Online DDL: dynamic cut-over threshold via ALTER VITESS_MIGRATION ... CUTOVER_THRESHOLD ... command

[shlomi-noach]

Description

This PR introduces the following command (example):

ALTER VITESS_MIGRATION '03740aca_9771_11ef_a0c8_b27afeff3c83' CUTOVER_THRESHOLD '15s'

This can be applied to any migration, either just submitted, scheduled to run, running, cancelled, ... (in short: at any state). However the case, the value gets persisted to a new column: schema_migrations.cutover_threshold_seconds.

The value in that column is an integer, and is the truncated value of the given argument. e.g. if given CUTOVER_THRESHOLD '7500ms' then the value stored is 7 (seconds).

The value is further sanitized: it must be in the range 5s..30s and forced into that range if outside. A value 0, or if never indicated in the first place, implies the default cut-over threshold which is 10s.

The value can be updated at any time and as many times. As a migration goes into cut-over, it reads and uses the current value from the table.

Related Issue(s)

Fixes #17123

Docs: vitessio/website#1879

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[shlomi-noach]

Docs: vitessio/website#1879

[shlomi-noach]

Code Coverage fails -- this functionality is tested in e2e tests.

[codecov]

Codecov Report

Attention: Patch coverage is 14.47368% with 65 lines in your changes missing coverage. Please review.

Project coverage is 67.40%. Comparing base (4980d6f) to head (8fa16d5).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	14.49%	59 Missing ⚠️
go/vt/sqlparser/ast_format.go	20.00%	4 Missing ⚠️
go/vt/vttablet/tabletserver/query_executor.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17126   +/-   ##
=======================================
  Coverage   67.39%   67.40%           
=======================================
  Files        1570     1570           
  Lines      252892   252944   +52     
=======================================
+ Hits       170437   170493   +56     
+ Misses      82455    82451    -4     

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[mattlord]

IMO it's better to return an error if the user is e.g. trying to set it to 90s to avoid user confusion — especially since this isn't driven by a flag where we can explicitly tell the user about the valid range and also potentially check the input value on the client side. I don't feel strongly about it though (I do see that we're documenting it).

[shlomi-noach]

We now return an error if the user's value is out of bounds (< 5s or > 30s). 0 is allowed and presets the threshold value to the factory default.

[shlomi-noach]

Docs PR updated.