prometheus-blackbox-exporter: fixing path issue

This fixes an issue with a recent addition of a config file
check in c28ded36ef.

Previously it was possible to supply a path as a string
to `configFile`. Now it will fail checking the config file
during evaluation of the module due to sandboxing.

A toggle to disable the check, more informative log messages
and handling for various configFile values are added.

nixos/modules/services/monitoring/prometheus/exporters/blackbox.nix

@@ -3,16 +3,34 @@
 with lib;
 
 let
+  logPrefix = "services.prometheus.exporter.blackbox";
   cfg = config.services.prometheus.exporters.blackbox;
 
-  checkConfig = file: pkgs.runCommand "checked-blackbox-exporter.conf" {
-    preferLocalBuild = true;
-    buildInputs = [ pkgs.buildPackages.prometheus-blackbox-exporter ]; } ''
-    ln -s ${file} $out
-    blackbox_exporter --config.check --config.file $out
-  '';
-in
-{
+  # This ensures that we can deal with string paths, path types and
+  # store-path strings with context.
+  coerceConfigFile = file:
+    if (builtins.isPath file) || (lib.isStorePath file) then
+      file
+    else
+      (lib.warn ''
+        ${logPrefix}: configuration file "${file}" is being copied to the nix-store.
+        If you would like to avoid that, please set enableConfigCheck to false.
+      '' /. + file);
+  checkConfigLocation = file:
+    if lib.hasPrefix "/tmp/" file then
+      throw
+      "${logPrefix}: configuration file must not reside within /tmp - it won't be visible to the systemd service."
+    else
+      true;
+  checkConfig = file:
+    pkgs.runCommand "checked-blackbox-exporter.conf" {
+      preferLocalBuild = true;
+      buildInputs = [ pkgs.buildPackages.prometheus-blackbox-exporter ];
+    } ''
+      ln -s ${coerceConfigFile file} $out
+      blackbox_exporter --config.check --config.file $out
+    '';
+in {
   port = 9115;
   extraOpts = {
     configFile = mkOption {
@@ -21,14 +39,29 @@ in
         Path to configuration file.
       '';
     };
+    enableConfigCheck = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to run a correctness check for the configuration file. This depends
+        on the configuration file residing in the nix-store. Paths passed as string will
+        be copied to the store.
+      '';
+    };
   };
-  serviceOpts = {
+
+  serviceOpts = let
+    adjustedConfigFile = if cfg.enableConfigCheck then
+      checkConfig cfg.configFile
+    else
+      checkConfigLocation cfg.configFile;
+  in {
     serviceConfig = {
       AmbientCapabilities = [ "CAP_NET_RAW" ]; # for ping probes
       ExecStart = ''
         ${pkgs.prometheus-blackbox-exporter}/bin/blackbox_exporter \
           --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
-          --config.file ${checkConfig cfg.configFile} \
+          --config.file ${adjustedConfigFile} \
           ${concatStringsSep " \\\n  " cfg.extraFlags}
       '';
       ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";