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Fixes LPOP/RPOP wrong replies when count is 0 #9692

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oranagra merged 2 commits into from
Nov 4, 2021
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Fixes LPOP/RPOP wrong replies when count is 0 #9692

oranagra merged 2 commits into from
Nov 4, 2021

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@itamarhaber
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@itamarhaber itamarhaber commented Oct 27, 2021 •
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Fixes #9680.

Introduced in #8179, this fixes the command's replies in the 0 count edge case.

  • [BREAKING] changes the reply type when count is 0 to an empty array (instead of nil)
  • Moves LPOP ... 0 fast exit path after type check to reply with WRONGTYPE

TOOD:

  • Update docs with reply type

@itamarhaber
Fixes redis#9680
a1174d1 
* [BREAKING] changes the reply type when count is 0
* Moves LPOP ... 0 fast exit path after type check to reply with WRONGTYPE
@itamarhaber itamarhaber added release-notes indication that this issue needs to be mentioned in the release notes breaking-change This change can potentially break existing application state:needs-doc-pr requires a PR to redis-doc repository labels Oct 27, 2021
zuiderkwast
zuiderkwast approved these changes Oct 27, 2021
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Very nice. It's a breaking change indeed but it's a bugfix.

This is a typical bug which can be blamed partly on TCL's types (null-string, null-array, empty array and empty string are all represented as an empty TCL string) and partly on the existence of null-arrays all togther...

madolson
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Lgtm

@itamarhaber
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itamarhaber commented Oct 27, 2021

@zuiderkwast thanks for the kind words, but the RCA points at layer 8.

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enjoy-binbin commented Oct 28, 2021 •
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Another thought, why should we allow count == 0?
The use case count == 0 will always result empty, i don't see any usage scenarios.
In LMPOP/ZMPOP, I made a restriction that count can only be greater than 0 ....
If we already a breaking change, I think we can discuss one more time

The behavior of LMPOP is as follows

// unstable branch
127.0.0.1:6379> lmpop 2 non1 non2 left count 2
(nil)
127.0.0.1:6379> lmpop 2 non1 non2 left count 0
(error) ERR count should be greater than 0

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I agree with @enjoy-binbin this is not a real use case for this command and we can chose to return an error.
however, i also agree with the fix in this PR, we can also chose to return an empty array.

Nil is certainly a bug though, and since this is a new variant for the command, and a bug in a non-common use case, i think we should also backport it to 6.2.

@oranagra oranagra merged commit 06dd202 into redis:unstable Nov 4, 2021
@enjoy-binbin enjoy-binbin mentioned this pull request Jan 11, 2022
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@itamarhaber @oranagra
Fixes LPOP/RPOP wrong replies when count is 0 (#9692)
968cd2b 
Introduced in #8179, this fixes the command's replies in the 0 count edge case.
[BREAKING] changes the reply type when count is 0 to an empty array (instead of nil)
Moves LPOP ... 0 fast exit path after type check to reply with WRONGTYPE

(cherry picked from commit 06dd202)
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breaking-change This change can potentially break existing application release-notes indication that this issue needs to be mentioned in the release notes state:needs-doc-pr requires a PR to redis-doc repository

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[BUG] lpop/pop with count 0 returning null

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@itamarhaber @enjoy-binbin @zuiderkwast @oranagra @madolson