ADR 0119: a taken tunnel's predecessor is retired once the take is proven

02-DECISIONS/0119-a-taken-tunnels-predecessor-is-retired.md

@@ -0,0 +1,78 @@
+---
+topic: the mesh
+status: accepted
+date: 2026-09-27
+deciders: jochen
+reconstructed: false
+extends: 0105-the-mesh-adopts-the-predecessors-tunnel-in-place.md
+---
+
+# 119. A taken tunnel's predecessor is retired once the take is proven
+
+## Context
+
+[ADR 0105](0105-the-mesh-adopts-the-predecessors-tunnel-in-place.md) has the private network take
+over the tunnel it finds: the found unit stopped and disabled, never flushed, and **its
+configuration left on disk, kept like any held file.** That was the right caution for the take
+itself — if the mesh's interface failed to come up, the host starts the found unit again and the
+peers never notice — and every apply since stops the found unit again should anyone start it.
+
+What it leaves is a predecessor that never finishes leaving. On every machine that has enrolled,
+the tunnel is the mesh's and has been proven so — its interface up with the found key, the peers
+handshaking, the machines resolving and reaching each other over it — and still the predecessor's
+configuration sits where its unit reads it, held for a module that has long since replaced it.
+The predecessor itself is being deprecated. A tunnel that can be started again by one command, with
+a configuration nothing maintains any more, is not a rollback path; it is a second way onto the
+network that nobody is watching. And the hold never ends, so every node report keeps listing it.
+
+## Considered Options
+
+**1. Keep it, as 0105 says.** Rejected: the caution it bought is spent once the take is proven, and
+what remains is a live, unmaintained way back onto the network.
+
+**2. Delete it at the take.** Rejected: the take is exactly the moment the fallback is needed. If
+the mesh's interface does not come up, the host must still be able to raise the found one.
+
+**3. Retire it once the take is proven.** Chosen.
+
+## Decision
+
+**Once the mesh's interface has proven it carries the tunnel, the found interface's configuration
+is removed from where its unit reads it.**
+
+- **Proven means:** the tunnel's state is *taken* — the found unit down and disabled, the mesh's
+  interface up with the found key — and the mesh's interface has completed a handshake with at
+  least one peer. Not before: until then, a failed take still falls back to the found unit.
+- **Retired means:** the configuration file the found unit reads is removed. Its original was
+  already kept, before anything happened to it
+  ([ADR 0100](0100-a-node-in-use-is-adopted-before-it-is-converged.md)), and stays kept; that copy
+  is the record of what the predecessor was, and a person's way back if one is ever wanted.
+- The found unit stays disabled. Without its configuration it cannot raise the interface, so the
+  every-apply stop that guarded against it becomes a check that finds nothing to do.
+- **The hold ends.** What was held for the private network has been replaced; the node stops
+  reporting it.
+- **The mesh never brings it back.** Undeclaring the private network does not restore the found
+  tunnel: the mesh stopped it, and nothing is started on the way out
+  (ADR 0118, in review). A machine whose
+  private network is unassigned has no tunnel until it is assigned again — which is what
+  unassigning it means.
+
+## Consequences
+
+- On every machine that took a tunnel, the predecessor's tunnel configuration disappears at the
+  first apply after the take is proven. Nothing a peer sees changes; the mesh's interface already
+  carries the same key, port, address and peers.
+- A take that is never proven — no peer ever handshakes — keeps the found configuration, and the
+  node says so, so a broken take is visible rather than silently retired.
+- Rolling back to the predecessor's tunnel becomes a deliberate act: copy the kept original back
+  and start its unit. The mesh does neither.
+- 0105's "its configuration stays on disk, kept like any held file" holds until the take is proven,
+  and not after.
+
+## References
+
+- [ADR 0105](0105-the-mesh-adopts-the-predecessors-tunnel-in-place.md): the take, and why it keeps
+  the found configuration during it
+- [ADR 0100](0100-a-node-in-use-is-adopted-before-it-is-converged.md): kept originals
+- ADR 0118 (in review): nothing is started on the way out
+- mesh-host `internal/apply/takeover.go`

02-DECISIONS/README.md

@@ -133,6 +133,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0105** — [The mesh adopts the predecessor's tunnel in place](0105-the-mesh-adopts-the-predecessors-tunnel-in-place.md)
 - **0106** — [The bus is NATS](0106-the-bus-is-nats.md)
 - **0116** — [The bus is built in five steps, and the protocol moves with it](0116-the-bus-is-built-in-five-steps.md)
+- **0119** — [A taken tunnel's predecessor is retired once the take is proven](0119-a-taken-tunnels-predecessor-is-retired.md)
 
 ### Its tiers, from the bottom up