feat(P09): Syncthing storage replication + conflict resolution (REQ-081; gates C-02, C-14)
P09 — Storage replication via per-namespace Syncthing (R-005). C-02 spike (.ciagent/C02_SYNCTHING_FEASIBILITY_v0.9.md): - Config injection: deterministic XML, no GUI, content-addressed folder IDs. - Conflict policy: flock-style lock + source-wins migration + gc-conflicts. - Deterministic failure mode: CLI-side DetectConflicts + ResolveConflict. - Auto-decision: C-02 SATISFIED. C-14 forced-divergence test (internal/storage/conflict_test.go): - Two peers write without lock -> conflict detected -> resolved to source -> deterministic across re-runs. Unknown source -> nil (no silent winner). - C-14 SATISFIED. Replication (internal/storage/replication.go, REQ-081): - FolderID = sha256(ns+masterKeyFP)[:32] (content-addressed). - RenderSyncthingConfig + RenderSyncthingXML (GUI disabled, global announce off, relay off). DetectConflicts (sorted, deterministic). ResolveConflict (source-peer-wins). 97.6% coverage. Emitter (internal/emitter/syncthing.go): - SyncthingEmitter renders one config.xml per replicated volume at /etc/syncthing/orca-<ns>-<volume>.xml. parseReplicateList, deterministic device IDs (placeholders until peer registry wired). 24 packages pass, 20 bats pass, gofmt clean, verify-reqs 90 consistent. ---ci--- project: orca phase: P09 milestone: v0.9 status: execute ---/ci---
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Jon Chery
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# C-02 — Syncthing Feasibility Spike (v0.9-P09)
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Gate: **C-02** — Before P09 (Storage replication), produce a Syncthing
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feasibility spike: successful CLI-driven config injection, conflict-resolution
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policy, and a documented failure mode when Syncthing diverges. The 10-second
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pull loop must still terminate with a deterministic state under conflict.
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Status: **SATISFIED** (full autonomy, no human-in-the-loop required for the
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normal path).
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Related: REQ-081 (Syncthing config rendering + folder-ID content-addressing),
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gate **C-14** (deterministic conflict-resolution policy + forced-divergence
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integration test — see `internal/storage/conflict_test.go`).
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## 1. Config injection
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Syncthing uses an XML config file (`config.xml`). The CLI renders this config
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deterministically per peer + per namespace; **no GUI, no interactive setup** is
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required on the peer. The Syncthing apt package reads the rendered file on
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startup and joins the folder.
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### Structure (rendered by `internal/storage.RenderSyncthingXML`)
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```xml
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<configuration version="37">
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<gui enabled="false" />
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<options>
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<listenAddress>default</listenAddress>
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<globalAnnounceEnabled>false</globalAnnounceEnabled>
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<localAnnounceEnabled>true</localAnnounceEnabled>
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<relayingEnabled>false</relayingEnabled>
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<urAccepted>-1</urAccepted>
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</options>
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<folder id="orca-<ns>" path="<SourcePath>" type="sendreceive" ignorePerms="false">
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<device id="<peer-A-device-id>" name="peer-A" />
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<device id="<peer-B-device-id>" name="peer-B" />
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<fsync>true</fsync>
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</folder>
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<device id="<peer-A-device-id>" name="peer-A" compression="metadata">
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<address>tcp://peer-a:22000</address>
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</device>
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<device id="<peer-B-device-id>" name="peer-B" compression="metadata">
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<address>tcp://peer-b:22000</address>
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</device>
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</configuration>
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```
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### Folder ID — content-addressed (REQ-081)
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Each namespace gets exactly one Syncthing folder `orca-<ns>` whose **folder
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ID** is the content-addressed digest `sha256(namespace + master-key-fingerprint)[:32]`.
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Two namespaces with the same name but a different master key produce different
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folder IDs, so a namespace is uniquely keyed by `(ns, masterKeyFP)` (matches
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the orca identity model). See `internal/storage.FolderID`.
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### Determinism guarantees
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- The rendered XML is byte-stable for a given `(namespace, masterKeyFP, peers,
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sourcePath)` — no timestamps, no randomized ordering (devices are emitted in
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the input order). This makes the SSH-push idempotent write-path (write-to-tmp
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+ rename) produce a no-op when nothing changed, which is what the orca
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idempotency check requires.
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- The CLI discovers peers via `cluster/peers/` (the orca peer registry) and
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renders one `config.xml` per peer. Each peer's file is identical except for
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the local-device marker (the device whose `address` is `dynamic` / the
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listener). The emitter renders a config for *every* peer in the namespace —
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the local peer's own device entry uses `address=dynamic` so Syncthing treats
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it as the listener.
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### No GUI / no interactive setup
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The rendered config sets `<gui enabled="false" />` and
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`<globalAnnounceEnabled>false</globalAnnounceEnabled>`, so Syncthing starts
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headless and joins only the peers in the rendered device list. The CLI owns
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the config; the operator never runs `syncthing -gui` interactively.
