[BUG] Attempting to read last entry of empty stream throws error

[thearchitector]

Describe the bug

If you attempt to read the last entry of an empty stream using XREAD STREAMS blah +, you get an error. It should be returning nil instead.

To reproduce

127.0.0.1:6379> xgroup create blah blah 0 MKSTREAM
OK
127.0.0.1:6379> xread STREAMS blah +
(error) ERR the stream last element ID is 0-0

Expected behavior

Valkey should return a nil response instead of an error, as with Redis:

127.0.0.1:6379> xgroup create blah blah 0 MKSTREAM
OK
127.0.0.1:6379> xread STREAMS blah +
(nil)
127.0.0.1:6379>

Additional information

This happens use the latest valkey/valkey:alpine image.

The source indicates this error should be impossible.

[diego-ciciani01]

Hi, @thearchitector

When you run XREAD STREAMS blah + on an empty stream, the code:

• Retrieves s->last_id of the empty stream (which is 0-0)

• Attempts to decrement this ID by calling streamDecrID(&ids[id_idx])

• Since 0-0 is the minimum possible ID, streamDecrID() returns C_ERR

This error is caught and converted into the message: “ERR the stream last element ID is 0-0” .

So I think the problem is that the condition does not check whether the stream is empty or not

[diego-ciciani01]

hi @thearchitector, i open a PR for this tiny bug