[Libguestfs] [libnbd PATCH v2 12/13] wip: api: Give aio_opt_go a completion callback

[Eric Blake]

Squash this into opt_go? into opt_info? Standalone?

Testing: why does python not throw an exception:
$ ./run nbdsh

Welcome to nbdsh, the shell for interacting with
Network Block Device (NBD) servers.

The ‘nbd’ module has already been imported and there
is an open NBD handle called ‘h’.

h.connect_tcp ("remote", "10809")   # Connect to a remote server.
h.get_size ()                       # Get size of the remote disk.
buf = h.pread (512, 0, 0)           # Read the first sector.
exit() or Ctrl-D                    # Quit the shell
help (nbd)                          # Display documentation

nbd> h.set_opt_mode(True)
nbd> h.connect_command(["nbdkit","-s","--exit-","eval",
"open=echo ENOENT >&2; exit 1"])
nbd> h.aio_is_negotiating()
True
nbd> h.set_export_name('a')
nbd> h.opt_go()
nbdkit: eval: error: /tmp/nbdkitNyQD8J/open: ENOENT
nbd> h.set_export_name('b')
nbd> h.opt_go()
nbdkit: eval: error: /tmp/nbdkitNyQD8J/open: ENOENT
nbd> h.set_opt_mode(True)
Traceback (most recent call last):
  File "/usr/lib64/python3.8/code.py", line 90, in runcode
    exec(code, self.locals)
  File "<console>", line 1, in <module>
  File "/home/eblake/libnbd/python/nbd.py", line 570, in set_opt_mode
    return libnbdmod.set_opt_mode (self._o, enable)
nbd.Error: nbd_set_opt_mode: invalid state: NEGOTIATING: the handle must be newly created: Invalid argument (EINVAL)
nbd> quit()
---
 generator/API.ml                   | 12 ++++++++++--
 generator/states-newstyle-opt-go.c | 19 +++++++++++++------
 lib/opt.c                          | 18 ++++++++++++++++--
 3 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/generator/API.ml b/generator/API.ml
index 52970d3..bf50030 100644
--- a/generator/API.ml
+++ b/generator/API.ml
@@ -1876,7 +1876,9 @@ on the connection.";

   "aio_opt_go", {
     default_call with
-    args = []; ret = RErr;
+    args = [];
+    optargs = [ OClosure completion_closure ];
+    ret = RErr;
     permitted_states = [ Negotiating ];
     shortdesc = "end negotiation and move on to using an export";
     longdesc = "\
@@ -1885,7 +1887,13 @@ export previously specified by L<nbd_set_export_name(3)>.  This can only
 be used if L<nbd_set_opt_mode(3)> enabled option mode.

 To determine when the request completes, wait for
-L<nbd_aio_is_connecting(3)> to return false.";
+L<nbd_aio_is_connecting(3)> to return false.  Or supply the optional
+C<completion_callback> which will be invoked as described in
+L<libnbd(3)/Completion callbacks>, except that it is automatically
+retired regardless of return value.  Note that detecting whether the
+server returns an error (as is done by the return value of the
+synchronous counterpart) is only possible with a completion
+callback.";
     see_also = [Link "set_opt_mode"; Link "opt_go"];
   };

diff --git a/generator/states-newstyle-opt-go.c b/generator/states-newstyle-opt-go.c
index 0568695..95cc041 100644
--- a/generator/states-newstyle-opt-go.c
+++ b/generator/states-newstyle-opt-go.c
@@ -124,6 +124,7 @@ STATE_MACHINE {
   uint64_t exportsize;
   uint16_t eflags;
   uint32_t min, pref, max;
+  int err;

   reply = be32toh (h->sbuf.or.option_reply.reply);
   len = be32toh (h->sbuf.or.option_reply.replylen);
@@ -241,15 +242,21 @@ STATE_MACHINE {
                  reply);
     }
     nbd_internal_reset_size_and_flags (h);
-    if (h->opt_mode)
-      SET_NEXT_STATE (%.NEGOTIATING);
-    else
-      SET_NEXT_STATE (%^PREPARE_OPT_ABORT);
+    err = nbd_get_errno ();
     break;
   case NBD_REP_ACK:
+    err = 0;
+    break;
+  }
+
+  CALL_CALLBACK (h->opt_cb.completion, &err);
+  nbd_internal_free_option (h);
+  if (err == 0)
     SET_NEXT_STATE (%^FINISHED);
-    break;
-  }
+  else if (h->opt_mode)
+    SET_NEXT_STATE (%.NEGOTIATING);
+  else
+    SET_NEXT_STATE (%^PREPARE_OPT_ABORT);
   return 0;

 } /* END STATE MACHINE */
diff --git a/lib/opt.c b/lib/opt.c
index 1a5f645..17f2508 100644
--- a/lib/opt.c
+++ b/lib/opt.c
@@ -60,16 +60,28 @@ wait_for_option (struct nbd_handle *h)
   return 0;
 }

+static int
+go_complete (void *opaque, int *err)
+{
+  int *i = opaque;
+  *i = *err;
+  return 0;
+}
+
 /* Issue NBD_OPT_GO (or NBD_OPT_EXPORT_NAME) and wait for the reply. */
 int
 nbd_unlocked_opt_go (struct nbd_handle *h)
 {
-  int r = nbd_unlocked_aio_opt_go (h);
+  int err;
+  nbd_completion_callback c = { .callback = go_complete, .user_data = &err };
+  int r = nbd_unlocked_aio_opt_go (h, c);

   if (r == -1)
     return r;

   r = wait_for_option (h);
+  if (r == 0)
+    r = err;
   if (r == 0 && nbd_internal_is_state_negotiating (get_next_state (h)))
     return -1; /* NBD_OPT_GO failed, but can be attempted again */
   return r;
@@ -132,9 +144,11 @@ nbd_unlocked_opt_list (struct nbd_handle *h, nbd_list_callback list)

 /* Issue NBD_OPT_GO (or NBD_OPT_EXPORT_NAME) without waiting. */
 int
-nbd_unlocked_aio_opt_go (struct nbd_handle *h)
+nbd_unlocked_aio_opt_go (struct nbd_handle *h,
+                         nbd_completion_callback complete)
 {
   h->current_opt = NBD_OPT_GO;
+  h->opt_cb.completion = complete;

   if (nbd_internal_run (h, cmd_issue) == -1)
     debug (h, "option queued, ignoring state machine failure");
-- 
2.28.0