packaging: ship fips.yaml as an example, not a dpkg conf-file

Installing /etc/fips/fips.yaml as a live dpkg conf-file collides with a
configuration-management-rendered or operator-edited config on upgrade:
dpkg either prompts interactively (keep/replace), stalling unattended
upgrades, or clobbers the local file. Ship the default config as
/usr/share/doc/fips/fips.yaml.example (mode 644) and drop it from
conf-files. postinst now seeds /etc/fips/fips.yaml from the example only
when it does not already exist (mode 600), yielding to any existing
config without a prompt or clobber. Add ConditionPathExists for the
config to the service unit so a missing config skips the unit cleanly
rather than crash-looping.

Cargo.toml

@@ -71,7 +71,7 @@ assets = [
     ["target/release/fips", "/usr/bin/", "755"],
     ["target/release/fipsctl", "/usr/bin/", "755"],
     ["target/release/fipstop", "/usr/bin/", "755"],
-    ["packaging/common/fips.yaml", "/etc/fips/fips.yaml", "600"],
+    ["packaging/common/fips.yaml", "/usr/share/fips/fips.yaml.example", "644"],
     ["packaging/common/hosts", "/etc/fips/hosts", "644"],
     ["packaging/common/fips.nft", "/etc/fips/fips.nft", "644"],
     ["packaging/debian/fips.service", "/lib/systemd/system/fips.service", "644"],
@@ -84,7 +84,7 @@ assets = [
     ["packaging/debian/fips-gateway.service", "/lib/systemd/system/fips-gateway.service", "644"],
     ["docs/design/fips-security.md", "/usr/share/doc/fips/fips-security.md", "644"],
 ]
-conf-files = ["/etc/fips/fips.yaml", "/etc/fips/hosts", "/etc/fips/fips.nft"]
+conf-files = ["/etc/fips/hosts", "/etc/fips/fips.nft"]
 
 [dev-dependencies]
 tempfile = "3.15"

packaging/README.md

@@ -33,10 +33,20 @@ packaging/
 ### Debian/Ubuntu (`.deb`)
 
 Built with [cargo-deb](https://github.com/kornelski/cargo-deb). Installs
-`fips`, `fipsctl`, and `fipstop` to `/usr/bin/`, places config at
+`fips`, `fipsctl`, and `fipstop` to `/usr/bin/`, and enables the systemd
-`/etc/fips/fips.yaml` (preserved on upgrade), and enables the systemd
 service.
 
+The default configuration ships as an example at
+`/usr/share/fips/fips.yaml.example` and is **not** a dpkg conf-file.
+(It is deliberately **not** under `/usr/share/doc`, which minimal and
+container installs path-exclude, since the postinst reads it at install
+time.)
+On install, `postinst` seeds `/etc/fips/fips.yaml` (mode 600) from the
+example **only if it does not already exist**, so a configuration that
+was rendered by configuration management or edited by an operator is
+never prompted for or clobbered on upgrade. To reset to defaults, remove
+`/etc/fips/fips.yaml` and reinstall, or copy the example back manually.
+
 ```sh
 # Build
 make deb

packaging/debian/fips.service

@@ -3,6 +3,11 @@ Description=FIPS Mesh Network Daemon
 After=network-online.target
 Wants=network-online.target
 
+# The config file is no longer a packaged conf-file; postinst seeds it
+# if absent. Skip the unit (inactive, not failed) rather than crash-loop
+# if it is ever missing.
+ConditionPathExists=/etc/fips/fips.yaml
+
 [Service]
 Type=simple
 ExecStart=/usr/bin/fips --config /etc/fips/fips.yaml

packaging/debian/postinst

@@ -9,6 +9,16 @@ case "$1" in
             groupadd --system fips
         fi
 
+        # Seed /etc/fips/fips.yaml from the shipped example only if it
+        # does not already exist. The live config is no longer a dpkg
+        # conf-file; this copy-if-absent yields to any operator- or
+        # configuration-management-rendered file and never clobbers it.
+        if [ ! -e /etc/fips/fips.yaml ]; then
+            install -m 600 -o root -g root \
+                /usr/share/fips/fips.yaml.example \
+                /etc/fips/fips.yaml
+        fi
+
         # Drop-in directory for operator nftables rules included by
         # /etc/fips/fips.nft. Empty by default; the include glob matches
         # nothing cleanly out of the box.