[BUG] Kvrocks SCAN does not match with Redis SCAN

[VictorCam]

Describe the bug
Scanning on Kvrocks produces incorrect responses when compared to Redis.

To Reproduce
Steps to reproduce the behavior:

1. run docker
2. get images "docker pull redis" and "docker pull kvrocks/kvrocks"
3. "docker run --rm -d -p 6666:6666/tcp kvrocks/kvrocks:latest"
4. "docker run --rm -d -p 6379:6379/tcp redis:latest"
5. open two redis-cli instances one on port 6739 and 6666
6. set "hi" and set "hello there" on both instances
7. preform the same queries as shown in the image.

Expected behavior
I believe scan should be the same as how redis is preforming the scan.

[ShooterIT]

@VictorCam Thanks for your report
there are two difference with redis, i find

• return value is string instead of integer, the reason is here *SCAN and iterator as a string - Redis Python library fix #417
• From code, Kvrocks currently only support "prefix" match, so it return 0 when match *he*, for this, we can support it i think, even it may be inefficient.

[VictorCam]

Thank you for the quick response and explanation. I'd be nice to have that implemented. I believe it can be useful in certain scenarios. Though feel free to decide if you'd like to implement it or not considering that fact that it will be inefficient.

[adulau]

and if you are curious, we maintain a fork of the Redis Python library to make the iterator works on kvrocks.

[tisonkun]

@ShooterIT Another concern is that on Kvrocks, SCAN return the cursor "_hello" but actually if you want to iterate on next element, it requires a number based cursor.

For example:

127.0.0.1:6666> set "hi there" v0
OK
127.0.0.1:6666> set "hello" v1
OK
127.0.0.1:6666> SCAN 0 MATCH hel* COUNT 10000
1) "_hello"
2) 1) "hello"
127.0.0.1:6666> SCAN 0 MATCH he* COUNT 10000
1) "_hello"
2) 1) "hello"
127.0.0.1:6666> SCAN 0 MATCH h* COUNT 10000
1) "_hi there"
2) 1) "hello"
   2) "hi there"
127.0.0.1:6666> SCAN "_hi there" MATCH h* COUNT 10000
1) "0"
2) (empty array)
127.0.0.1:6666> SCAN "hi there" MATCH h* COUNT 10000
1) "0"
2) (empty array)
127.0.0.1:6666> SCAN 1 MATCH h* COUNT 10000
1) "_hi there"
2) 1) "hi there"

Note that SCAN "_hi there" ... returns "0".

[tisonkun]

Sorry. I get it wrong.

127.0.0.1:6666> SCAN 1 MATCH h* COUNT 1
1) "_hi there"
2) 1) "hi there"
127.0.0.1:6666> SCAN 0 MATCH h* COUNT 1
1) "_hello"
2) 1) "hello"
127.0.0.1:6666> SCAN "_hello" MATCH h* COUNT 1
1) "_hi there"
2) 1) "hi there"

The cursor should work.

[PragmaTwice]

See #1489.