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Re: [PATCH 08/10] tools/xenstored: Extend restore code to handle multiple input buffer

On 24.06.21 10:42, Julien Grall wrote:
Hi Juergen,

On 24/06/2021 10:30, Juergen Gross wrote:
On 16.06.21 16:43, Julien Grall wrote:
From: Julien Grall <jgrall@xxxxxxxxxx>

Currently, the restore code is considering the stream will contain at
most one in-flight request per connection. In a follow-up changes, we
will want to transfer multiple in-flight requests.

The function read_state_buffered() is now extended to restore multiple
in-flight request. Complete requests will be queued as delayed
requests, if there a partial request (only the last one can) then it
will used as the current in-flight request.

Note that we want to bypass the quota check for delayed requests as
the new Xenstore may have a lower limit.

Lastly, there is no need to change the specification as there was
no restriction on the number of in-flight requests preserved.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
  tools/xenstore/xenstored_core.c | 56 ++++++++++++++++++++++++++++-----
  1 file changed, 48 insertions(+), 8 deletions(-)

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index a5084a5b173d..5b7ab7f74013 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -1486,6 +1486,10 @@ static void process_message(struct connection *conn, struct buffered_data *in)
      enum xsd_sockmsg_type type = in->hdr.msg.type;
      int ret;
+    /* At least send_error() and send_reply() expects
conn->in == in */
+    assert(conn->in == in);
+    trace_io(conn, in, 0);
      if ((unsigned int)type >= XS_TYPE_COUNT|| !wire_funcs[type].func)
          eprintf("Client unknown operation %i", type);
          send_error(conn, ENOSYS);
@@ -1515,6 +1519,23 @@ static void process_message(struct connection *conn, struct buffered_data *in)
      conn->transaction = NULL;
+static bool process_delayed_message(struct delayed_request *req)
+    struct connection *conn = req->data;
+    struct buffered_data *saved_in = conn->in;
+    /*
+     * Part of process_message() expects conn->in to contains the
+     * processed response. So save the current conn->in and restore it
+     * afterwards.
+     */
+    conn->in = req->in;
+    process_message(req->data, req->in);
+    conn->in = saved_in;

This pattern was added to do_lu_start() already, and it will be needed
for each other function being called via call_delayed().

Maybe it would be better to add conn explicitly to struct
delayed_request (or even better: replace data with conn) and to do the
conn->in saving/setting/restoring in call_delayed() instead?

This was my original approach, but I abandoned it because in the case of do_lu_start() we need to save the original conn->in in the stream (see patch #3 for more details).

If we overwrite conn->in in call_delayed(), then we need a different way to find the original conn->in. I couldn't find a nice way and decided to settle with the duplication.

Ah, okay, understood. Even in case we'd be able to restore conn->in it
would be the same amount of code again, but harder to follow.


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