From 81ecac11cb0620ba5db6e3fcb3d3877fadbda54d Mon Sep 17 00:00:00 2001
From: Marek Siarkowicz <siarkowicz@google.com>
Date: Fri, 4 Aug 2023 15:58:52 +0200
Subject: [PATCH] server: Separate txnRead from txnWrite

Signed-off-by: Marek Siarkowicz <siarkowicz@google.com>
---
 server/etcdserver/txn/txn.go | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/server/etcdserver/txn/txn.go b/server/etcdserver/txn/txn.go
index 6070389fd..d694f2ea3 100644
--- a/server/etcdserver/txn/txn.go
+++ b/server/etcdserver/txn/txn.go
@@ -242,31 +242,35 @@ func Txn(ctx context.Context, lg *zap.Logger, rt *pb.TxnRequest, txnModeWriteWit
 	isWrite := !IsTxnReadonly(rt)
 	// When the transaction contains write operations, we use ReadTx instead of
 	// ConcurrentReadTx to avoid extra overhead of copying buffer.
-	var txnWrite mvcc.TxnWrite
+	var mode mvcc.ReadTxMode
 	if isWrite && txnModeWriteWithSharedBuffer /*a.s.Cfg.ExperimentalTxnModeWriteWithSharedBuffer*/ {
-		txnWrite = mvcc.NewReadOnlyTxnWrite(kv.Read(mvcc.SharedBufReadTxMode, trace))
+		mode = mvcc.SharedBufReadTxMode
 	} else {
-		txnWrite = mvcc.NewReadOnlyTxnWrite(kv.Read(mvcc.ConcurrentReadTxMode, trace))
+		mode = mvcc.ConcurrentReadTxMode
 	}
+	txnRead := kv.Read(mode, trace)
 	var txnPath []bool
 	trace.StepWithFunction(
 		func() {
-			txnPath = compareToPath(txnWrite, rt)
+			txnPath = compareToPath(txnRead, rt)
 		},
 		"compare",
 	)
-	err := checkTxn(ctx, txnWrite, rt, isWrite, lessor, txnPath)
+	err := checkTxn(ctx, txnRead, rt, isWrite, lessor, txnPath)
 	if err != nil {
-		txnWrite.End()
+		txnRead.End()
 		return nil, nil, err
 	}
 	// When executing mutable txnWrite ops, etcd must hold the txnWrite lock so
 	// readers do not see any intermediate results. Since writes are
 	// serialized on the raft loop, the revision in the read view will
 	// be the revision of the write txnWrite.
+	var txnWrite mvcc.TxnWrite
 	if isWrite {
-		txnWrite.End()
+		txnRead.End()
 		txnWrite = kv.Write(trace)
+	} else {
+		txnWrite = mvcc.NewReadOnlyTxnWrite(txnRead)
 	}
 	txnResp, err := txn(ctx, lg, txnWrite, rt, isWrite, txnPath)
 	txnWrite.End()
@@ -278,16 +282,16 @@ func Txn(ctx context.Context, lg *zap.Logger, rt *pb.TxnRequest, txnModeWriteWit
 	return txnResp, trace, err
 }
 
-func checkTxn(ctx context.Context, txnWrite mvcc.TxnWrite, rt *pb.TxnRequest, isWrite bool, lessor lease.Lessor, txnPath []bool) error {
+func checkTxn(ctx context.Context, txnRead mvcc.TxnRead, rt *pb.TxnRequest, isWrite bool, lessor lease.Lessor, txnPath []bool) error {
 	trace := traceutil.Get(ctx)
 	if isWrite {
 		trace.AddField(traceutil.Field{Key: "read_only", Value: false})
-		if _, err := checkRequests(txnWrite, rt, txnPath,
+		if _, err := checkRequests(txnRead, rt, txnPath,
 			func(rv mvcc.ReadView, ro *pb.RequestOp) error { return checkRequestPut(rv, lessor, ro) }); err != nil {
 			return err
 		}
 	}
-	if _, err := checkRequests(txnWrite, rt, txnPath, checkRequestRange); err != nil {
+	if _, err := checkRequests(txnRead, rt, txnPath, checkRequestRange); err != nil {
 		return err
 	}
 	trace.Step("check requests")