Remove v2 apply logic

[serathius]

v2 store is no longer available in v3.6.
We can remove apply logic for it as they will never be used.

Only V2 PUT is needed as it applies to v3 storage and etcd v3.5 uses it for setting member attributes and cluster version.

I'm proposing etcd v3.6 to panic when encountering v2 store change. This can only happened if user has run v3.5 with --enable-v2. As documented in --v2-deprecation=write-only (default in etcd v3.6) will not allow custom v2 state.

etcd/server/etcdmain/help.go

Line 169 in fd0882b

'write-only' // Custom v2 state is not allowed (planned default in v3.6)

If etcd still has some v2 data, users are expected to either:

• Halt the upgrade and reconsider what to do with v2 state.
• Run etcd with --v2-deprecation=write-only-drop-data which is expected to delete the data. Implementation pending.

Note for reviewers:

• We have dedicated tests for v2 cluster membership change and v2 cluster version setting. Add tests for setting cluster version using v2 request #16994
• We have dedicated tests for running hybrid clusters (mixed v3.6 and v3.5)

  etcd/tests/common/e2e_test.go

  Lines 55 to 73 in 28d9564

  	if fileutil.Exist(e2e.BinPath.EtcdLastRelease) {
  	tcs = append(tcs, testCase{
  	name: "MinorityLastVersion",
  	config: config.ClusterConfig{
  	ClusterSize: 3,
  	ClusterContext: &e2e.ClusterContext{
  	Version: e2e.MinorityLastVersion,
  	},
  	},
  	}, testCase{
  	name: "QuorumLastVersion",
  	config: config.ClusterConfig{
  	ClusterSize: 3,
  	ClusterContext: &e2e.ClusterContext{
  	Version: e2e.QuorumLastVersion,
  	},
  	},
  	})
  	}

cc @ahrtr @fuweid @chaochn47 @siyuanfoundation

[fuweid]

LGTM

[fuweid]

FAIL: (code:1):
  % (cd server && 'golangci-lint' 'run' '--config' '/home/runner/work/etcd/etcd/tools/.golangci.yaml' './...')
Error: etcdserver/apply_v2.go:119:6: func `toResponse` is unused (unused)
func toResponse(ev *v2store.Event, err error) Response {
FAIL: 'run golangci-lint run --config /home/runner/work/etcd/etcd/tools/.golangci.yaml ./...' checking failed (!=0 return code)

Need to remove the unused function toResponse.

[ahrtr]

Could you breakdown this PR into 2+ PRs?

• This PR only focus on removing v2 apply logic (excluding Only V2 PUT, because it's still needed when in mix-version cluster). It should can be merged soon once the workflow is green.
• Keep all other changes in separate PRs, which need careful review.

[serathius]

Could you breakdown this PR into 2+ PRs?

• This PR only focus on removing v2 apply logic (excluding Only V2 PUT, because it's still needed when in mix-version cluster). It should can be merged soon once the workflow is green.
• Keep all other changes in separate PRs, which need careful review.

Done

[serathius]

All the subsequent PRs were merged. ping @ahrtr for review.

For simplicity I have opted to not delete the code, just to an early check and panic. This allows to make logic change very clear and visible.

We can clean up the code slowly in subsequent PR.

[upodroid]

/retest

[serathius]

/retest

[serathius]

/retest

[ahrtr]

Note: this case won't work anymore if we bootstrap etcd based on v3store. But that isn't finalized yet, and may not be done in 3.6. But please let's add a comment/note.

[serathius]

We might need to update the test, however the test is still relevant.