Function updateSettingsForSource

src/utils/settings/settings.ts:416–524

(
  source: EditableSettingSource,
  settings: SettingsJson,
)

 * the key is present with an explicit `undefined` value.
 */
export function updateSettingsForSource(
  source: EditableSettingSource,
  settings: SettingsJson,
): { error: Error | null } {
  if (
    (source as unknown) === 'policySettings' ||
    (source as unknown) === 'flagSettings'
  ) {
    return { error: null }
  }

  // Create the folder if needed
  const filePath = getSettingsFilePathForSource(source)
  if (!filePath) {
    return { error: null }
  }

  try {
    getFsImplementation().mkdirSync(dirname(filePath))

    // Try to get existing settings with validation. Bypass the per-source
    // cache — mergeWith below mutates its target (including nested refs),
    // and mutating the cached object would leak unpersisted state if the
    // write fails before resetSettingsCache().
    let existingSettings = getSettingsForSourceUncached(source)

    // If validation failed, check if file exists with a JSON syntax error
    if (!existingSettings) {
      let content: string | null = null
      try {
        content = readFileSync(filePath)
      } catch (e) {
        if (!isENOENT(e)) {
          throw e
        }
        // File doesn't exist — fall through to merge with empty settings
      }
      if (content !== null) {
        const rawData = safeParseJSON(content)
        if (rawData === null) {
          // JSON syntax error - return validation error instead of overwriting
          // safeParseJSON will already log the error, so we'll just return the error here
          return {
            error: new Error(
              `Invalid JSON syntax in settings file at ${filePath}`,
            ),
          }
        }
        if (rawData && typeof rawData === 'object') {
          existingSettings = rawData as SettingsJson
          logForDebugging(
            `Using raw settings from ${filePath} due to validation failure`,
          )
        }
      }
    }

    const updatedSettings = mergeWith(