When Inflights to a particular node is full, i.e. MaxInflightMsgs for the
append messages flow is saturated, it is still necessary to continue sending
MsgApp to ensure progress. Currently this is achieved by "forgetting" the first
in-flight message in the window, which frees up quota for one new MsgApp.
This new message is constructed in such a way that it potentially has multiple
entries, or a large entry. The effect of this is that the in-flight limitations
can be exceeded arbitrarily, for as long as the flow to this node continues
being saturated. In particular, if a follower is stuck, the leader will keep
sending entries to it.
This commit makes the MsgApp empty when Inflights is saturated, and prevents
the described leakage of Entries to slow followers.
Signed-off-by: Pavel Kalinnikov <pavel@cockroachlabs.com>
This commit adds a data-driven test which simulates conditions under which Raft
messages flow to a particular node is throttled while in StateReplicate. The
test demonstrates that MsgApp messages with non-empty Entries may "leak" to a
paused stream every time there is successful heartbeat exchange.
Signed-off-by: Pavel Kalinnikov <pavel@cockroachlabs.com>
