python: backport 'feat: allow setting read buffer limit'

Expose the limit parameter of the underlying StreamReader and StreamWriter
instances.

This is helpful for the use case of transferring files in and out of a VM
via the QEMU guest agent's guest-file-open, guest-file-read, guest-file-write,
and guest-file-close methods, as it allows pushing the buffer size up to the
guest agent's limit of 48MB per transfer.

Signed-off-by: Adam Dorsey <adam@dorseys.email>
cherry picked from commit python-qemu-qmp@9ba6a698344eb3b570fa4864e906c54042824cd6
cherry picked from commit python-qemu-qmp@e4d0d3f835d82283ee0e48438d1b154e18303491
[Squashed in linter fixups. --js]
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

python/qemu/qmp/protocol.py

@@ -53,6 +53,9 @@ InternetAddrT = Tuple[str, int]
 UnixAddrT = str
 SocketAddrT = Union[UnixAddrT, InternetAddrT]
 
+# Maximum allowable size of read buffer, default
+_DEFAULT_READBUFLEN = 64 * 1024
+
 
 class Runstate(Enum):
     """Protocol session runstate."""
@@ -202,22 +205,26 @@ class AsyncProtocol(Generic[T]):
         will log to 'qemu.qmp.protocol', but each individual connection
         can be given its own logger by giving it a name; messages will
         then log to 'qemu.qmp.protocol.${name}'.
+    :param readbuflen:
+        The maximum read buffer length of the underlying StreamReader
+        instance.
     """
     # pylint: disable=too-many-instance-attributes
 
     #: Logger object for debugging messages from this connection.
     logger = logging.getLogger(__name__)
 
-    # Maximum allowable size of read buffer
-    _limit = 64 * 1024
-
     # -------------------------
     # Section: Public interface
     # -------------------------
 
-    def __init__(self, name: Optional[str] = None) -> None:
+    def __init__(
+        self, name: Optional[str] = None,
+        readbuflen: int = _DEFAULT_READBUFLEN
+    ) -> None:
         self._name: Optional[str]
         self.name = name
+        self.readbuflen = readbuflen
 
         # stream I/O
         self._reader: Optional[StreamReader] = None
@@ -574,7 +581,7 @@ class AsyncProtocol(Generic[T]):
                 port=address[1],
                 ssl=ssl,
                 backlog=1,
-                limit=self._limit,
+                limit=self.readbuflen,
             )
         else:
             coro = asyncio.start_unix_server(
@@ -582,7 +589,7 @@ class AsyncProtocol(Generic[T]):
                 path=address,
                 ssl=ssl,
                 backlog=1,
-                limit=self._limit,
+                limit=self.readbuflen,
             )
 
         # Allow runstate watchers to witness 'CONNECTING' state; some
@@ -637,7 +644,7 @@ class AsyncProtocol(Generic[T]):
                               "fd=%d, family=%r, type=%r",
                               address.fileno(), address.family, address.type)
             connect = asyncio.open_connection(
-                limit=self._limit,
+                limit=self.readbuflen,
                 ssl=ssl,
                 sock=address,
             )
@@ -647,14 +654,14 @@ class AsyncProtocol(Generic[T]):
                 address[0],
                 address[1],
                 ssl=ssl,
-                limit=self._limit,
+                limit=self.readbuflen,
             )
         else:
             self.logger.debug("Connecting to file://%s ...", address)
             connect = asyncio.open_unix_connection(
                 path=address,
                 ssl=ssl,
-                limit=self._limit,
+                limit=self.readbuflen,
             )
 
         self._reader, self._writer = await connect

python/qemu/qmp/qmp_client.py

@@ -170,6 +170,12 @@ class QMPClient(AsyncProtocol[Message], Events):
 
     :param name: Optional nickname for the connection, used for logging.
 
+    :param readbuflen:
+        The maximum buffer length for reads and writes to and from the QMP
+        server, in bytes. Default is 10MB. If `QMPClient` is used to
+        connect to a guest agent to transfer files via ``guest-file-read``/
+        ``guest-file-write``, increasing this value may be required.
+
     Basic script-style usage looks like this::
 
       qmp = QMPClient('my_virtual_machine_name')
@@ -203,14 +209,18 @@ class QMPClient(AsyncProtocol[Message], Events):
     #: Logger object used for debugging messages.
     logger = logging.getLogger(__name__)
 
-    # Read buffer limit; 10MB like libvirt default
-    _limit = 10 * 1024 * 1024
+    # Read buffer default limit; 10MB like libvirt default
+    _readbuflen = 10 * 1024 * 1024
 
     # Type alias for pending execute() result items
     _PendingT = Union[Message, ExecInterruptedError]
 
-    def __init__(self, name: Optional[str] = None) -> None:
-        super().__init__(name)
+    def __init__(
+        self,
+        name: Optional[str] = None,
+        readbuflen: int = _readbuflen
+    ) -> None:
+        super().__init__(name, readbuflen)
         Events.__init__(self)
 
         #: Whether or not to await a greeting after establishing a connection.