Add default response of DetermineStrategy() of DeploymentPlugin

[t-kikuc]

What this PR does:

• DetermineStrategy() returns PipelineSync by default
• Describe the spec of DetermineStrategy()

cf. pipedv0 uses pipelineSync by default.

pipecd/pkg/app/piped/planner/terraform/terraform.go

Lines 103 to 115 in a19732a

	// Force to use pipeline when the alwaysUsePipeline field was configured.
	if cfg.Planner.AlwaysUsePipeline {
	out.SyncStrategy = model.SyncStrategy_PIPELINE
	out.Stages = buildProgressivePipeline(cfg.Pipeline, cfg.Input.AutoRollback, time.Now())
	out.Summary = "Sync with the specified pipeline (alwaysUsePipeline was set)"
	return
	}
	out.SyncStrategy = model.SyncStrategy_PIPELINE
	out.Stages = buildProgressivePipeline(cfg.Pipeline, cfg.Input.AutoRollback, now)
	out.Summary = "Sync with the specified progressive pipeline"
	return
	}

Why we need it:

• Prevent plugin developers from getting confused. They can just return (nil, nil) for DetermineStrategy().
  • Plugins other than the k8s plugin probably do not have the specific logic.

Which issue(s) this PR fixes:

Part of #5530

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 28.45%. Comparing base (0afedaa) to head (c5f6d75).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/sdk/deployment.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5929      +/-   ##
==========================================
- Coverage   28.46%   28.45%   -0.01%     
==========================================
  Files         520      520              
  Lines       56408    56416       +8     
==========================================
  Hits        16055    16055              
- Misses      39087    39095       +8     
  Partials     1266     1266              

Flag	Coverage Δ
.	23.73% <ø> (ø)
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	49.20% <0.00%> (-0.22%)	⬇️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ffjlabo]

@t-kikuc
I think we need to modify the caller logic to handle nil.
This might cause panic because of nil pointer dereference.
https://github.com/pipe-cd/pipecd/blob/a19732a292b9407c8ac03963d71e39a0347329d8/pkg/app/pipedv1/controller/planner.go#L375C1-L388C3

[t-kikuc]

@ffjlabo
I think that should be done in another PR as a controller bug.

In this PR, even if DetermineStrategy() of a plugin returns nil, the SDK will not return nil to piped.
(Now, if a plugin returns nil, the SDK will panic.)

[ffjlabo]

@t-kikuc
Sorry, I misunderstood. 🙏 I got your point.

[ffjlabo]

👍

[khanhtc1202]

Suggested change

	// when a plugin does not have specific logic.
	// If the plugin does not have specific logic to determine strategy
	// returns PipelineSync as default.

nits, the comment should be clear, not half-phase, to reduce confusion.

[t-kikuc]

@khanhtc1202
Thanks, I updated!
c5f6d75

[khanhtc1202]

Thanks 👍

[t-kikuc]

@ffjlablo
Sorry, please approve again 🙏
And I opened a PR for the issue you mentioned: #5930