Current File : //lib/python3/dist-packages/landscape/lib/message.py
"""Helpers for reliable persistent message queues."""
RESYNC = object() # Used as a flag indicating a resync is needed
def got_next_expected(store, next_expected):
"""Our peer has told us what it expects our next message's sequence to be.
Call this with the message store and sequence number that the peer
wants next; this will do various things based on what *this* side
has in its outbound queue store.
0. The peer expects a sequence number from the server that is too high, the
difference greater than pending messages in the peer. We flush the older
messages in queue b/c the server does not want old or ancient messages,
however we also reset the offset so that the pending messages are
resent. Then we resynchronize by returning RESYNC. See LP: #1917540
1. The peer expects a sequence greater than what we last
sent. This is the common case and generally it should be
expecting last_sent_sequence+len(messages_sent)+1.
2. The peer expects a sequence number our side has already sent,
and we no longer have that message. In this case, just send
*all* messages we have, including the previous generation,
starting at the sequence number the peer expects (meaning that
messages have probably been lost).
3. The peer expects a sequence number we already sent, and we
still have that message cached. In this case, we send starting
from that message.
If the next expected sequence from the server refers to a message
older than we have, then L{RESYNC} will be returned.
"""
ret = None
old_sequence = store.get_sequence()
if (next_expected - old_sequence) > store.count_pending_messages():
store.delete_old_messages() # Flush queue from previous iteration
pending_offset = 0 # This means current messages will be resent
ret = RESYNC
elif next_expected > old_sequence:
store.delete_old_messages()
pending_offset = next_expected - old_sequence
elif next_expected < (old_sequence - store.get_pending_offset()):
# "Ancient": The other side wants messages we don't have,
# so let's just reset our counter to what it expects.
pending_offset = 0
ret = RESYNC
else:
# No messages transferred, or
# "Old": We'll try to send these old messages that the
# other side still wants.
pending_offset = (
store.get_pending_offset() + next_expected - old_sequence
)
store.set_pending_offset(pending_offset)
store.set_sequence(next_expected)
return ret
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