blockjob: Add block_job_yield()

This will unset busy flag and put coroutine to sleep, can be used to
wait for QMP complete/cancel.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockjob.c

@@ -210,6 +210,20 @@ void block_job_sleep_ns(BlockJob *job, QEMUClockType type, int64_t ns)
     job->busy = true;
 }
 
+void block_job_yield(BlockJob *job)
+{
+    assert(job->busy);
+
+    /* Check cancellation *before* setting busy = false, too!  */
+    if (block_job_is_cancelled(job)) {
+        return;
+    }
+
+    job->busy = false;
+    qemu_coroutine_yield();
+    job->busy = true;
+}
+
 BlockJobInfo *block_job_query(BlockJob *job)
 {
     BlockJobInfo *info = g_new0(BlockJobInfo, 1);

include/block/blockjob.h

@@ -146,6 +146,14 @@ void *block_job_create(const BlockJobDriver *driver, BlockDriverState *bs,
  */
 void block_job_sleep_ns(BlockJob *job, QEMUClockType type, int64_t ns);
 
+/**
+ * block_job_yield:
+ * @job: The job that calls the function.
+ *
+ * Yield the block job coroutine.
+ */
+void block_job_yield(BlockJob *job);
+
 /**
  * block_job_completed:
  * @job: The job being completed.