etcd: fix dereferncing issue in etcd.CommitQueue causing contention

This commit fixes an issue where subsequent transaction retries may have
changed the read/write sets inside the STM which in turn left junk
references to these keys in the CommitQueue. The left keys potentially
conflicted with subsequent transactions, queueing them up causing
througput degradation.

kvdb/etcd/commit_queue.go

@@ -54,14 +54,14 @@ func (c *commitQueue) Wait() {
 // Add increases lock counts and queues up tx commit closure for execution.
 // Transactions that don't have any conflicts are executed immediately by
 // "downgrading" the count mutex to allow concurrency.
-func (c *commitQueue) Add(commitLoop func(), rset readSet, wset writeSet) {
+func (c *commitQueue) Add(commitLoop func(), rset []string, wset []string) {
 	c.mx.Lock()
 	blocked := false
 
 	// Mark as blocked if there's any writer changing any of the keys in
 	// the read set. Do not increment the reader counts yet as we'll need to
 	// use the original reader counts when scanning through the write set.
-	for key := range rset {
+	for _, key := range rset {
 		if c.writerMap[key] > 0 {
 			blocked = true
 			break
@@ -70,7 +70,7 @@ func (c *commitQueue) Add(commitLoop func(), rset readSet, wset writeSet) {
 
 	// Mark as blocked if there's any writer or reader for any of the keys
 	// in the write set.
-	for key := range wset {
+	for _, key := range wset {
 		blocked = blocked || c.readerMap[key] > 0 || c.writerMap[key] > 0
 
 		// Increment the writer count.
@@ -78,7 +78,7 @@ func (c *commitQueue) Add(commitLoop func(), rset readSet, wset writeSet) {
 	}
 
 	// Finally we can increment the reader counts for keys in the read set.
-	for key := range rset {
+	for _, key := range rset {
 		c.readerMap[key] += 1
 	}
 
@@ -108,18 +108,18 @@ func (c *commitQueue) Add(commitLoop func(), rset readSet, wset writeSet) {
 }
 
 // Done decreases lock counts of the keys in the read/write sets.
-func (c *commitQueue) Done(rset readSet, wset writeSet) {
+func (c *commitQueue) Done(rset []string, wset []string) {
 	c.mx.Lock()
 	defer c.mx.Unlock()
 
-	for key := range rset {
+	for _, key := range rset {
 		c.readerMap[key] -= 1
 		if c.readerMap[key] == 0 {
 			delete(c.readerMap, key)
 		}
 	}
 
-	for key := range wset {
+	for _, key := range wset {
 		c.writerMap[key] -= 1
 		if c.writerMap[key] == 0 {
 			delete(c.writerMap, key)