etcdserver: special apply for readonly transactions

[lavacat]

#14110

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[ptabor]

Please work on the main branch first. It should be also simpler, as the apply code-path is already separated from the serving:

I think that we can change both:

• etcd/server/etcdserver/txn/txn.go

  Line 220 in 71bba3c

  func Txn(ctx context.Context, lg *zap.Logger, rt *pb.TxnRequest, txnModeWriteWithSharedBuffer bool, kv mvcc.KV, lessor lease.Lessor) (*pb.TxnResponse, *traceutil.Trace, error) {

• etcd/server/etcdserver/txn/txn.go

  Line 268 in 71bba3c

  applyTxn(ctx, lg, kv, lessor, txnWrite, rt, txnPath, txnResp)

• etcd/server/etcdserver/txn/txn.go

  Line 314 in 71bba3c

  func applyTxn(ctx context.Context, lg *zap.Logger, kv mvcc.KV, lessor lease.Lessor, txnWrite mvcc.TxnWrite, rt *pb.TxnRequest, txnPath []bool, tresp *pb.TxnResponse) (txns int) {

I would consider passing there additional argument func criticalErrorHandler(err Error) Error that has 2 implementations:

• panic() based provided for the apply call:
  

  etcd/server/etcdserver/apply/apply.go

  Line 172 in 71bba3c

  return mvcctxn.Txn(ctx, a.lg, rt, a.txnModeWriteWithSharedBuffer, a.kv, a.lessor)

• return err based for the v3server codepath:
  

  etcd/server/etcdserver/v3_server.go

  Line 185 in 71bba3c

  resp, _, err = txn.Txn(ctx, s.Logger(), r, s.Cfg.ExperimentalTxnModeWriteWithSharedBuffer, s.KV(), s.lessor)

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[ahrtr]

THe issue has already been resolved in #14149