Valkey 9.1.0-rc2 release notes

Upgrade urgency LOW: This is the second release candidate of Valkey 9.1.0.

Behavior Changes

• Revert strict TLS certificate validation at config load as it is a breaking change, deferred to next major version (#3572)

New Features and enhanced behavior

• Do the failover immediately if the replica is the best ranked replica by @enjoy-binbin (#2227)
• Add cluster-config-save-behavior option to control nodes.conf save behavior by @enjoy-binbin (#3372)
• Lua scripting engine is now statically linked by default instead of dynamically linked by @eifrah-aws (#3392)
• Module command result callback addition by @martinrvisser (#2936)

Performance and Efficiency improvements

• Redesign IO threading communication model with lock-free queues (8-17% throughput gain) by @akashkgit (#3324)
• Increase embedded string threshold from 64 to 128 bytes (30% GET throughput gain) by @Nikhil-Manglore (#3397)
• ARM NEON SIMD optimization for pvFind() in vset.c (2-3x speedup) by @ahmadbelb (#3033)
• Optimize WATCH duplicate key check from O(N) to O(1) using per-db hashtable by @enjoy-binbin (#3360)
• Optimize CLUSTERSCAN MATCH so that it uses a specific slot if given by @nmvk (#3380)
• Improve COB memory tracking with copy avoidance by @dvkashapov (#3306)

Bug Fixes

• Fix valkey-cli --cluster del-node for unreachable nodes by @yang-z-o (#3209)
• Enhance cluster stale packet detection to prevent sub-replica and empty primary by @zhijun42 (#2811)
• Big endian bitmap byte order mismatch fix by @nmvk (#3401)
• Fix slot-migration-max-failover-repl-bytes unable to accept -1 by @enjoy-binbin (#3443)
• Fix config rewrite producing negative values for unsigned memory configs by @enjoy-binbin (#3440)
• Fix HPERSIST RESP protocol violation on wrong-type key by @madolson (#3516)
• Fix lua-enable-insecure-api default value cannot be changed to yes by @enjoy-binbin (#3548)

Full Changelog: 9.1.0-rc1...9.1.0-rc2