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4969aa81ae
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>