Implement SCAN cursor [TYPE type] modifier suggested in issue #6107.

Add tests to check basic functionality of this optional keyword, and also tested with
a module (redisgraph). Checked quickly with valgrind, no issues.

Copies name the type name canonicalisation code from `typeCommand`, perhaps this would
be better factored out to prevent the two diverging and both needing to be edited to
add new `OBJ_*` types, but this is a little fiddly with C strings.

The [redis-doc](https://github.com/antirez/redis-doc/blob/master/commands.json) repo
will need to be updated with this new arg if accepted.

A quirk to be aware of here is that the GEO commands are backed by zsets not their own
type, so they're not distinguishable from other zsets.

Additionally, for sparse types this has the same behaviour as `MATCH` in that it may
return many empty results before giving something, even for large `COUNT`s.

src/db.c

@@ -613,7 +613,7 @@ int parseScanCursorOrReply(client *c, robj *o, unsigned long *cursor) {
 }
 
 /* This command implements SCAN, HSCAN and SSCAN commands.
- * If object 'o' is passed, then it must be a Hash or Set object, otherwise
+ * If object 'o' is passed, then it must be a Hash, Set or Zset object, otherwise
  * if 'o' is NULL the command will operate on the dictionary associated with
  * the current database.
  *
@@ -629,6 +629,7 @@ void scanGenericCommand(client *c, robj *o, unsigned long cursor) {
     listNode *node, *nextnode;
     long count = 10;
     sds pat = NULL;
+    sds typename = NULL;
     int patlen = 0, use_pattern = 0;
     dict *ht;
 
@@ -665,6 +666,10 @@ void scanGenericCommand(client *c, robj *o, unsigned long cursor) {
             use_pattern = !(pat[0] == '*' && patlen == 1);
 
             i += 2;
+        } else if (!strcasecmp(c->argv[i]->ptr, "type") && o == NULL && j >= 2) {
+            /* SCAN for a particular type only applies to the db dict */
+            typename = c->argv[i+1]->ptr;
+            i+= 2;
         } else {
             addReply(c,shared.syntaxerr);
             goto cleanup;
@@ -759,6 +764,31 @@ void scanGenericCommand(client *c, robj *o, unsigned long cursor) {
             }
         }
 
+        /* Filter an element if it isn't the type we want. */
+        if (!filter && o == NULL && typename){
+            robj* typecheck;
+            char *type;
+            typecheck = lookupKeyReadWithFlags(c->db, kobj, LOOKUP_NOTOUCH);
+            if (typecheck == NULL) {
+                type = "none";
+            } else {
+                switch(typecheck->type) {
+                case OBJ_STRING: type = "string"; break;
+                case OBJ_LIST: type = "list"; break;
+                case OBJ_SET: type = "set"; break;
+                case OBJ_ZSET: type = "zset"; break;
+                case OBJ_HASH: type = "hash"; break;
+                case OBJ_STREAM: type = "stream"; break;
+                case OBJ_MODULE: {
+                    moduleValue *mv = typecheck->ptr;
+                    type = mv->type->name;
+                }; break;
+                default: type = "unknown"; break;
+                }
+            }
+            if (strcasecmp((char*) typename, type)) filter = 1;
+        }
+
         /* Filter element if it is an expired key. */
         if (!filter && o == NULL && expireIfNeeded(c->db, kobj)) filter = 1;
 

tests/unit/scan.tcl

@@ -53,6 +53,51 @@ start_server {tags {"scan"}} {
         assert_equal 100 [llength $keys]
     }
 
+    test "SCAN TYPE" {
+        r flushdb
+        # populate only creates strings
+        r debug populate 1000
+
+        # Check non-strings are excluded
+        set cur 0
+        set keys {}
+        while 1 {
+            set res [r scan $cur type "list"]
+            set cur [lindex $res 0]
+            set k [lindex $res 1]
+            lappend keys {*}$k
+            if {$cur == 0} break
+        }
+
+        assert_equal 0 [llength $keys]
+
+        # Check strings are included
+        set cur 0
+        set keys {}
+        while 1 {
+            set res [r scan $cur type "string"]
+            set cur [lindex $res 0]
+            set k [lindex $res 1]
+            lappend keys {*}$k
+            if {$cur == 0} break
+        }
+
+        assert_equal 1000 [llength $keys]
+
+        # Check all three args work together
+        set cur 0
+        set keys {}
+        while 1 {
+            set res [r scan $cur type "string" match "key:*" count 10]
+            set cur [lindex $res 0]
+            set k [lindex $res 1]
+            lappend keys {*}$k
+            if {$cur == 0} break
+        }
+
+        assert_equal 1000 [llength $keys]
+    }
+
     foreach enc {intset hashtable} {
         test "SSCAN with encoding $enc" {
             # Create the Set