refactor: split line manipulation should use invoice API

[turip]

Overview

The InvoicePendingLines call is one of the most complex ones in the existing invoicing stack, due to algorithmical complexity, but also the code was scattered in multiple places with different layers handling different parts of the flow.

This patch makes sure that the whole logic is:

• Handled by the service
• Contained in a single file
• Well documented

As a side-effect we can remove the adapter dependency from the line service, making it closer to a "calculation" only package.

I have also removed the AssociateLinesToInvoice as extra care was needed to correct the inconsistencies caused by that call.

Summary by CodeRabbit

• New Features

  • Added InvoicePendingLines workflow to gather and process pending invoice lines across multiple currencies
  • Enhanced validation ensuring pending line selections contain at least one line ID

• Bug Fixes

  • Improved line cloning mechanism with customizable edit functions

• Refactor

  • Simplified invoice line management API by removing direct line association method
  • Streamlined public line operation interface

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Removes adapter-based line-association APIs and legacy invoice-pending orchestration; introduces a new, service-level gathering InvoicePendingLines workflow, simplifies line service interfaces, and tightens invoice input validation.

Changes

Cohort / File(s)	Summary
Adapter Interface Cleanup openmeter/billing/adapter.go, openmeter/billing/adapter/invoicelines.go	Removed AssociateLinesToInvoice from InvoiceLineAdapter and deleted its implementation.
Invoice Input Validation openmeter/billing/invoice.go	InvoicePendingLinesInput.Validate now requires IncludePendingLines to contain at least one ID if present.
Line Type & Cloning Behavior openmeter/billing/invoiceline.go	Added LineEditFunction type; Line.CloneWithoutDependencies now accepts variadic edits, resets timestamps (CreatedAt, UpdatedAt) and clears DeletedAt; removed AssociateLinesToInvoiceAdapterInput.
New Gathering Invoice Pending Lines Workflow openmeter/billing/service/gatheringinvoicependinglines.go	Added comprehensive service-level implementation for InvoicePendingLines (per-currency processing, splitting, invoice creation, moving lines, and event publishing).
Old Workflow Removal openmeter/billing/service/invoice.go	Removed previous InvoicePendingLines implementation and associated helper types.
Line Service Helper Removal openmeter/billing/service/invoiceline.go	Removed internal associateLinesToInvoice helper (splitting, validation, association, snapshotting, state transitions).
Line Service Interface Simplification openmeter/billing/service/lineservice/linebase.go	Removed several LineBase methods: IsSplitLineGroupMember, CloneForCreate, Update, Save, Delete, Split, and Service.
Lineservice Config & Validation openmeter/billing/service/lineservice/service.go, openmeter/billing/service/service.go	Removed BillingAdapter from Config and constructor wiring; removed AssociateLinesToInvoice on Service; added Lines.ValidateForInvoice(ctx, invoice) to validate collections before invoicing.

Sequence Diagram

sequenceDiagram
    participant Client
    participant Service as Service (InvoicePendingLines)
    participant Repo as Repository
    participant LineService as Line Service
    participant EventPub as EventPublisher

    Client->>Service: InvoicePendingLines(ctx, input)
    activate Service
    Service->>Service: Validate input & namespace lock
    Service->>Repo: Resolve customer/profile
    Service->>Repo: Fetch gathering invoices (per currency)
    Service->>LineService: Compute in-scope billable lines
    loop per currency
        Service->>Service: prepareLinesToBill (validate, split decisions)
        alt split required
            Service->>Service: splitGatheringInvoiceLine
            Service->>Repo: Update gathering invoice (post-split)
        end
        Service->>Repo: Create standard invoice
        Service->>Repo: Move/associate lines to new invoice
        Service->>Repo: Update gathering invoice state (or delete if empty)
    end
    Service->>EventPub: Publish InvoiceCreatedEvent(s)
    Service-->>Client: Return created invoices
    deactivate Service

Loading

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• refactor: use invoicing api for creating upcoming changes #3773 — Modifies invoice line handling and gathering-invoice workflows (overlaps with line/invoice movement changes).
• feat: custom invocing app #2705 — Related changes to invoice-pending lines and association flows.

Suggested labels

kind/refactor, area/billing, release-note/breaking-change

Suggested reviewers

• tothandras
• hekike
• chrisgacsal

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main refactoring objective: moving line manipulation logic to use the invoice API instead of separate adapter methods.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch refactor/split-lines

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[coderabbitai]

Actionable comments posted: 4

🤖 Fix all issues with AI agents

In `@openmeter/billing/service/gatheringinvoicependinglines.go`:
- Around line 292-321: After filtering res by the requested IDs (using
LinesToInclude.OrEmpty(), linesShouldBeIncluded, and lo.Filter), remove any
currency entries that end up with zero lines so empty currencies don't remain in
res; update the loop that sets res[currency] to compute the filtered slice and
if its length is zero delete(res, currency) else assign the filtered slice back
to res[currency], ensuring downstream validation only sees currencies with at
least one line.
- Around line 377-403: The code currently appends a pre‑split line even when
splitGatheringInvoiceLine marks it deleted and also logs the same "period_end"
key twice; update the block handling splitGatheringInvoiceLine so that after the
nil check you also skip adding the pre‑split line if
splitLine.PreSplitAtLine.Deleted is true (i.e., do not append to invoiceLines or
include its ID in LineIDsToBill), and fix the duplicate log key in the
logger.WarnContext call (replace the second "period_end" key with a distinct key
such as "period_original_end" or remove the duplicate) — references:
splitGatheringInvoiceLine, splitGatheringInvoiceLineInput,
splitLine.PreSplitAtLine, invoiceLines, LineIDsToBill, logger.WarnContext.
- Around line 148-157: The transaction callback in gatheringInvoicePendingLines
(inside transaction.Run) currently calls s.publisher.Publish for each invoice,
which can emit events for data that may be rolled back; instead, collect the
created invoices inside the callback (e.g., append to createdInvoices slice
returned from the transaction) and return successfully, then after
transaction.Run completes (i.e., after commit) iterate over the committed
invoices, create events via billing.NewInvoiceCreatedEvent(invoice) and call
s.publisher.Publish(ctx, event) outside the transaction; update the code path
around createdInvoices and the transaction.Run call so publishing only happens
after a successful commit.

In `@openmeter/billing/service/lineservice/service.go`:
- Around line 189-202: ValidateForInvoice currently assumes each element in the
Lines slice is non-nil and will panic if a nil Line is present; update the
mapping function inside ValidateForInvoice to first check if the `line` is nil
(type `Line`) and return a descriptive error (e.g. including the index or
identifier) instead of calling `line.Validate` or `line.ID()`, then proceed to
call `line.Validate(ctx, invoice)` and `line.ID()` only when `line` is non-nil;
reference the ValidateForInvoice function and the Line.Validate / Line.ID
methods when making this change.

🧹 Nitpick comments (2)

openmeter/billing/invoiceline.go (2)

251-252: Consider exporting the edit type (or adding helpers) if external callers should use it.

CloneWithoutDependencies is public, but lineEditFunction isn’t, so packages outside billing can’t pass edits. If you want external customization, exporting the type (or adding exported helper funcs) would keep the API usable. If not, all good.

---

255-277: Skip nil edits to avoid a panic.

A nil edit function will panic on call. A tiny guard keeps this safe and costs nothing.

🛠️ Suggested tweak

-	for _, edit := range edits {
-		edit(clone)
-	}
+	for _, edit := range edits {
+		if edit == nil {
+			continue
+		}
+		edit(clone)
+	}