From 004efaf015bc0379ceb62846a5e06b5c0d598d3f Mon Sep 17 00:00:00 2001
From: Tim Wong <tim@nextdoor.com>
Date: Wed, 9 Aug 2017 17:10:04 -0700
Subject: [PATCH] ZUNIONSTORE overwrites a key after potentially reading it.

---
 cmd_sorted_set.go      | 10 +++++++++-
 cmd_sorted_set_test.go | 14 ++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/cmd_sorted_set.go b/cmd_sorted_set.go
index 564b9e25..8a89ca82 100644
--- a/cmd_sorted_set.go
+++ b/cmd_sorted_set.go
@@ -1153,7 +1153,15 @@ func (m *Miniredis) cmdZunionstore(c *server.Peer, cmd string, args []string) {
 
 	withTx(m, c, func(c *server.Peer, ctx *connCtx) {
 		db := m.db(ctx.selectedDB)
-		db.del(destination, true)
+		deleteDest := true
+		for _, key := range keys {
+			if destination == key {
+				deleteDest = false
+			}
+		}
+		if deleteDest {
+			db.del(destination, true)
+		}
 
 		sset := sortedSet{}
 		for i, key := range keys {
diff --git a/cmd_sorted_set_test.go b/cmd_sorted_set_test.go
index 8211d833..7bbb6325 100644
--- a/cmd_sorted_set_test.go
+++ b/cmd_sorted_set_test.go
@@ -1106,6 +1106,20 @@ func TestZunionstore(t *testing.T) {
 		equals(t, map[string]float64{"field1": 2, "field2": 4}, ss)
 	}
 
+	// Merge destination with itself.
+	{
+		s.ZAdd("h3", 1.0, "field1")
+		s.ZAdd("h3", 3.0, "field3")
+
+		res, err := redis.Int(c.Do("ZUNIONSTORE", "h3", 2, "h1", "h3"))
+		ok(t, err)
+		equals(t, 3, res)
+
+		ss, err := s.SortedSet("h3")
+		ok(t, err)
+		equals(t, map[string]float64{"field1": 2, "field2": 2, "field3": 3}, ss)
+	}
+
 	// WEIGHTS
 	{
 		res, err := redis.Int(c.Do("ZUNIONSTORE", "weighted", 2, "h1", "h2", "WeIgHtS", "4.5", "12"))