ADR 0086: a secret reaches a process as a file, and an exception is declared

Closes issue 041 by decision and by code on the same branch: the catalogue
engine refuses a secret in a container's env, and a secret-carrying env-file
unless the container declares its reason; the controller reads all six of
its credentials from files; design 13 states the rule and how it is checked.

02-DECISIONS/0086-a-secret-reaches-a-process-as-a-file.md

@@ -0,0 +1,75 @@
+---
+topic: building it
+status: accepted
+date: 2026-09-21
+deciders: jochen
+reconstructed: false
+extends: 02-DECISIONS/0085-a-secret-is-a-provision.md
+---
+
+# 86. A secret reaches a process as a file, and an exception is declared
+
+## Context
+
+The mesh seals a secret to the machine that uses it and discards the plaintext; the host unseals it
+into a file at 0600 ([ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md)). Then
+the module hands it to its container through an env-file, and the runtime puts it where anything
+on the machine that can talk to the runtime can read it: `docker inspect` prints it, and
+`/proc/<pid>/environ` holds it for the life of the process
+([issue 041](../04-ISSUES/041-a-sealed-credential-ends-up-in-the-process-environment/00-report.md)).
+
+Two facts made this a decision rather than a fix. **It is a supported shape**: placing
+`${secret:…}` in a file a container reads as its environment is what playbook 06 shows, and 36
+containers in 25 catalogue modules do it, the controller's own among them. And **nothing says which
+secrets are exposed**: a reader cannot tell from a manifest whether a credential is protected from
+`inspect` or not, because the manifest looks the same either way. The vault
+([ADR 0085](0085-a-secret-is-a-provision.md)) rests on the seal this undoes.
+
+## Considered Options
+
+1. **Leave it: the environment is where configuration goes.** Rejected — it gives the sealed value
+   back to a routine operation, and the care taken to seal it is then theatre.
+2. **Refuse every secret in an environment.** Rejected — some software reads its configuration
+   from the environment and nothing else, and a rule the catalogue cannot obey is a rule that gets
+   switched off.
+3. **A secret reaches a process as a file; an environment exception is declared, with a reason,
+   and refused otherwise.** Adopted.
+
+## Decision
+
+**A secret reaches a process as a file.** A module mounts the file the host wrote and points the
+program at it; the mesh's own programs accept a `_FILE` twin for every variable that carries a
+credential, the way the controller's store connections already did.
+
+**A container that reads a secret from its environment says so.** The manifest key
+`secrets-in-environment` on the container carries the reason. It is catalogue-level — the host
+never sees it — and it exists so a reader can tell from the manifest which secrets are exposed
+that way and why.
+
+**Everything else is refused.** A `${secret:…}` placeholder inside a container's `env` is never
+filled and is refused outright. A file carrying a secret that a container names in `env-file` is
+refused unless the container declares the exception.
+
+## Consequences
+
+The property *a sealed credential is readable only where it is used* becomes a manifest-level
+fact: true where no exception is declared, stated where one is. The controller reads all six of
+its credentials from files; the 35 other containers are marked with their reason and convert one
+by one where their software accepts a path, which is per-module work under
+[issue 041](../04-ISSUES/041-a-sealed-credential-ends-up-in-the-process-environment/00-report.md).
+
+What got harder: a module author meets one more refusal, and the reason they write is only as
+honest as they are. What is not decided: the reach of the exposure on a node — which identities can
+talk to the runtime — which decides whether a declared exception is a hardening item or something
+sharper.
+
+## How it is checked
+
+The catalogue engine refuses at composition, before anything reaches a machine, and its tests
+refuse both shapes and prove the reason is stripped from what is sent.
+
+## References
+
+- [issue 041](../04-ISSUES/041-a-sealed-credential-ends-up-in-the-process-environment/00-report.md)
+- [ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md), [ADR 0085](0085-a-secret-is-a-provision.md)
+- [`03-DESIGN/01-to-be/13-credentials-and-their-rotation.md`](../03-DESIGN/01-to-be/13-credentials-and-their-rotation.md)