chore: forbid API based flat fee line manipulation if ubp fs is active

[turip]

Overview

This ensures that we are not generating new flat fee lines from the user interface.

Notes for reviewer

Summary by CodeRabbit

• New Features
  • Introduced a feature switch to control the use of usage-based flat fee invoice lines.
• Bug Fixes
  • Added validation to prevent creation or modification of flat fee invoice lines when the usage-based flat fee feature is enabled, ensuring only usage-based lines are allowed under this setting.

[coderabbitai]

📝 Walkthrough

Walkthrough

This change introduces a feature switch for usage-based flat fee invoice lines by adding a new configuration field and propagating it through the server, router, and billing HTTP handler layers. Validation logic is added to prevent creation or modification of flat fee invoice lines when the relevant feature flag is enabled.

Changes

Files/Paths	Change Summary
cmd/server/main.go	Passes BillingFeatureSwitches from config to RouterConfig during server setup.
openmeter/server/router/router.go	Adds BillingFeatureSwitches field to Config struct; passes it to billing handler initialization.
openmeter/billing/httpdriver/handler.go	Adds featureSwitches field to handler struct; updates New constructor to accept and assign it.
openmeter/billing/httpdriver/invoiceline.go	Adds validation to disallow flat fee invoice lines when UseUsageBasedFlatFeeLines feature switch is enabled.

Possibly related PRs

• feat: add config for ubp flat fee lines #2902: Also adds and propagates the BillingFeatureSwitchesConfiguration for feature flagging in billing components.
• fix: only support qty=1 for flat fee lines #2893: Modifies invoice line validation logic for flat fee lines, related to the feature switch-based validation in this PR.
• feat: use UBP flat fee lines #2898: Implements logic and synchronization for usage-based flat fee lines, addressing the same feature flag in a different billing subsystem.

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (1.64.8)

Error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2
Failed executing command with error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2

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📜 Recent review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between deef967 and b321c6a.

📒 Files selected for processing (4)

• cmd/server/main.go (1 hunks)
• openmeter/billing/httpdriver/handler.go (4 hunks)
• openmeter/billing/httpdriver/invoiceline.go (3 hunks)
• openmeter/server/router/router.go (3 hunks)

🧰 Additional context used

🧬 Code Graph Analysis (3)

openmeter/billing/httpdriver/handler.go (1)

app/config/billing.go (1)

• BillingFeatureSwitchesConfiguration (35-38)

openmeter/server/router/router.go (1)

app/config/billing.go (1)

• BillingFeatureSwitchesConfiguration (35-38)

openmeter/billing/httpdriver/invoiceline.go (3)

openmeter/billing/invoiceline.go (2)

• InvoiceLineTypeFee (31-31)
• LineChildren (693-695)

openmeter/billing/errors.go (1)

• ValidationError (119-119)

openmeter/ent/db/ent.go (1)

• ValidationError (255-258)

⏰ Context from checks skipped due to timeout of 90000ms (7)

• GitHub Check: Test
• GitHub Check: Quickstart
• GitHub Check: Lint
• GitHub Check: E2E
• GitHub Check: Developer environment
• GitHub Check: CI
• GitHub Check: Analyze (go)

🔇 Additional comments (9)

cmd/server/main.go (1)

164-164: LGTM - Configuration propagation is correct.

The addition of BillingFeatureSwitches to the router configuration properly propagates the billing feature switches from the main configuration through to the router layer.

openmeter/server/router/router.go (3)

15-15: LGTM - Required import for configuration type.

The config package import is necessary for the BillingFeatureSwitchesConfiguration type.

---

90-90: LGTM - Config struct properly extended.

The addition of the BillingFeatureSwitches field correctly extends the router configuration to support billing feature switches.

---

349-349: LGTM - Feature switches correctly passed to billing handler.

The billing feature switches are properly passed to the billing HTTP handler during initialization, completing the configuration propagation chain.

openmeter/billing/httpdriver/handler.go (2)

9-9: LGTM - Required import for configuration type.

The config package import is necessary for the BillingFeatureSwitchesConfiguration type used in the handler struct.

---

56-56: LGTM - Feature switches properly integrated into handler.

The handler struct is correctly extended with the featureSwitches field, the constructor accepts the configuration parameter, and the field is properly assigned. This enables the handler to access feature switch settings for validation logic.

Also applies to: 72-72, 81-81

openmeter/billing/httpdriver/invoiceline.go (3)

72-80: LGTM - Validation correctly prevents flat fee line creation.

The validation logic properly checks the UseUsageBasedFlatFeeLines feature switch and prevents creation of flat fee lines when the feature is enabled. The error message is clear and informative.

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955-959: LGTM - Consistent validation for new lines in merge operation.

The validation is correctly applied when creating new lines through the merge operation, maintaining consistency with the direct creation path.

---

981-985: LGTM - Validation prevents updates to existing flat fee lines.

The validation correctly prevents updates to existing flat fee lines when the feature switch is enabled, ensuring comprehensive protection against flat fee line manipulation.

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