[Libguestfs] [PATCH nbdkit 1/2] server: Call .get_ready before redirecting stdin/stdout to /dev/null.

[Richard W.M. Jones]

VDDK plugin + --run was broken because of the following sequence of
events:

(1) After .config_complete, server redirects stdin/stdout to /dev/null.

(2) Server then calls vddk_get_ready which reexecs.

(3) We restart the server from the top, but with stdin/stdout
    redirected to /dev/null.  So saved_stdin/saved_stdout save
    /dev/null.

(4) run_command is called which "restores" /dev/null as stdin/stdout.

(5) The output of the --run option is sent to /dev/null.

In addition to fixing this problem with VDDK, it also makes general
sense not to redirect stdin/stdout before calling .get_ready since
this callback is supposed to be the last chance before the server
daemonizes, and redirecting stdin/stdout is part of daemonization.

Before this commit:

  $ ./nbdkit vddk libdir=tests/.libs /dev/null --run 'qemu-img info $nbd'
  [no output]

After this commit:

  $ ./nbdkit vddk libdir=tests/.libs /dev/null --run 'qemu-img info $nbd'
  image: nbd://localhost:10809
  file format: raw
  virtual size: 512 KiB (524288 bytes)
  disk size: unavailable

One test (test-stdio.sh) made the assumption that stdin/stdout had
already been redirected to /dev/null in .get_ready.  I adjusted this
test so it checks for redirection in .after_fork instead.
---
 server/main.c             | 10 +++++-----
 tests/test-stdio-plugin.c |  9 +++++++++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/server/main.c b/server/main.c
index 2f0aaf07..11c1614b 100644
--- a/server/main.c
+++ b/server/main.c
@@ -693,6 +693,11 @@ main (int argc, char *argv[])
   /* Select the correct thread model based on config. */
   lock_init_thread_model ();
 
+  /* Tell the plugin that we are about to start serving.  This must be
+   * called before we change user, fork, or open any sockets.
+   */
+  top->get_ready (top);
+
   /* Sanitize stdin/stdout to /dev/null, after saving the originals
    * when needed.  We are still single-threaded at this point, and
    * already checked that stdin/out were open, so we don't have to
@@ -726,12 +731,7 @@ main (int argc, char *argv[])
     perror ("open");
     exit (EXIT_FAILURE);
   }
-
-  /* Tell the plugin that we are about to start serving.  This must be
-   * called before we change user, fork, or open any sockets.
-   */
   configured = true;
-  top->get_ready (top);
 
   start_serving ();
 
diff --git a/tests/test-stdio-plugin.c b/tests/test-stdio-plugin.c
index 618eae83..86447278 100644
--- a/tests/test-stdio-plugin.c
+++ b/tests/test-stdio-plugin.c
@@ -122,6 +122,14 @@ stdio_config_complete (void)
 
 static int
 stdio_get_ready (void)
+{
+  bool check = stdio_check ();
+  assert (check == false);
+  return 0;
+}
+
+static int
+stdio_after_fork (void)
 {
   bool check = stdio_check ();
   assert (check == true);
@@ -163,6 +171,7 @@ static struct nbdkit_plugin plugin = {
   .config            = stdio_config,
   .config_complete   = stdio_config_complete,
   .get_ready         = stdio_get_ready,
+  .after_fork        = stdio_after_fork,
   .open              = stdio_open,
   .get_size          = stdio_get_size,
   .pread             = stdio_pread,
-- 
2.27.0