Fix zero-ordering SORT when called against lists

People mostly use SORT against lists, but our prior
behavior was pretending lists were an unordered bag
requiring a forced-sort when no sort was requested.

We can just use the native list ordering to ensure
consistency across replicaion and scripting calls.

Closes #2079
Closes #545 (again)

src/sort.c

@@ -285,16 +285,13 @@ void sortCommand(redisClient *c) {
         return;
     }
 
-    /* For the STORE option, or when SORT is called from a Lua script,
+    /* When sorting a set with no sort specified, we must sort the output
-     * we want to force a specific ordering even when no explicit ordering
+     * so the result is consistent across scripting and replication.
-     * was asked (SORT BY nosort). This guarantees that replication / AOF
-     * is deterministic.
      *
-     * However in the case 'dontsort' is true, but the type to sort is a
+     * The other types (list, sorted set) will retain their native order
-     * sorted set, we don't need to do anything as ordering is guaranteed
+     * even if no sort order is requested, so they remain stable across
-     * in this special case. */
+     * scripting and replication. */
-    if ((storekey || c->flags & REDIS_LUA_CLIENT) &&
+    if ((dontsort && sortval->type == REDIS_SET))
-        (dontsort && sortval->type != REDIS_ZSET))
     {
         /* Force ALPHA sorting */
         dontsort = 0;

tests/unit/sort.tcl

@@ -246,6 +246,24 @@ start_server {
         r sort mylist by num get x:*->
     } {100}
 
+    test "SORT by nosort retains native order for lists" {
+        r del testa
+        r lpush testa 2 1 4 3 5
+        r sort testa by nosort
+    } {5 3 4 1 2}
+
+    test "SORT by nosort plus store retains native order for lists" {
+        r del testa
+        r lpush testa 2 1 4 3 5
+        r sort testa by nosort store testb
+        r lrange testb 0 -1
+    } {5 3 4 1 2}
+
+    test "SORT by nosort with limit returns based on original list order" {
+        r sort testa by nosort limit 0 3 store testb
+        r lrange testb 0 -1
+    } {5 3 4}
+
     tags {"slow"} {
         set num 100
         set res [create_random_dataset $num lpush]