fix(ops/irccat): Avoid permissions issue with LoadCredentials= r/3191

The DynamicUser + Group configuration does not work as planned, thus
the systemd LoadCredentials feature is used instead which makes the
file (which itself is only readable by root) available in a
memory-backed location only readable by the service.

The secret is only available to `ExecStart` commands, so units using
this feature can not be used with pre/post units and the like if those
commands need secrets.

To accommodate this, the merge of configuration files has been moved
into the service launch script, which is now the ExecStart= process.

For details take a look at https://www.freedesktop.org/software/systemd/man/systemd.exec.html#LoadCredential=ID:PATH

Change-Id: I693fe5677cc0d63c7aa485c2c7472457c5262166

2 files changed, 8 insertions, 15 deletions

diff --git a/ops/machines/whitby/default.nix b/ops/machines/whitby/default.nix
index 381980fd37..41b53fa984 100644
--- a/ops/machines/whitby/default.nix
+++ b/ops/machines/whitby/default.nix
@@ -209,6 +209,7 @@ in {
     in {
       clbot.file = secretFile "clbot";
       gerrit-queue.file = secretFile "gerrit-queue";
+      irccat.file = secretFile "irccat";
       owothia.file = secretFile "owothia";
 
       buildkite-agent-token = {
@@ -221,12 +222,6 @@ in {
         file = secretFile "clbot-ssh";
         owner = "clbot";
       };
-
-      irccat = {
-        file = secretFile "irccat";
-        mode = "0440";
-        group = "irccat";
-      };
     };
 
   # Automatically collect garbage from the Nix store.
diff --git a/ops/modules/irccat.nix b/ops/modules/irccat.nix
index 9d3eea53c0..9b4b96d3ad 100644
--- a/ops/modules/irccat.nix
+++ b/ops/modules/irccat.nix
@@ -11,15 +11,17 @@ let
   # then recursively merge it with an on-disk secret using jq on
   # service launch.
   configJson = pkgs.writeText "irccat.json" (builtins.toJSON cfg.config);
-  configMerge = pkgs.writeShellScript "merge-irccat-config" ''
-    if [ ! -f "${cfg.secretsFile}" ]; then
+  mergeAndLaunch = pkgs.writeShellScript "merge-irccat-config" ''
+    if [ ! -f "$CREDENTIALS_DIRECTORY/secrets" ]; then
       echo "irccat secrets file is missing"
       exit 1
     fi
 
     # jq's * is the recursive merge operator
-    ${pkgs.jq}/bin/jq -s '.[0] * .[1]' ${configJson} ${cfg.secretsFile} \
+    ${pkgs.jq}/bin/jq -s '.[0] * .[1]' ${configJson} "$CREDENTIALS_DIRECTORY/secrets" \
       > /var/lib/irccat/irccat.json
+
+    exec ${depot.third_party.irccat}/bin/irccat
   '';
 in {
   options.services.depot.irccat = {
@@ -40,20 +42,16 @@ in {
   config = lib.mkIf cfg.enable {
     systemd.services.irccat = {
       inherit description;
-      preStart = "${configMerge}";
-      script = "${depot.third_party.irccat}/bin/irccat";
       wantedBy = [ "multi-user.target" ];
 
       serviceConfig = {
+        ExecStart = "${mergeAndLaunch}";
         DynamicUser = true;
-        Group = "irccat";
         StateDirectory = "irccat";
         WorkingDirectory = "/var/lib/irccat";
+        LoadCredential = "secrets:${cfg.secretsFile}";
         Restart = "always";
       };
     };
-
-    # Create a real group to grant access to secrets to.
-    users.groups.irccat = {};
   };
 }