NM-273: Vnat pool assignments fix

[abhishek9686]

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Checklist before requesting a review

• My changes affect only 10 files or less.
• I have performed a self-review of my code and tested it.
• If it is a new feature, I have added thorough tests, my code is <= 1450 lines.
• If it is a bugfix, my code is <= 200 lines.
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[tenki-reviewer]

Tenki Code Review - Complete

Files Reviewed: 4
Findings: 2

By Severity:

• 🟠 Medium: 2

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This PR refactors Virtual NAT (VNAT) pool allocation by centralizing the logic in logic/networks.go and reusing it from both migrate/migrate.go and controllers/network.go, while also fixing the updateNetwork handler to return the correct server-side network object. Two minor correctness issues were identified: a missing overflow truncation in the new bigIntToIP helper (compared to the equivalent in pro/logic/egress.go), and a potentially unintentional removal of a feature-flag guard in the migration path.

Files Reviewed (4 files)

controllers/network.go
logic/networks.go
migrate/migrate.go
pro/logic/egress.go

[tenki-reviewer]

Overview

The PR consolidates VNAT pool allocation logic — previously duplicated between migrate/migrate.go and pro/logic/egress.go — into exportable functions in logic/networks.go (AllocateUniqueVNATPool, AllocateUniquePoolFromFallback, NthSubnet). It also fixes a response encoding bug in updateNetwork and cleans up the migration function.

Positive Changes

• updateNetwork response fix (controllers/network.go:826): The handler previously returned the raw client payload, missing server-managed fields (ID, CreatedAt, VNAT settings). Switching to netOld — which is mutated in-place by UpdateNetwork() — correctly returns the persisted DB state.
• Refactoring: Centralizing AllocateUniquePoolFromFallback, NthSubnet, and related helpers avoids duplication between migrate/ and logic/.
• Uniqueness guarantee: AllocateUniqueVNATPool now loads all existing network pools before allocating, preventing conflicts at network creation time.

Issues Found

bigIntToIP missing overflow guard (logic/networks.go:374)

The bigIntToIP function in logic/networks.go pads short byte slices but does not truncate when big.Int.Bytes() returns more bytes than expected (overflow). The identical function in pro/logic/egress.go includes this truncation guard. Without it, an overflowed IPv4 address could produce a nil result from To4(), causing NthSubnet to return a nil IP — which the pool allocation loop correctly skips, but means a valid pool slot is silently lost.

Migration no longer gated by feature flag (migrate/migrate.go:79)

The PR removes the check if !logic.GetFeatureFlags().EnableOverlappingEgressRanges { return } from initializeVirtualNATSettings(). This means the migration now populates VNAT fields for all Pro networks on startup, regardless of whether the overlapping egress feature is enabled. If intentional (pre-populating for future feature activation), this should be documented with a comment. If unintentional, the guard should be restored.