kv: don't serve non-locking, read-write requests on followers #60765
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Discovered while investigating a test failure in cockroachdb#59566. In 278a21b, we shifted from talking about read and write requests to locking and non-locking requests when deciding whether a request could be served on a follower. This prevented locking scans and gets from being served on followers. However, it began letting lone HeartbeatTxn and EndTxn requests past the old `!IsReadOnly()` check. Luckily, these were still prevented from being served on followers because they are only sent in read-write transactions, which were also prevented from performing follower reads. Yesterday, in 0ac8ab9, we lifted this second limitation, allowing read-write transactions to perform follower reads for non-locking batches. However, this no longer prevented HeartbeatTxn and EndTxn requests from being routed and served on follower replicas. This resulted in a pretty disastrous situation where in very rare cases, a follower was proposing a write under a lease that it did not own. Luckily, new assertions added in cockroachdb#59566 caught this. This commit fixes this oversight be re-introducing "read-only" as a condition for serving follower reads. Release note: None
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complete! 1 of 0 LGTMs obtained (waiting on @andreimatei)
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This was referenced Feb 23, 2021
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Discovered while investigating a test failure in #59566.
In 278a21b, we shifted from talking about read and write requests to
locking and non-locking requests when deciding whether a request could
be served on a follower. This prevented locking scans and gets from
being served on followers. However, it began letting lone HeartbeatTxn
and EndTxn requests past the old
!IsReadOnly()check. Luckily, thesewere still prevented from being served on followers because they are
only sent in read-write transactions, which were also prevented from
performing follower reads.
Yesterday, in 0ac8ab9, we lifted this second limitation, allowing
read-write transactions to perform follower reads for non-locking
batches. However, this no longer prevented HeartbeatTxn and EndTxn
requests from being routed and served on follower replicas. This
resulted in a pretty disastrous situation where in very rare cases, a
follower was proposing a write under a lease that it did not own.
Luckily, new assertions added in #59566 caught this.
This commit fixes this oversight be re-introducing "read-only" as a
condition for serving follower reads.
Release note: None