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Re: [Xen-devel] Xen 4.4, xenstored and WATCH requests

Hi Wei,

Yes, the Unix socket interface works okay. Here’s a relevant part of the script changed to use the socket:

    import socket
    sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    fd = os.dup(sock.fileno())
        send_packet(fd, WRITE, 24, 0, b"/foo\x00bar\x00")
        print_reply(fd)  # Responds with rq_id = 24.
        send_packet(fd, WATCH, 42, 0, b"/foo\x00token\x00")
        print_reply(fd)  # ACK for WATCH with rq_id = 42.

BTW can you paste in the result of

 $ ps aux | grep xenstored


$ ps aux | grep xenstored
root       807  0.0  0.2  10932  2132 ?        S    00:34   0:01 /usr/lib/xen-4.4/bin/xenstored --p


On 26 Feb 2016, at 14:26, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:


On Thu, Feb 25, 2016 at 03:54:14PM +0300, Sergei Lebedev wrote:
Hello list,

I’m working on a Python client library for XenStore [1]. The library
implements two ways to access XenStore: via Unix socket and via /dev.
The /dev interface turned out to be a bit problematic, because it
ignores the req_id field for WATCH requests.

So the unix socket interface worked fine for you?

The spec [2] requires all responses (except WATCH_EVENT) to copy
req_id from request. So I wonder if it’s possible that the behaviour I
observe is in fact a bug in xenstored?

Below is a Python script demonstrating the issue:

   import os
   import struct

   WATCH = 4
   WRITE = 11

   def send_packet(fd, op, rq_id, tx_id, payload):
       data = "" op, rq_id, tx_id, len(payload)) + payload
       os.write(fd, data)

   def print_reply(fd):
       op, rq_id, tx_id, size = struct.unpack(b"IIII", os.read(fd, 16))
       payload = os.read(fd, size) if size else b""
       print(op, rq_id, tx_id, payload)

   fd = os.open("/dev/xen/xenbus", os.O_RDWR)
       send_packet(fd, WRITE, 24, 0, b"/foo\x00bar\x00")
       print_reply(fd)  # ACK for WRITE with rq_id = 24.
       send_packet(fd, WATCH, 42, 0, b"/foo\x00token\x00")
       print_reply(fd)  # Spurious (?) WATCH_EVENT.
       print_reply(fd)  # ACK for WATCH with rq_id = 0.

Example output:

   (11, 24, 0, 'OK\x00')
   (15, 4294967295, 892960384, '/foo\x00token\x00')
   (4, 0, 0, 'OK\x00')

I’m running Xen 4.4 on Ubuntu 14.04 inside VirtualBox.

$ uname -a
Linux xen-devel 3.19.0-25-generic #26~14.04.1-Ubuntu SMP Fri Jul 24 21:16:20 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

FWIW I tried you program in my own testbox running xen-unstable with
Debian's stock 3.16 kernel, the result is the same.

And in xen.git

 $ git diff origin/stable-4.4..origin/staging -- docs/misc/xenstore.txt

shows nothing.

So this is not a problem in 4.4 only.



[1]: http://github.com/selectel/pyxs
[2]: http://xenbits.xen.org/docs/4.4-testing/misc/xenstore.txt

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