[PATCH 2/2] Prevent vhost-user-blk-test hang

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

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 via
a pipefd before allowing the test to proceed. 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/

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
---
 tests/qtest/vhost-user-blk-test.c | 33 ++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/tests/qtest/vhost-user-blk-test.c b/tests/qtest/vhost-user-blk-test.c
index 6f108a1b62..b62af449df 100644
--- a/tests/qtest/vhost-user-blk-test.c
+++ b/tests/qtest/vhost-user-blk-test.c
@@ -21,6 +21,8 @@
 #include "libqos/vhost-user-blk.h"
 #include "libqos/libqos-pc.h"
 
+const char *daemon_msg = "Block exports setup\n";
+
 #define TEST_IMAGE_SIZE         (64 * 1024 * 1024)
 #define QVIRTIO_BLK_TIMEOUT_US  (30 * 1000 * 1000)
 #define PCI_SLOT_HP             0x06
@@ -885,7 +887,8 @@ 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, err, pipe_fds[2];
+    char buf[32] = {0};
     gchar *img_path;
     GString *storage_daemon_command = g_string_new(NULL);
     QemuStorageDaemonState *qsd;
@@ -898,6 +901,12 @@ 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 ");
 
+    err = pipe(pipe_fds);
+    if (err != 0) {
+        fprintf(stderr, "start_vhost_user_blk: pipe() failed %m\n");
+        abort();
+    }
+
     for (i = 0; i < vus_instances; i++) {
         int fd;
         char *sock_path = create_listen_socket(&fd);
@@ -914,22 +923,40 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
                                i + 1, sock_path);
     }
 
+    g_string_append_printf(storage_daemon_command, "--printset");
+
     g_test_message("starting vhost-user backend: %s",
                    storage_daemon_command->str);
+
     pid_t pid = fork();
     if (pid == 0) {
+        close(pipe_fds[0]);
+
         /*
          * Close standard file descriptors so tap-driver.pl pipe detects when
          * our parent terminates.
          */
         close(0);
-        close(1);
         open("/dev/null", O_RDONLY);
-        open("/dev/null", O_WRONLY);
+        close(1);
+        dup2(pipe_fds[1], 1);
 
         execlp("/bin/sh", "sh", "-c", storage_daemon_command->str, NULL);
         exit(1);
     }
+
+    close(pipe_fds[1]);
+
+    err = read(pipe_fds[0], buf, 20);
+    if (err < 0) {
+        fprintf(stderr, "Failed to read from storage-daemon pipe %m\n");
+        abort();
+    } else if (strcmp(buf, daemon_msg) != 0) {
+        fprintf(stderr, "qemu-storage-daemon did not write expected messaage "
+                "to the pipe. Total bytes read: %d. Got: %s\n", err, buf);
+        abort();
+    }
+
     g_string_free(storage_daemon_command, true);
 
     qsd = g_new(QemuStorageDaemonState, 1);
-- 
2.20.1