un-implent "SCAN COUNT" so cursors work as they should (#418)

This works according to the specs, and avoids having to implement proper cursors.

Author: alicebob

cmd_generic.go

@@ -650,7 +650,8 @@ func (m *Miniredis) cmdScan(c *server.Peer, cmd string, args []string) {
 
 	withTx(m, c, func(c *server.Peer, ctx *connCtx) {
 		db := m.db(ctx.selectedDB)
-		// We return _all_ (matched) keys every time.
+		// We return _all_ (matched) keys every time, so that cursors work.
+		// We ignore "COUNT", which is allowed according to the Redis docs.
 		var keys []string
 
 		if opts.withType {
@@ -671,25 +672,14 @@ func (m *Miniredis) cmdScan(c *server.Peer, cmd string, args []string) {
 			keys, _ = matchKeys(keys, opts.match)
 		}
 
-		low := opts.cursor
-		high := low + opts.count
-		// validate high is correct
-		if high > len(keys) || high == 0 {
-			high = len(keys)
-		}
-		if opts.cursor > high {
-			// invalid cursor
+		// we only ever return all at once, so no non-zero cursor can every be valid
+		if opts.cursor != 0 {
 			c.WriteLen(2)
 			c.WriteBulk("0") // no next cursor
 			c.WriteLen(0)    // no elements
 			return
 		}
-		cursorValue := low + opts.count
-		if cursorValue >= len(keys) {
-			cursorValue = 0 // no next cursor
-		}
-		keys = keys[low:high]
-
+		cursorValue := 0 // we don't use cursors
 		c.WriteLen(2)
 		c.WriteBulk(fmt.Sprintf("%d", cursorValue))
 		c.WriteLen(len(keys))

cmd_generic_test.go

@@ -758,27 +758,31 @@ func TestScan(t *testing.T) {
 		s.Set("v8", "value")
 		s.Set("v9", "value")
 
+		// count is ignored
 		mustDo(t, c,
 			"SCAN", "0", "COUNT", "3",
 			proto.Array(
-				proto.String("3"),
+				proto.String("0"),
 				proto.Array(
 					proto.String("key"),
 					proto.String("v1"),
 					proto.String("v2"),
+					proto.String("v3"),
+					proto.String("v4"),
+					proto.String("v5"),
+					proto.String("v6"),
+					proto.String("v7"),
+					proto.String("v8"),
+					proto.String("v9"),
 				),
 			),
 		)
 
 		mustDo(t, c,
 			"SCAN", "3", "COUNT", "3",
 			proto.Array(
-				proto.String("6"),
-				proto.Array(
-					proto.String("v3"),
-					proto.String("v4"),
-					proto.String("v5"),
-				),
+				proto.String("0"),
+				proto.Array(),
 			),
 		)
 	})