placement: supports follower role with evict-leader hint

[nolouch]

What problem does this PR solve?

Issue Number: close #46304

Problem Summary:

Currently, all FOLLOWER_CONSTRAINTS will set the role in the rule to be voter. It can cover major scenarios and reduce the user's burden. but in some scenarios cannot cover, like:

there are three az, zone1, zone2, zone3. the user wants the replicas in zone 1 and zone 2 can be the leader, and normal situations, the leader should be placed in zone 1, if zone 1 is gone, the leader is better in zone 2. and zone3 reject be the leader, zone 3 can be seen as a backup zone, it's not used to serve.

maybe we can enhance the FOLLOWER_CONSTRAINTS, like

add attribute

Dictionary:  e.g. {"+region=us-east": 2, "+region=us-west,#evict-leader": 1}

#evict-leader explains voters should be followers, and reject becoming leaders.

full SQL will like this:

CREATE PLACEMENT POLICY `mydeploy`  LEADER_CONSTRAINTS='{"+region=us-east-1": 1}' FOLLOWER_CONSTRAINTS='{"+region=us-east-1": 1,  "+region=use-east-2":2, "+region=us-west,#evict-leader": 1};

alternative plan

warn: this is the current easy way to achieve this issue scenario. we would be supported leader_preferences in the future to define the leader choice priority.

What is changed and how it works?

add new keywords.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[nolouch]

/retest-required

[codecov]

Codecov Report

Merging #47307 (f6a1f59) into master (4cc6c18) will increase coverage by 0.0541%.
Report is 71 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #47307        +/-   ##
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+ Coverage   72.9347%   72.9888%   +0.0541%     
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  Files          1340       1373        +33     
  Lines        400476     409956      +9480     
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+ Hits         292086     299222      +7136     
- Misses        89447      91903      +2456     
+ Partials      18943      18831       -112     

Flag	Coverage Δ
integration	39.5136% <0.0000%> (?)
unit	72.6237% <100.0000%> (-0.3110%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9913% <ø> (ø)
parser	84.7651% <ø> (-0.1581%)	⬇️
br	49.5258% <ø> (-3.5594%)	⬇️

[nolouch]

/retest-required

[nolouch]

PTAL @CabinfeverB

[rleungx]

LGTM, but I was wondering if the user uses # as part of the label, it may have a problem. It's better to add the notice in the doc.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rleungx, xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-28 06:43:34.556889105 +0000 UTC m=+84212.143999250: ☑️ agreed by xhebox.
• 2023-10-09 04:20:40.557003946 +0000 UTC m=+1026038.144114091: ☑️ agreed by rleungx.