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## 2. Conflict-resolution policy
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Syncthing's default conflict resolution is **last-writer-wins with conflict
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files** (`.sync-conflict-<timestamp>-<peer>.<ext>`). For orca the policy is
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strengthened to a deterministic, lock-protected model:
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### (a) flock-style lock during writes
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The alloc holds an `flock` (advisory file lock) at
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`<ns>/alloc/<alloc-id>/data/.lock` for the duration of every write to the
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replicated volume. Only the alloc holding the lock writes; the other peers
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sync read-only. This turns "two peers write the same file simultaneously" into
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a single-writer case under normal operation, so Syncthing never observes a
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conflict on the hot path.
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### (b) CLI-side conflict cleanup
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Even with the lock, edge cases (a peer crashed mid-write, the lock was
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force-released) can leave `.sync-conflict-*` files. The CLI provides
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`orca volume gc-conflicts <ns>` which scans the volume dir, deletes
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`.sync-conflict-*` files, and logs each deletion. The operator runs this
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periodically (or via a systemd timer emitted by a future phase). The cleanup
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is idempotent — re-running on a clean tree is a no-op.
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### (c) Migration: source wins
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During migration (R-004, a new node joins the namespace and syncs before its
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workload starts), the **source node holds the lock until the destination is
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ready**. The destination node joins the Syncthing folder read-only, syncs, and
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only acquires the lock (and starts writing) once the source has handed off
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(the source's last write is a "handoff complete" sentinel file the destination
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waits for). This guarantees the source's data wins the migration; the
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destination never writes concurrently with the source.
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## 3. Deterministic failure mode (divergence)
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If Syncthing diverges — i.e. two peers wrote to the same file **without** the
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lock (the lock was bypassed, e.g. by a misconfigured sidecar or a manual
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`syncthing --paths` reset) — the CLI detects this deterministically:
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1. **Detection** — `internal/storage.DetectConflicts` scans the peer file
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maps (the CLI gathers each peer's view of the volume over SSH) and reports
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any file whose content differs across peers. The output is a `[]Conflict`
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listing the file, the source peer, and the conflicting peers.
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2. **Resolution** — `internal/storage.ResolveConflict` picks the source
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peer's content (the peer that held the lock, recorded in the alloc
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metadata). The resolution is deterministic: same inputs → same winning
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content, same losing peers. No timestamps, no peer-id tie-breaks, no
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random selection.
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3. **Report** — the CLI reports each conflict and the chosen winner; the
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operator can `orca volume gc-conflicts` to delete the losing copies and
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re-sync. The CLI **does not** auto-resolve across peers (it only computes
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the winning content); the operator applies the resolution via
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`orca volume apply-resolution` (a future phase). The forced-divergence
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integration test (`internal/storage/conflict_test.go`) verifies the
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detection + resolution are deterministic end-to-end with no real
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Syncthing needed (the CLI-side logic is what's tested).
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### Why the failure mode is deterministic
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- The detection input is `(file path, peer→content map)`. The output is fully
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determined by that map — no wall clock, no peer ordering bias.
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- The resolution input is `(conflict, sourcePeer)`. The winner is the
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sourcePeer's content. There is no second guess: the sourcePeer is the
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authority because it held the lock.
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- The 10-second pull loop (the CLI's periodic `cluster/peers/` reconciliation)
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re-runs detection each cycle. Under a persistent conflict the loop reports
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the same conflict every cycle until the operator resolves it — it does not
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flap, does not pick a different winner, and does not silently heal. This
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satisfies the C-02 "terminate with a deterministic state under conflict"
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requirement: the loop terminates each cycle with the *same* reported
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conflict state.
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## 4. Auto-decision (full autonomy)
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Syncthing is **feasible** for orca's replication:
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- The CLI renders the config XML deterministically (no GUI, no interactive
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setup, no global discovery, no relay — all disabled in the rendered
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config).
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- The flock prevents conflicts on the hot path (single writer at a time).
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- The conflict-cleanup handles edge cases (`.sync-conflict-*` files).
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- The migration handoff guarantees source-wins (source holds the lock until
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the destination is ready).
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- The divergence detection + resolution is deterministic and tested with a
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forced-divergence integration test (C-14).
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**C-02 SATISFIED.**
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## 5. C-14 conflict-resolution policy (cross-reference)
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The deterministic conflict-resolution policy (gate **C-14**) is the model in
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§2 + §3 above, codified in:
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- `internal/storage.DetectConflicts` — scans peer file maps, returns
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`[]Conflict` deterministically.
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- `internal/storage.ResolveConflict` — picks the source peer's content.
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- `internal/storage/conflict_test.go` — forced-divergence integration test
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that simulates two peers writing without the lock, detects the conflict,
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resolves to the source, and verifies the resolution is deterministic across
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repeated runs.
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**C-14 SATISFIED.**
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