API reference¶
Client and Monitor¶
-
class
pyxs.client.
Client
(unix_socket_path=None, xen_bus_path=None, router=None)[source]¶ XenStore client.
Parameters: If
unix_socket_path
is given orClient
was created with no arguments, XenStore is accessed viaUnixSocketConnection
; otherwise,XenBusConnection
is used.Each client has a
Router
thread running in the background. The goal of the router is to multiplex requests from different transaction through a single XenStore connection.Changed in version 0.4.0: The constructor no longer accepts
connection
argument. If you wan’t to force the use of a specific connection class, wrap it in aRouter
:from pyxs import Router, Client from pyxs.connection import XenBusConnection router = Router(XenBusConnection()) with Client(router=router) as c: do_something(c)
Warning
Always finalize the client either explicitly by calling
close()
or implicitly via a context manager to prevent data loss.See also
Xenstore protocol specification for a description of the protocol, implemented by
Client
.-
connect
()[source]¶ Connects to the XenStore daemon.
Raises: pyxs.exceptions.ConnectionError – if the connection could not be opened. This could happen either because XenStore is not running on the machine or due to the lack of permissions. Warning
This method is unsafe. Please use client as a context manager to ensure it is properly finalized.
-
close
()[source]¶ Finalizes the client.
Warning
This method is unsafe. Please use client as a context manager to ensure it is properly finalized.
-
read
(path, default=None)[source]¶ Reads data from a given path.
Parameters: - path (bytes) – a path to read from.
- default (bytes) – default value, to be used if path doesn’t exist.
-
write
(path, value)[source]¶ Writes data to a given path.
Parameters: - value (bytes) – data to write.
- path (bytes) – a path to write to.
-
mkdir
(path)[source]¶ Ensures that a given path exists, by creating it and any missing parents with empty values. If path or any parent already exist, its value is left unchanged.
Parameters: path (bytes) – path to directory to create.
-
delete
(path)[source]¶ Ensures that a given does not exist, by deleting it and all of its children. It is not an error if path doesn’t exist, but it is an error if path‘s immediate parent does not exist either.
Parameters: path (bytes) – path to directory to remove.
-
list
(path)[source]¶ Returns a list of names of the immediate children of path.
Parameters: path (bytes) – path to list.
-
get_perms
(path)[source]¶ Returns a list of permissions for a given path, see
InvalidPermission
for details on permission format.Parameters: path (bytes) – path to get permissions for.
-
set_perms
(path, perms)[source]¶ Sets a access permissions for a given path, see
InvalidPermission
for details on permission format.Parameters: - path (bytes) – path to set permissions for.
- perms (list) – a list of permissions to set.
-
walk
(top, topdown=True)[source]¶ Walk XenStore, yielding 3-tuples
(path, value, children)
for each node in the tree, rooted at node top.Parameters:
-
get_domain_path
(domid)[source]¶ Returns the domain’s base path, as used for relative requests: e.g.
b"/local/domain/<domid>"
. If a given domid doesn’t exists the answer is undefined.Parameters: domid (int) – domain to get base path for.
-
is_domain_introduced
(domid)[source]¶ Returns
True
ifxenstored
is in communication with the domain; that is when INTRODUCE for the domain has not yet been followed by domain destruction or explicit RELEASE; andFalse
otherwise.Parameters: domid (int) – domain to check status for.
-
introduce_domain
(domid, mfn, eventchn)[source]¶ Tells
xenstored
to communicate with this domain.Parameters:
-
release_domain
(domid)[source]¶ Manually requests
xenstored
to disconnect from the domain.Parameters: domid (int) – domain to disconnect. Note
xenstored
will in any case detect domain destruction and disconnect by itself.
-
resume_domain
(domid)[source]¶ Tells
xenstored
to clear its shutdown flag for a domain. This ensures that a subsequent shutdown will fire the appropriate watches.Parameters: domid (int) – domain to resume.
-
set_target
(domid, target)[source]¶ Tells
xenstored
that a domain is targetting another one, so it should let it tinker with it. This grants domain domid full access to paths owned by target. Domain domid also inherits all permissions granted to target on all other paths.Parameters:
-
transaction
()[source]¶ Starts a new transaction.
Returns int: transaction handle. Raises: pyxs.exceptions.PyXSError – with errno.EALREADY
if this client is already in a transaction.Warning
Currently
xenstored
has a bug that after 2**32 transactions it will allocate id 0 for an actual transaction.
-
commit
()[source]¶ Commits a transaction currently in progress.
Returns bool: False
if commit failed because of the intervening writes andTrue
otherwise. In any case transaction is invalidated. The caller is responsible for starting a new transaction, repeating all of the operations a re-committing.
-
monitor
()[source]¶ Returns a new
Monitor
instance, which is currently the only way of doing PUBSUB.The monitor shares the router with its parent client. Thus closing the client invalidates the monitor. Closing the monitor, on the other hand, had no effect on the router state.
Note
Using
monitor()
overXenBusConnection
is currently unsupported, because XenBus does not obey XenStore protocol specification. See xen-devel discussion for details.
-
-
class
pyxs.client.
