[ENG-3999] Support IPv6 filer resolvers in drive mounts - #260

🧪 Testing Guide

What this PR addresses

Sandbox drive mounts failed on IPv6-only environments because the filer address parser in getFilerAddress() only accepted IPv4 nameservers (validated by checking for four dot-separated components). Production Blaxel sandboxes now receive IPv6 resolver addresses (e.g. 2600:1f14:c75:3900::301), causing blfs mount to never start.

This PR:

• Replaces the IPv4-only validation with net/netip.ParseAddr, accepting both IPv4 and IPv6.
• Extracts parseFilerAddress and formatFilerServerAddress as testable helpers.
• Preserves SeaweedFS's host:http.grpc address format for both address families.

Steps to reproduce the original issue

1. Deploy a sandbox in an environment where /etc/resolv.conf contains only an IPv6 nameserver (e.g. nameserver 2600:1f14:c75:3900::301).
2. Attempt to mount an Agent Drive (trigger MountDrive).
3. Before this PR: The mount fails with "no valid nameserver found in /etc/resolv.conf" because the IPv6 address doesn't have four dot-separated parts.

What to verify (expected behavior)

1. Unit tests pass:

   cd sandbox-api && go test ./src/handler/drive/ -run "TestParseFilerAddress|TestFormatFilerServerAddress" -v

   All 6 TestParseFilerAddress cases (IPv4, IPv6, zoned IPv6, malformed fallback, directive mismatch, missing nameserver) and both TestFormatFilerServerAddress cases (IPv4 and IPv6) should pass.

2. IPv4 environments still work: In a sandbox with an IPv4 resolver, drive mounts continue to work as before (no regression).

3. IPv6 environments now work: In a sandbox with an IPv6-only resolver, the filer address is correctly parsed and the -filer flag is formatted as <ipv6>:49200.49201 — SeaweedFS handles bracket-wrapping internally.

4. Format correctness: Verify that formatFilerServerAddress produces:

   • 172.16.1.126:49200.49201 for IPv4
   • 2600:1f14:c75:3900::301:49200.49201 for IPv6 (unbracketed, per SeaweedFS's parser convention)

5. No other /etc/resolv.conf directives are misinterpreted: Lines like nameserver-proxy or options should be skipped (covered by tests).

✅ Linked to Linear issue ENG-3999 — status already In Progress.

Interaction Flow

sequenceDiagram
    participant MD as MountDrive()
    participant GFA as getFilerAddress()
    participant PFA as parseFilerAddress()
    participant RC as /etc/resolv.conf
    participant FMT as formatFilerServerAddress()
    participant SW as SeaweedFS (blfs)

    MD->>GFA: resolve filer address
    GFA->>PFA: parse resolv.conf content
    PFA->>RC: read nameserver entries
    RC-->>PFA: nameserver lines (IPv4 or IPv6)
    PFA->>PFA: netip.ParseAddr() validates address
    PFA-->>GFA: valid IP string (v4 or v6)
    GFA-->>MD: filer address
    MD->>FMT: format server address
    FMT->>FMT: append ":httpPort.grpcPort"
    FMT-->>MD: "host:49200.49201"
    MD->>SW: mount with filer server address

Summary

The PR refactors the drive mount filer resolution to support IPv6 nameservers in /etc/resolv.conf. The key flow is:

1. MountDrive calls getFilerAddress() which now delegates to the new parseFilerAddress() for parsing resolv.conf entries.
2. parseFilerAddress() uses netip.ParseAddr() (replacing manual dot-split validation) to accept both IPv4 and IPv6 addresses, including zone identifiers.
3. The validated address is passed to the new formatFilerServerAddress() which appends SeaweedFS's host:http.grpc port format.
4. The formatted address is used to mount the drive via SeaweedFS (blfs).

This ensures sandboxes with IPv6-only resolvers can successfully mount Agent Drives.

Parking this pending owner review — @cploujoux, this is in your lane (ENG-3999), so handing it over rather than merging it myself.

What is verified:

• merges cleanly with current main; go build ./..., go vet ./... and go test ./src/handler/drive all pass
• the full sandbox-api suite shows only mkdir /var/log/sandbox-api: permission denied failures in src/handler/process, and those reproduce identically on origin/main (local non-root environment, unrelated to this change)
• the IPv4 path is unchanged: the format string is still %s:49200.49201 and netip.ParseAddr(...).String() returns the same literal for IPv4. The old four-dot check also accepted invalid input such as 1.2.3.junk, so this is strictly tighter
• no overlap with cploujoux/sandbox-api-ipv6-autoforward, which adds src/lib/networking/dualstack.go on the listener side and does not touch getFilerAddress

What is NOT verified, and why I stopped:

• that blfs/SeaweedFS actually mounts with an unbracketed IPv6 filer. The split-on-final-colon reasoning is sound but it is an argument, not an execution — and that is the entire point of the change
• I tried to verify on dev first and could not: three sandboxes in a dev workspace (blaxel/base-image:latest x2, blaxel/node:latest x1, eu-dub-1) all reached FAILED. Cause is scheduling, not this PR:

reason: FailedScheduling
0/9 nodes are available: 2 node(s) didn't match Pod's node affinity/selector,
7 node(s) had untolerated taint(s). no new claims to deallocate,
preemption: 0/9 nodes are available: 9 Preemption is not helpful for scheduling.

Pod sbx-eng3999-probe3-gokind, namespace dev-onboarder, cluster executionplanev3-dev-eu-west-1, label blaxel-node-type-override: kraft. All probe sandboxes were deleted.

One thing to weigh before merging: a push to main sets BL_ENV=prod in build.yaml, builds mk3 images into the prod image bucket, and runs publish-sandbox.sh with TAG: latest, which controlplane picks up automatically. Worst case if the IPv6 assumption is wrong is that IPv6 mounts keep failing; IPv4 behaviour is provably unchanged.