xenbus: Fix handling of requests with xsd_sockmsg.len = 0 for socket connections

Following the report at https://lore.kernel.org/xen-devel/CAFLBxZaeTMcF4tcV45MJdCVx4A6qbzQdjKei_Quh_iLrtARVFA@mail.gmail.com/

Without this fix, the backend would be stuck waiting on the zero-sized
body until the next request comes in.

Instead return the request immediately after reading the header, there's
no need to wait for another call to .has_more_input and .input

Reported-by: George Dunlap <dunlapg@umich.edu>
Signed-off-by: Andrii Sultanov <andriy.sultanov@vates.tech>

Author: last-genius

xenbus/xb.ml

@@ -295,6 +295,11 @@ let can_input con = Queue.can_push con.pkt_out CommandReply
 
 (* NB: can throw Reconnect *)
 let input con =
+  let reset_and_return partial_pkt =
+    let pkt = Packet.of_partialpkt partial_pkt in
+    con.partial_in <- init_partial_in () ;
+    Some pkt
+  in
   if not (can_input con) then
     None
   else
@@ -309,23 +314,27 @@ let input con =
         (* we complete the data *)
         if sz > 0 then
           Partial.append partial_pkt (Bytes.to_string b) sz ;
-        if Partial.to_complete partial_pkt = 0 then (
-          let pkt = Packet.of_partialpkt partial_pkt in
-          con.partial_in <- init_partial_in () ;
-          Some pkt
-        ) else
+        if Partial.to_complete partial_pkt = 0 then
+          reset_and_return partial_pkt
+        else
           None
     | NoHdr (i, buf) ->
         (* we complete the partial header *)
         if sz > 0 then
           Bytes.blit b 0 buf (Partial.header_size () - i) sz ;
-        con.partial_in <-
-          ( if sz = i then
-              HaveHdr (Partial.of_string (Bytes.to_string buf))
-            else
-              NoHdr (i - sz, buf)
-          ) ;
-        None
+        if sz = i then
+          let partial_pkt = Partial.of_string (Bytes.to_string buf) in
+          (* If there is no body, we can return the full request immediately *)
+          if Partial.to_complete partial_pkt = 0 then
+            reset_and_return partial_pkt
+          else (
+            con.partial_in <- HaveHdr partial_pkt ;
+            None
+          )
+        else (
+          con.partial_in <- NoHdr (i - sz, buf) ;
+          None
+        )
 
 let classify t =
   match t.Packet.ty with Op.Watchevent -> Watchevent | _ -> CommandReply