[Experimental] Add new hmsetnx lua command

Would be nice to have this in Redis core, but doesn't look like
it's likely to happen. Ref. redis/redis#542

Note that there's another Lua implementation floating around
(https://redisgreen.net/library/hmsetnx.html), but that doesn't account
for a pre-existing hash key without the given fields.

src/lua/hmsetnx.lua

@@ -0,0 +1,23 @@
+local hkey = KEYS[1];
+
+if redis.call('exists', hkey) == 0 then
+    redis.call('hmset', hkey, unpack(ARGV));
+    return 1;
+else
+   local proceed = true;
+   for i,x in ipairs(ARGV) do
+      if (i % 2 ~= 0) then -- odd i => `x` is field
+	 if redis.call('hexists', hkey, x) == 1 then
+	    proceed = false;
+	    break;
+	 end
+      end
+   end
+
+   if proceed then
+      redis.call('hmset', hkey, unpack(ARGV));
+      return 1;
+   else
+      return 0;
+   end
+end

src/taoensso/carmine.clj

@@ -635,6 +635,17 @@
 
 ;;;;
 
+(def hmsetnx "Experimental."
+  (let [script (enc/slurp-resource "lua/hmsetnx.lua")]
+    (fn [key field value & more]
+      (-eval* script 1 (into [key field value] more)))))
+
+(comment
+  (wcar {} (hgetall "foo"))
+  (wcar {} (hmsetnx "foo" "f1" "v1" "f2" "v2")))
+
+;;;;
+
 (defn reduce-scan
   "For use with `scan`, `zscan`, etc. Takes:
     - (fn rf      [acc scan-result]) -> next accumulator