block/export: Conditionally ignore set-context error

When invoking block-export-add with some iothread and
fixed-iothread=false, and changing the node's iothread fails, the error
is supposed to be ignored.

However, it is still stored in *errp, which is wrong.  If a second error
occurs, the "*errp must be NULL" assertion in error_setv() fails:

  qemu-system-x86_64: ../util/error.c:59: error_setv: Assertion
  `*errp == NULL' failed.

So if fixed-iothread=false, we should ignore the error by passing NULL
to bdrv_try_set_aio_context().

Fixes: f51d23c
       ("block/export: add iothread and fixed-iothread options")
Signed-off-by: Max Reitz <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

block/export/export.c

@@ -111,6 +111,7 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     if (export->has_iothread) {
         IOThread *iothread;
         AioContext *new_ctx;
+        Error **set_context_errp;
 
         iothread = iothread_by_id(export->iothread);
         if (!iothread) {
@@ -120,7 +121,9 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
 
         new_ctx = iothread_get_aio_context(iothread);
 
-        ret = bdrv_try_set_aio_context(bs, new_ctx, errp);
+        /* Ignore errors with fixed-iothread=false */
+        set_context_errp = fixed_iothread ? errp : NULL;
+        ret = bdrv_try_set_aio_context(bs, new_ctx, set_context_errp);
         if (ret == 0) {
             aio_context_release(ctx);
             aio_context_acquire(new_ctx);