Share the retry rule and copy budget helpers

[kvch]

Description

Three small extractions so that a second component can reuse the postgres target behaviour the bulk ingest writer already implements: how a failure is classified as retriable, which retry policy applies once the default is resolved, and how large the concurrent COPY budget may be.

Nothing changes about what any of them decide. Each is a move or an extraction, verified by the existing tests.

This is the first of four stacked PRs splitting #1129. On its own it adds no functionality; the consumer arrives in the branch above it.

Related Issue(s)

• Related to Copy passthrough during snapshot mode if no transformers are configured #1129

Type of Change

• 🔧 Refactoring (no functional changes)

Changes Made

• Extract the retrying querier's classification into an exported retrier.IsRetriableError
• Move CopyBudgetReserve and CopyBudgetSize from pkg/wal/processor/postgres to internal/sync, next to the weighted semaphore they size
• Add Config.EffectiveRetryPolicy, returning the postgres writer's retry policy once its default has been applied. retryPolicy stays unexported and remains the single definition.

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass

Checklist

• Code follows project style guidelines
• Self-review completed
• Code is well-commented
• Documentation updated where necessary

[xata-bot]

✅ Approved by the pgstream review agent.

This is a pure mechanical extraction/export of existing logic (retry classification, copy budget sizing, retry policy accessor) with no behavioral change; tests were updated in lockstep and verify identical outputs.

_{tier T1-agent · 4 files / 28 substantive lines · risk low. This is an automated review and does not replace human judgement on anything it escalates.}

[github-actions]

Coverage

Total: 62.7% (±0.0% vs main)

Coverage in packages changed by this PR:

Package	Coverage	Δ
internal/postgres/retrier	92.0%	+0.1%
internal/sync	5.0%	+5.0%
pkg/wal/processor/postgres	88.8%	-0.1%