[PATCH 3/4] server: Allow calling async_handoff() with status code STATUS_ALERTED.

[Jinoh Kang]

If the server detects that an I/O request could be completed immediately
(e.g. reading from socket that already has incoming data), it can now
return STATUS_ALERTED to allow opportunistic synchronous I/O.
The client will then attempt to perform I/O in nonblocking mode and
report back the I/O status to the server with signal_wait_async().

If the operation returns e.g. EAGAIN or EWOULDBLOCK, the client can opt
to either abandon the request (by specifying an error status) or poll
for it in the server as usual (by specifying STATUS_PENDING).

Without this mechanism the client cannot safely perform immediately
satiable I/O operations synchronously, since it can potentially
conflict with other pending I/O operations that have already been
queued.

Signed-off-by: Jinoh Kang <jinoh.kang.kr at gmail.com>
---
 server/async.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/server/async.c b/server/async.c
index 339d050b086..e74a141c459 100644
--- a/server/async.c
+++ b/server/async.c
@@ -128,7 +128,8 @@ static void async_satisfied( struct object *obj, struct wait_queue_entry *entry
     if (async->direct_result)
         async_set_result( &async->obj, async->iosb->status, async->iosb->result );
 
-    if (async->initial_status == STATUS_PENDING && async->blocking)
+    if ((async->initial_status == STATUS_PENDING && async->blocking) ||
+        async->initial_status == STATUS_ALERTED)
         set_wait_status( entry, async->iosb->status );
     else
         set_wait_status( entry, async->initial_status );
@@ -471,6 +472,17 @@ void async_set_result( struct object *obj, unsigned int status, apc_param_t tota
     {
         async->terminated = 0;
         async->alerted = 0;
+
+        /* If the client attempted to complete synchronously and failed,
+         * then it would have called signal_wait_async() to restart the
+         * operation in the full asynchronous mode.  In this case, we set
+         * the pending flag so that the completion port notification and
+         * APC call will be triggered appropriately.  Also, the async
+         * object is currently in signaled state; unset the signaled flag
+         * if the client wants to block on this async. */
+        async->pending = 1;
+        if (async->blocking) async->signaled = 0;
+
         async_reselect( async );
     }
     else
-- 
2.31.1