Make PFMERGE source key optional in docs, add tests with one input key, add tests on missing source keys (#11205)

The following two cases will create an empty destkey HLL:
1. called with no source keys, like `pfmerge destkey`
2. called with non-existing source keys, like `pfmerge destkey non-existing-source-key`

In the first case, in `PFMERGE`, the dest key is actually one of the source keys too.
So `PFMERGE k1 k2` is equivalent to `SUNIONSTORE k1 k1 k2`,
and `PFMERGE k1` is equivalent to `SUNIONSTORE k1 k1`.
So the first case is reasonable, the source key is actually optional.

And the second case, `PFMERGE` on missing keys should succeed and create an empty dest.
This is consistent with `PFCOUNT`, and also with `SUNIONSTORE`, no need to change.

src/commands.c

@@ -2608,7 +2608,7 @@ struct redisCommandArg PFDEBUG_Args[] = {
 /* PFMERGE argument table */
 struct redisCommandArg PFMERGE_Args[] = {
 {"destkey",ARG_TYPE_KEY,0,NULL,NULL,NULL,CMD_ARG_NONE},
-{"sourcekey",ARG_TYPE_KEY,1,NULL,NULL,NULL,CMD_ARG_MULTIPLE},
+{"sourcekey",ARG_TYPE_KEY,1,NULL,NULL,NULL,CMD_ARG_OPTIONAL|CMD_ARG_MULTIPLE},
 {0}
 };
 

src/commands/pfmerge.json

@@ -62,6 +62,7 @@
                 "name": "sourcekey",
                 "type": "key",
                 "key_spec_index": 1,
+                "optional": true,
                 "multiple": true
             }
         ]

src/help.h

@@ -1125,7 +1125,7 @@ struct commandHelp {
     11,
     "2.8.9" },
     { "PFMERGE",
-    "destkey sourcekey [sourcekey ...]",
+    "destkey [sourcekey [sourcekey ...]]",
     "Merge N different HyperLogLogs into a single one.",
     11,
     "2.8.9" },

tests/unit/hyperloglog.tcl

@@ -165,6 +165,33 @@ start_server {tags {"hll"}} {
         r pfcount hll{t}
     } {5}
 
+    test {PFMERGE on missing source keys will create an empty destkey} {
+        r del sourcekey{t} sourcekey2{t} destkey{t} destkey2{t}
+
+        assert_equal {OK} [r pfmerge destkey{t} sourcekey{t}]
+        assert_equal 1 [r exists destkey{t}]
+        assert_equal 0 [r pfcount destkey{t}]
+
+        assert_equal {OK} [r pfmerge destkey2{t} sourcekey{t} sourcekey2{t}]
+        assert_equal 1 [r exists destkey2{t}]
+        assert_equal 0 [r pfcount destkey{t}]
+    }
+
+    test {PFMERGE with one empty input key, create an empty destkey} {
+        r del destkey
+        assert_equal {OK} [r pfmerge destkey]
+        assert_equal 1 [r exists destkey]
+        assert_equal 0 [r pfcount destkey]
+    }
+
+    test {PFMERGE with one non-empty input key, dest key is actually one of the source keys} {
+        r del destkey
+        assert_equal 1 [r pfadd destkey a b c]
+        assert_equal {OK} [r pfmerge destkey]
+        assert_equal 1 [r exists destkey]
+        assert_equal 3 [r pfcount destkey]
+    }
+
     test {PFCOUNT multiple-keys merge returns cardinality of union #1} {
         r del hll1{t} hll2{t} hll3{t}
         for {set x 1} {$x < 10000} {incr x} {