info->actual_size     = bdrv_get_allocated_file_size(bs);
+    info->actual_size     = bdrv_get_actual_size(bs);
     info->has_actual_size = info->actual_size >= 0;
     if (bs->encrypted) {
         info->encrypted = true;
-- 
2.35.3