NixOS · Sep 6, 2019 · Sep 6, 2019 · Sep 6, 2019 · Sep 6, 2019 · Sep 6, 2019
diff --git a/nixos/lib/utils.nix b/nixos/lib/utils.nix
@@ -24,4 +24,116 @@ pkgs: with pkgs.lib;
       throw "${shell} is not a shell package"
     else
       shell;
+
+  /* Recurse into a list or an attrset, searching for attrs named like
+     the value of the "attr" parameter, and return an attrset where the
+     names are the corresponding jq path where the attrs were found and
+     the values are the values of the attrs.
+
+     Example:
+       recursiveGetAttrWithJqPrefix {
+         example = [
+           {
+             irrelevant = "not interesting";
+           }
+           {
+             ignored = "ignored attr";
+             relevant = {
+               secret = {
+                 _secret = "/path/to/secret";
+               };
+             };
+           }
+         ];
+       } "_secret" -> { ".example[1].relevant.secret" = "/path/to/secret"; }
+  */
+  recursiveGetAttrWithJqPrefix = item: attr:
+    let
+      recurse = prefix: item:
+        if item ? ${attr} then
+          nameValuePair prefix item.${attr}
+        else if isAttrs item then
+          map (name: recurse (prefix + "." + name) item.${name}) (attrNames item)
+        else if isList item then
+          imap0 (index: item: recurse (prefix + "[${toString index}]") item) item
+        else
+          [];
+    in listToAttrs (flatten (recurse "" item));
+
+  /* Takes an attrset and a file path and generates a bash snippet that
+     outputs a JSON file at the file path with all instances of
+
+     { _secret = "/path/to/secret" }
+
+     in the attrset replaced with the contents of the file
+     "/path/to/secret" in the output JSON.
+
+     When a configuration option accepts an attrset that is finally
+     converted to JSON, this makes it possible to let the user define
+     arbitrary secret values.
+
+     Example:
+       If the file "/path/to/secret" contains the string
+       "topsecretpassword1234",
+
+       genJqSecretsReplacementSnippet {
+         example = [
+           {
+             irrelevant = "not interesting";
+           }
+           {
+             ignored = "ignored attr";
+             relevant = {
+               secret = {
+                 _secret = "/path/to/secret";
+               };
+             };
+           }
+         ];
+       } "/path/to/output.json"
+
+       would generate a snippet that, when run, outputs the following
+       JSON file at "/path/to/output.json":
+
+       {
+         "example": [
+           {
+             "irrelevant": "not interesting"
+           },
+           {
+             "ignored": "ignored attr",
+             "relevant": {
+               "secret": "topsecretpassword1234"
+             }
+           }
+         ]
+       }
+  */
+  genJqSecretsReplacementSnippet = genJqSecretsReplacementSnippet' "_secret";
+
+  # Like genJqSecretsReplacementSnippet, but allows the name of the
+  # attr which identifies the secret to be changed.
+  genJqSecretsReplacementSnippet' = attr: set: output:
+    let
+      secrets = recursiveGetAttrWithJqPrefix set attr;
+    in ''
+      if [[ -h '${output}' ]]; then
+        rm '${output}'
+      fi
+    ''
+    + concatStringsSep
+        "\n"
+        (imap1 (index: name: "export secret${toString index}=$(<'${secrets.${name}}')")
+               (attrNames secrets))
+    + "\n"
+    + "${pkgs.jq}/bin/jq >'${output}' '"
+    + concatStringsSep
+      " | "
+      (imap1 (index: name: ''${name} = $ENV.secret${toString index}'')
+             (attrNames secrets))
+    + ''
+      ' <<'EOF'
+      ${builtins.toJSON set}
+      EOF
+    '';
 }