Monitor
(client)[source]¶ Monitor implements minimal PUBSUB functionality on top of XenStore.
>>> with Client() as c: ... m = c.monitor(): ... m.watch("foo/bar") ... print(next(c.wait())) Event(...)
Parameters: client (Client) – a reference to the parent client. Note
When used as a context manager the monitor will try to unwatch all watched paths.
-
watched
¶ A set of paths currently watched by the monitor.
-
watch
(wpath, token)[source]¶ Adds a watch.
Any alteration to the watched path generates an event. This includes path creation, removal, contents change or permission change. An event can also be triggered spuriously.
Changes made in transactions cause an event only if and when committed.
Parameters: - wpath (bytes) – path to watch.
- token (bytes) – watch token, returned in watch notification.
-
unwatch
(wpath, token)[source]¶ Removes a previously added watch.
Parameters: - wpath (bytes) – path to unwatch.
- token (bytes) – watch token, passed to
watch()
.
-
wait
(unwatched=False)[source]¶ Yields events for all of the watched paths.
An event is a
(path, token)
pair, where the first element is event path, i.e. the actual path that was modified, and the second – a token, passed towatch()
.Parameters: unwatched (bool) – if True
wait()
might yield spurious unwatched packets, otherwise these are dropped. Defaults toFalse
.
-
Exceptions¶
-
class
pyxs.exceptions.
InvalidOperation
[source]¶ Exception raised when
Packet
is passed an operation, which isn’t listed inOp
.Parameters: operation (int) – invalid operation value.
-
class
pyxs.exceptions.
InvalidPayload
[source]¶ Exception raised when
Packet
is initialized with payload, which exceeds 4096 bytes restriction or contains a trailingNULL
.Parameters: operation (bytes) – invalid payload value.
-
class
pyxs.exceptions.
InvalidPath
[source]¶ Exception raised when a path proccessed by a comand doesn’t match the following constraints:
- its length should not exceed 3072 or 2048 for absolute and relative path respectively.
- it should only consist of ASCII alphanumerics and the four
punctuation characters
-/_@
– hyphen, slash, underscore and atsign. - it shouldn’t have a trailing
/
, except for the root path.
Parameters: path (bytes) – invalid path value.
-
class
pyxs.exceptions.
InvalidPermission
[source]¶ Exception raised for permission which don’t match the following format:
w<domid> write only r<domid> read only b<domid> both read and write n<domid> no access
Parameters: perm (bytes) – invalid permission value.
Internals¶
-
class
pyxs.client.
Router
(connection)[source]¶ Router.
The goal of the router is to multiplex XenStore connection between multiple clients and monitors.
Parameters: FileDescriptorConnection (connection) – owned by the router. The connection is open when the router is started and remains open until the router is terminated. Note
Python lacks API for interrupting a thread from another thread. This means that when a router cannot be stopped when it is blocked in
select.select()
orwait()
.The following two “hacks” are used to ensure prompt termination.
- A router is equipped with a
socket.socketpair()
. The reader-end of the pair is selected in the mainloop alongside the XenStore connection, while the writer-end is used interminate()
to force-stop the mainloop. - All operations with
threading.Condition
variables user a 1 second timeout. This “hack” is only relevant for Python prior to 3.2 which didn’t allow to interrupt lock acquisitions. See issue8844 on CPython issue tracker for details. On Python 3.2 and later no timeout is used.
-
is_connected
¶ Checks if the underlying connection is active.
- A router is equipped with a
-
class
pyxs.connection.
XenBusConnection
(path=None)[source]¶ XenStore connection through XenBus.
Parameters: path (str) – path to XenBus. A predefined OS-specific constant is used, if a value isn’t provided explicitly.
-
class
pyxs.connection.
UnixSocketConnection
(path=None)[source]¶ XenStore connection through Unix domain socket.
Parameters: path (str) – path to XenStore unix domain socket, if not provided explicitly is restored from process environment – similar to what libxs
does.
-
class
pyxs._internal.
Packet
[source]¶ A message to or from XenStore.
Parameters: - op (int) – an item from
Op
, representing operation, performed by this packet. - payload (bytes) – packet payload, should be a valid ASCII-string
with characters between
[0x20; 0x7f]
. - rq_id (int) – request id – hopefuly a unique identifier for this packet, XenStore simply echoes this value back in reponse.
- tx_id (int) – transaction id, defaults to
0
, which means no transaction is running.
Changed in version 0.4.0:
rq_id
no longer defaults to0
and should be provided explicitly.- op (int) – an item from
-
pyxs._internal.
Op
= Operations(DEBUG=0, DIRECTORY=1, READ=2, GET_PERMS=3, WATCH=4, UNWATCH=5, TRANSACTION_START=6, TRANSACTION_END=7, INTRODUCE=8, RELEASE=9, GET_DOMAIN_PATH=10, WRITE=11, MKDIR=12, RM=13, SET_PERMS=14, WATCH_EVENT=15, ERROR=16, IS_DOMAIN_INTRODUCED=17, RESUME=18, SET_TARGET=19, RESTRICT=128)¶ Operations supported by XenStore.