[RFC PATCH v3 1/5] 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. the socket to read from already has incoming data), it can now
return STATUS_ALERTED to allow opportunistic synchronous I/O.
The Unix side 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 waiting on the wait handle).

Without such mechanism in place, 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>
---

Notes:
    v2 -> v3: set async->alerted = 1

 server/async.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/server/async.c b/server/async.c
index 7aef28355f0..e169bb23225 100644
--- a/server/async.c
+++ b/server/async.c
@@ -338,6 +338,30 @@ obj_handle_t async_handoff( struct async *async, data_size_t *result, int force_
         return async->wait_handle;
     }
 
+    if (!async->pending && get_error() == STATUS_ALERTED)
+    {
+        /* Give the client opportunity to complete synchronously.
+         * If it turns out that the I/O request is not actually immediately satiable,
+         * the client may then choose to re-queue the async (with STATUS_PENDING).
+         */
+        async->unknown_status = 1;
+
+        /* Deactivate async so that it can be safely queued.
+         * Otherwise, the async will be erroneously alerted the next time
+         * someone calls async_wake_up() on the queue.
+         *
+         * Don't use async_terminate() here since we'd like to leave the IOSB
+         * status as STATUS_PENDING.  Also we certainly don't want APC_ASYNC_IO
+         * to fire in any circumstances.
+         */
+        async->terminated = 1;
+        async->alerted = 1;
+
+        async_reselect( async );
+
+        return async->wait_handle;
+    }
+
     async->initial_status = get_error();
 
     if (!async->pending && NT_ERROR( get_error() ))
-- 
2.34.1