Re: [PATCH v5] Prevent vhost-user-blk-test hang

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Sep 27, 2021 at 05:17:01PM +0000, Raphael Norwitz wrote:
> In the vhost-user-blk-test, as of now there is nothing stoping
> vhost-user-blk in QEMU writing to the socket right after forking off the
> storage daemon before it has a chance to come up properly, leaving the
> test hanging forever. This intermittently hanging test has caused QEMU
> automation failures reported multiple times on the mailing list [1].
> 
> This change makes the storage-daemon notify the vhost-user-blk-test
> that it is fully initialized and ready to handle client connections by
> creating a pidfile on initialiation. This ensures that the storage-daemon
> backend won't miss vhost-user messages and thereby resolves the hang.
> 
> [1] https://lore.kernel.org/qemu-devel/CAFEAcA8kYpz9LiPNxnWJAPSjc=nv532bEdyfynaBeMeohqBp3A@mail.gmail.com/

The following hack triggers the hang. The stack traces match what Peter
posted in the thread linked above. I'll take a closer look.

---

diff --git a/storage-daemon/qemu-storage-daemon.c b/storage-daemon/qemu-storage-daemon.c
index 10a1a33761..50c9caa1b2 100644
--- a/storage-daemon/qemu-storage-daemon.c
+++ b/storage-daemon/qemu-storage-daemon.c
@@ -316,6 +316,8 @@ int main(int argc, char *argv[])
     signal(SIGPIPE, SIG_IGN);
 #endif

+    sleep(5);
+
     error_init(argv[0]);
     qemu_init_exec_dir(argv[0]);
     os_setup_signal_handling();

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