Send primary-only commands to the primary when the caller prefers a replica

src/StackExchange.Redis/RedisBase.cs

@@ -68,6 +68,13 @@ internal virtual Task<T> ExecuteAsync<T>(Message? message, ResultProcessor<T>? p
 
         internal virtual RedisFeatures GetFeatures(in RedisKey key, CommandFlags flags, RedisCommand command, out ServerEndPoint? server)
         {
+            // the server picked here is the one the message is sent to, so a primary-only command has to pick the primary
+            // here as well, the same way Message.SetPrimaryOnly does when the message is created; otherwise PreferReplica
+            // picks a replica that then rejects it (DemandReplica is left alone so that it still fails the same way)
+            if (command.IsPrimaryOnly() && Message.GetPrimaryReplicaFlags(flags) != CommandFlags.DemandReplica)
+            {
+                flags = Message.SetPrimaryReplicaFlags(flags, CommandFlags.DemandMaster);
+            }
             server = multiplexer.SelectServer(command, flags, key);
             var version = server == null ? multiplexer.RawConfig.DefaultVersion : server.Version;
             return new RedisFeatures(version);

tests/StackExchange.Redis.Tests/KeyTests.cs

@@ -75,6 +75,37 @@ public async Task KeyTypeOfMissingKeyIsNone() // see #3156
         Assert.Equal(RedisType.None, await db.KeyTypeAsync(key));
     }
 
+    [Fact]
+    public async Task PrimaryOnlyCommandsWithPreferReplicaRunOnThePrimary() // see #2821
+    {
+        await using var conn = Create(configuration: TestConfig.Current.PrimaryServerAndPort + "," + TestConfig.Current.ReplicaServerAndPort);
+        Assert.SkipUnless(
+            conn.GetEndPoints().Any(ep => conn.GetServer(ep).IsReplica),
+            "No replica in this configuration");
+
+        var db = conn.GetDatabase();
+        var key = Me();
+        RedisValue token = Guid.NewGuid().ToString();
+
+        // these look at the server's features before building the message, which used to pick the replica;
+        // the fractional-second expiry forces the PEXPIRE path, which is the one that checks features
+        db.StringSet(key, "abc");
+        Assert.True(db.KeyExpire(key, TimeSpan.FromMilliseconds(100_500), CommandFlags.PreferReplica));
+        Assert.True(db.KeyDelete(key, CommandFlags.PreferReplica));
+
+        db.StringSet(key, "abc");
+        Assert.True(await db.KeyExpireAsync(key, TimeSpan.FromMilliseconds(100_500), CommandFlags.PreferReplica));
+        Assert.True(await db.KeyDeleteAsync(key, CommandFlags.PreferReplica));
+
+        Assert.True(db.LockTake(key, token, TimeSpan.FromMinutes(1)));
+        Assert.True(db.LockExtend(key, token, TimeSpan.FromMinutes(1), CommandFlags.PreferReplica));
+        Assert.True(await db.LockExtendAsync(key, token, TimeSpan.FromMinutes(1), CommandFlags.PreferReplica));
+        Assert.True(db.LockRelease(key, token, CommandFlags.PreferReplica));
+
+        Assert.True(db.LockTake(key, token, TimeSpan.FromMinutes(1)));
+        Assert.True(await db.LockReleaseAsync(key, token, CommandFlags.PreferReplica));
+    }
+
     [Fact]
     public async Task Zeros()
     {