nixos/postgresql: check config file syntax at build time

nixos/doc/manual/release-notes/rl-2009.xml

@@ -543,6 +543,14 @@ systemd.services.nginx.serviceConfig.ReadWritePaths = [ "/var/www" ];
       will have changed.
     </para>
    </listitem>
+   <listitem>
+    <para>
+      The syntax of the PostgreSQL configuration file is now checked at build
+      time. If your configuration includes a file inaccessible inside the build
+      sandbox, set <varname>services.postgresql.checkConfig</varname> to
+        <literal>false</literal>.
+    </para>
+   </listitem>
    <listitem>
     <para>
       The rkt module has been removed, it was archived by upstream.

nixos/modules/services/databases/postgresql.nix

@@ -18,7 +18,12 @@ let
     else toString value;
 
   # The main PostgreSQL configuration file.
-  configFile = pkgs.writeText "postgresql.conf" (concatStringsSep "\n" (mapAttrsToList (n: v: "${n} = ${toStr v}") cfg.settings));
+  configFile = pkgs.writeTextDir "postgresql.conf" (concatStringsSep "\n" (mapAttrsToList (n: v: "${n} = ${toStr v}") cfg.settings));
+
+  configFileCheck = pkgs.runCommand "postgresql-configfile-check" {} ''
+    ${cfg.package}/bin/postgres -D${configFile} -C config_file >/dev/null
+    touch $out
+  '';
 
   groupAccessAvailable = versionAtLeast postgresql.version "11.0";
 
@@ -53,6 +58,12 @@ in
         '';
       };
 
+      checkConfig = mkOption {
+        type = types.bool;
+        default = true;
+        description = "Check the syntax of the configuration file at compile time";
+      };
+
       dataDir = mkOption {
         type = types.path;
         defaultText = "/var/lib/postgresql/\${config.services.postgresql.package.psqlSchema}";
@@ -309,6 +320,8 @@ in
      "/share/postgresql"
     ];
 
+    system.extraDependencies = lib.optional (cfg.checkConfig && pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) configFileCheck;
+
     systemd.services.postgresql =
       { description = "PostgreSQL Server";
 
@@ -332,7 +345,7 @@ in
               touch "${cfg.dataDir}/.first_startup"
             fi
 
-            ln -sfn "${configFile}" "${cfg.dataDir}/postgresql.conf"
+            ln -sfn "${configFile}/postgresql.conf" "${cfg.dataDir}/postgresql.conf"
             ${optionalString (cfg.recoveryConfig != null) ''
               ln -sfn "${pkgs.writeText "recovery.conf" cfg.recoveryConfig}" \
                 "${cfg.dataDir}/recovery.conf"