UPSTREAM: 21443: Revert "Reuse events between sync loops"

[redhat-chai-bot]

Cherry-pick: etcd memory regression fix for 4.21

Cherry-pick of upstream etcd-io/etcd#21443 — Revert "Reuse events between sync loops"

Problem

etcd 3.6 introduced a memory regression that scales with cluster size (OCPBUGS-76329). The rangeEventsWithReuse optimization held event objects across sync loops, causing unbounded memory growth:

Cluster Size	etcd 3.5 Max RSS	etcd 3.6 Max RSS	Max Change
24 nodes	468 MiB	541 MiB	+15.6%
120 nodes	893 MiB	1.24 GiB	+42.2%
250 nodes	1.13 GiB	2.96 GiB	+162%

Fix

This cherry-picks the upstream revert which:

• Removes the rangeEventsWithReuse function
• Removes the shared evs variable from syncWatchersLoop
• Restores the original rangeEvents and kvsToEvents signatures with contains filtering

Why cherry-pick instead of full rebase

OCP 4.21 etcd is on 3.6.5. A full rebase to 3.6.9+ is complicated by Go version constraints (see openshift/etcd#377 which is held). This targeted cherry-pick addresses the critical memory regression without introducing other changes.

Build and test verification

• go build ./... ✅
• go test ./server/storage/mvcc/... ✅ (11.7s)

References

• Upstream issue: Increased memory usage with 3.6 event reuse etcd-io/etcd#21355
• Upstream fix: [release-3.6] Revert "Reuse events between sync loops" etcd-io/etcd#21443
• Jira: https://redhat.atlassian.net/browse/OCPBUGS-76329
• 4.22 rebase (includes this fix): OCPBUGS-82497: Rebase v3.6.13 for openshift-4.22 #393

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@sdodson requested in Slack thread

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ci/prow/upstream-integration	0ff063b	link	false	/test upstream-integration
ci/prow/upstream-e2e	0ff063b	link	false	/test upstream-e2e

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