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_test.go

@@ -39,28 +39,6 @@ func TestCommitQueue(t *testing.T) {
 		}
 	}
 
-	// Helper function to create a read set from the passed keys.
-	makeReadSet := func(keys []string) readSet {
-		rs := make(map[string]stmGet)
-
-		for _, key := range keys {
-			rs[key] = stmGet{}
-		}
-
-		return rs
-	}
-
-	// Helper function to create a write set from the passed keys.
-	makeWriteSet := func(keys []string) writeSet {
-		ws := make(map[string]stmPut)
-
-		for _, key := range keys {
-			ws[key] = stmPut{}
-		}
-
-		return ws
-	}
-
 	ctx := context.Background()
 	ctx, cancel := context.WithCancel(ctx)
 	q := NewCommitQueue(ctx)
@@ -73,26 +51,26 @@ func TestCommitQueue(t *testing.T) {
 	// Tx1: reads: key1, key2, writes: key3, conflict: none
 	q.Add(
 		commit("free", true),
-		makeReadSet([]string{"key1", "key2"}),
-		makeWriteSet([]string{"key3"}),
+		[]string{"key1", "key2"},
+		[]string{"key3"},
 	)
 	// Tx2: reads: key1, key2, writes: key3, conflict: Tx1
 	q.Add(
 		commit("blocked1", false),
-		makeReadSet([]string{"key1", "key2"}),
-		makeWriteSet([]string{"key3"}),
+		[]string{"key1", "key2"},
+		[]string{"key3"},
 	)
 	// Tx3: reads: key1, writes: key4, conflict: none
 	q.Add(
 		commit("free", true),
-		makeReadSet([]string{"key1", "key2"}),
-		makeWriteSet([]string{"key4"}),
+		[]string{"key1", "key2"},
+		[]string{"key4"},
 	)
 	// Tx4: reads: key2, writes: key4 conflict: Tx3
 	q.Add(
 		commit("blocked2", false),
-		makeReadSet([]string{"key2"}),
-		makeWriteSet([]string{"key4"}),
+		[]string{"key2"},
+		[]string{"key4"},
 	)
 
 	// Wait for all commits.