Document deployment-managed cache permissions

docs/operations.md

@@ -50,11 +50,45 @@ directory selects a per-user root:
 
 The field definitions and configuration examples are in [Configuration](config.md).
 On supported Unix systems, output directories and files are created with
-requested modes **0755** and **0644**. Chunk-plan, checkpoint, and debug
-directories and files use **0700** and **0600**. The operating system's umask
-may impose stricter output modes. Cache and debug roots may contain sensitive
-source-derived data, so provision them for one trusted account or service. An
-output bundle can also contain source content when its JSON output enables
+requested modes **0755** and **0644**; debug directories and files use
+**0700** and **0600**. The operating system's umask may impose stricter output
+or debug modes. Chunk-plan and checkpoint caches instead request maximum modes
+of **0777** for new directories and **0666** for new files, including atomic
+temporary files. The kernel applies the service's umask and applicable default
+ACLs when those entries are created.
+
+## Cache Provisioning
+
+Cache locations select where Notarius writes, not who can access them. A
+private service can use a restrictive umask:
+
+~~~sh
+umask 0077
+notarius run example --config /etc/notarius/config.yml --input /srv/input.txt
+~~~
+
+A trusted-group deployment can provision a setgid cache root and use a
+group-oriented umask (or an equivalent default ACL):
+
+~~~sh
+install -d -m 2770 -g notarius-operators /srv/notarius/chunk-plans /srv/notarius/checkpoints
+umask 0007
+notarius run example --config /etc/notarius/config.yml --input /srv/input.txt
+~~~
+
+Setgid and default ACLs control inherited group ownership and access policy;
+Notarius does not copy a parent directory's literal mode bits. Cache state is
+reconstructible but can contain source-derived data, so operators own its
+access and retention policy. Existing cache inodes retain their prior modes
+after an upgrade. Recreate a cache beneath correctly provisioned parents, or
+stop writers and perform a deliberate administrative migration when a
+whole-tree change is required.
+
+Cache publication remains atomic and stored records remain validated. Those
+guarantees do not add cross-process locking or transactional coordination for
+simultaneous writers to one checkpoint identity.
+
+Output bundles can also contain source content when JSON output enables
 evidence publication. Apply an appropriate umask and output-root access policy
 before enabling that option; the requested output modes alone may not be
 suitable for transcript-bearing bundles.