[PATCH v6] Work around vhost-user-blk-test hang

[Raphael Norwitz <raphael.norwitz@nutanix.com>]

The vhost-user-blk-test qtest has been hanging intermittently for a
while. The root cause is not yet fully understood, but the hang is
impacting enough users that it is important to merge a workaround for
it.

The race which causes the hang occurs early on in vhost-user setup,
where a vhost-user message is never received by the backend. Forcing
QEMU to wait until the storage-daemon has had some time to initialize
prevents the hang. Thus the existing storage-daemon pidfile option can
be used to implement a workaround cleanly and effectively, since it
creates a file only once the storage-daemon initialization is complete.

This change implements a workaround for the vhost-user-blk-test hang by
making QEMU wait until the storage-daemon has written out a pidfile
before attempting to connect and send messages over the vhost-user
socket.

Some relevent mailing list discussions:

[1] https://lore.kernel.org/qemu-devel/CAFEAcA8kYpz9LiPNxnWJAPSjc=nv532bEdyfynaBeMeohqBp3A@mail.gmail.com/
[2] https://lore.kernel.org/qemu-devel/YWaky%2FKVbS%2FKZjlV@stefanha-x1.localdomain/

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 tests/qtest/vhost-user-blk-test.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/tests/qtest/vhost-user-blk-test.c b/tests/qtest/vhost-user-blk-test.c
index 6f108a1b62..c6626a286b 100644
--- a/tests/qtest/vhost-user-blk-test.c
+++ b/tests/qtest/vhost-user-blk-test.c
@@ -24,6 +24,7 @@
 #define TEST_IMAGE_SIZE         (64 * 1024 * 1024)
 #define QVIRTIO_BLK_TIMEOUT_US  (30 * 1000 * 1000)
 #define PCI_SLOT_HP             0x06
+#define PIDFILE_RETRIES         5
 
 typedef struct {
     pid_t pid;
@@ -885,7 +886,8 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
                                  int num_queues)
 {
     const char *vhost_user_blk_bin = qtest_qemu_storage_daemon_binary();
-    int i;
+    int i, retries;
+    char *daemon_pidfile_path;
     gchar *img_path;
     GString *storage_daemon_command = g_string_new(NULL);
     QemuStorageDaemonState *qsd;
@@ -898,6 +900,8 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
             " -object memory-backend-memfd,id=mem,size=256M,share=on "
             " -M memory-backend=mem -m 256M ");
 
+    daemon_pidfile_path = g_strdup_printf("/tmp/daemon-%d", getpid());
+
     for (i = 0; i < vus_instances; i++) {
         int fd;
         char *sock_path = create_listen_socket(&fd);
@@ -914,6 +918,9 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
                                i + 1, sock_path);
     }
 
+    g_string_append_printf(storage_daemon_command, "--pidfile %s ",
+                           daemon_pidfile_path);
+
     g_test_message("starting vhost-user backend: %s",
                    storage_daemon_command->str);
     pid_t pid = fork();
@@ -930,7 +937,27 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
         execlp("/bin/sh", "sh", "-c", storage_daemon_command->str, NULL);
         exit(1);
     }
+
+    /*
+     * FIXME: The loop here ensures the storage-daemon has come up properly
+     *        before allowing the test to proceed. This is a workaround for
+     *        a race which used to cause the vhost-user-blk-test to hang. It
+     *        should be deleted once the root cause is fully understood and
+     *        fixed.
+     */
+    retries = 0;
+    while (access(daemon_pidfile_path, F_OK) != 0) {
+        g_assert_cmpint(retries, <, PIDFILE_RETRIES);
+
+        retries++;
+        g_usleep(1000);
+    }
+
     g_string_free(storage_daemon_command, true);
+    if (access(daemon_pidfile_path, F_OK) == 0) {
+        unlink(daemon_pidfile_path);
+    }
+    g_free(daemon_pidfile_path);
 
     qsd = g_new(QemuStorageDaemonState, 1);
     qsd->pid = pid;
-- 
2.20.1