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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

Hello

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 = struct.pack(b"IIII", 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)
>     try:
>         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.
>     finally:
>         os.close(fd)
> 

> 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.

Wei.

> Regards,
> Sergei
> 
> [1]: http://github.com/selectel/pyxs
> [2]: http://xenbits.xen.org/docs/4.4-testing/misc/xenstore.txt
> 
> 
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> http://lists.xen.org/xen-devel

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