VReplication: Force flag for traffic switching

[mattlord]

Description

This PR adds an expert only --force flag to the SwitchTraffic and ReverseTraffic workflow sub-commands that currently exist for:

• Reshard: https://vitess.io/docs/reference/programs/vtctldclient/vtctldclient_reshard
• MoveTables: https://vitess.io/docs/20.0/reference/programs/vtctldclient/vtctldclient_movetables

When specified, potentially non-critical failures such as partial tablet refreshes are considered non-fatal and the traffic switching work continues.

An example use case is:

• You've switched traffic from the source keyspace to the target keyspace as part of a MoveTables workflow
• One of the target shard's replica tablets is currently down
• You need to switch traffic back before addressing that tablet issue as e.g. you need to ReverseTraffic ASAP because application errors are occurring and it's causing a [partial] outage

Related Issue(s)

• Fixes: Feature Request: VReplication: add force flag for SwitchTraffic and ReverseTraffic #16528

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: Update docs for 16654 and 16709 website#1835

[vitess-bot]

Review Checklist

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• Protobuf changes should be wire-compatible.
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[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 35 lines in your changes missing coverage. Please review.

Project coverage is 68.93%. Comparing base (8816a2d) to head (161da76).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/traffic_switcher.go	46.87%	17 Missing ⚠️
go/vt/vtctl/workflow/server.go	64.10%	14 Missing ⚠️
go/vt/vtctl/grpcvtctldserver/server.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16709      +/-   ##
==========================================
- Coverage   68.94%   68.93%   -0.01%     
==========================================
  Files        1566     1566              
  Lines      201983   202036      +53     
==========================================
+ Hits       139252   139273      +21     
- Misses      62731    62763      +32     

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[deepthi]

This request keeps getting bigger. I wish we had implemented something similar to workflow options.

[mattlord]

WorkflowOptions are persisted in the workflow record as they affect the workflow generally (set on Create and used from then on). This flag only applies to a single traffic switch call and shouldn’t be persisted.

[mattlord]

I’m also not sure what’s bad about adding command flags and the corresponding proto fields for it? If this is about the actual wire protocol message size, then AFAIK embedding doesn't significantly impact that one way or the other. If that is the concern though, we could move all (non-repeated) fields to optional so that they are nil pointers instead of zero value structs (I'm actually not sure that this affects the over the wire bytes either way though, from my testing and experimentation I don't think it does, it only affects unmarshalling of the wire protocol message and the bytes used for that in-memory structure). I recently added one new optional field in #16654 and moved some existing ones to optional here: #16734

I can see e.g. that all of the (non-repeated) fields seem to be optional now in k8s: https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/api/core/v1/generated.proto