Settings: Support null and undefined env var substitutions

CHANGELOG.md

@@ -10,6 +10,19 @@
   connections to the MySQL/MariaDB server (by default) instead of 1. This might
   cause Etherpad to crash with a "ER_CON_COUNT_ERROR: Too many connections"
   error if your server is configured with a low connection limit.
+* Changes to environment variable substitution in `settings.json` (see the
+  documentation comments in `settings.json.template` for details):
+  * An environment variable set to the string "null" now becomes `null` instead
+    of the string "null". Similarly, if the environment variable is unset and
+    the default value is "null" (e.g., `"${UNSET_VAR:null}"`), the value now
+    becomes `null` instead of the string "null". It is no longer possible to
+    produce the string "null" via environment variable substitution.
+  * An environment variable set to the string "undefined" now causes the setting
+    to be removed instead of set to the string "undefined". Similarly, if the
+    environment variable is unset and the default value is "undefined" (e.g.,
+    `"${UNSET_VAR:undefined}"`), the setting is now removed instead of set to
+    the string "undefined". It is no longer possible to produce the string
+    "undefined" via environment variable substitution.
 
 ### Notable enhancements
 

settings.json.docker

@@ -24,6 +24,29 @@
  *
  * This is useful, for example, when running in a Docker container.
  *
+ * DETAILED RULES:
+ *   - If the environment variable is set to the string "true" or "false", the
+ *     value becomes Boolean true or false.
+ *   - If the environment variable is set to the string "null", the value
+ *     becomes null.
+ *   - If the environment variable is set to the string "undefined", the setting
+ *     is removed entirely, except when used as the member of an array in which
+ *     case it becomes null.
+ *   - If the environment variable is set to a string representation of a finite
+ *     number, the string is converted to that number.
+ *   - If the environment variable is set to any other string, including the
+ *     empty string, the value is that string.
+ *   - If the environment variable is unset and a default value is provided, the
+ *     value is as if the environment variable was set to the provided default:
+ *       - "${UNSET_VAR:}" becomes the empty string.
+ *       - "${UNSET_VAR:foo}" becomes the string "foo".
+ *       - "${UNSET_VAR:true}" and "${UNSET_VAR:false}" become true and false.
+ *       - "${UNSET_VAR:null}" becomes null.
+ *       - "${UNSET_VAR:undefined}" causes the setting to be removed (or be set
+ *         to null, if used as a member of an array).
+ *   - If the environment variable is unset and no default value is provided,
+ *     the value becomes null.
+ *
  * EXAMPLE:
  *    "port":     "${PORT:9001}"
  *    "minify":   "${MINIFY}"

settings.json.template

@@ -15,6 +15,29 @@
  *
  * This is useful, for example, when running in a Docker container.
  *
+ * DETAILED RULES:
+ *   - If the environment variable is set to the string "true" or "false", the
+ *     value becomes Boolean true or false.
+ *   - If the environment variable is set to the string "null", the value
+ *     becomes null.
+ *   - If the environment variable is set to the string "undefined", the setting
+ *     is removed entirely, except when used as the member of an array in which
+ *     case it becomes null.
+ *   - If the environment variable is set to a string representation of a finite
+ *     number, the string is converted to that number.
+ *   - If the environment variable is set to any other string, including the
+ *     empty string, the value is that string.
+ *   - If the environment variable is unset and a default value is provided, the
+ *     value is as if the environment variable was set to the provided default:
+ *       - "${UNSET_VAR:}" becomes the empty string.
+ *       - "${UNSET_VAR:foo}" becomes the string "foo".
+ *       - "${UNSET_VAR:true}" and "${UNSET_VAR:false}" become true and false.
+ *       - "${UNSET_VAR:null}" becomes null.
+ *       - "${UNSET_VAR:undefined}" causes the setting to be removed (or be set
+ *         to null, if used as a member of an array).
+ *   - If the environment variable is unset and no default value is provided,
+ *     the value becomes null.
+ *
  * EXAMPLE:
  *    "port":     "${PORT:9001}"
  *    "minify":   "${MINIFY}"

src/node/utils/Settings.js

@@ -509,17 +509,13 @@ const coerceValue = (stringValue) => {
     return +stringValue;
   }
 
-  // the boolean literal case is easy.
-  if (stringValue === 'true') {
-    return true;
+  switch (stringValue) {
+    case 'true': return true;
+    case 'false': return false;
+    case 'undefined': return undefined;
+    case 'null': return null;
+    default: return stringValue;
   }
-
-  if (stringValue === 'false') {
-    return false;
-  }
-
-  // otherwise, return this value as-is
-  return stringValue;
 };
 
 /**