Only one configuration should exist per service

Fixes #3514. Follow-up to pull request #3290.

packages/service-configuration/package.js

@@ -8,4 +8,5 @@ Package.onUse(function(api) {
   api.use('mongo', ['client', 'server']);
   api.export('ServiceConfiguration');
   api.addFiles('service_configuration_common.js', ['client', 'server']);
+  api.addFiles('service_configuration_server.js', 'server');
 });

packages/service-configuration/service_configuration_server.js

@@ -0,0 +1,30 @@
+// Only one configuration should ever exist for each service.
+// A unique index helps avoid various race conditions which could
+// otherwise lead to an inconsistent database state (when there are multiple
+// configurations for a single service, which configuration is correct?)
+try {
+    ServiceConfiguration.configurations._ensureIndex(
+        { "service": 1 },
+        { unique: true }
+    );
+} catch (err) {
+    console.error(
+        "The service-configuration package persists configuration in the " +
+        "meteor_accounts_loginServiceConfiguration collection in MongoDB. As " +
+        "each service should have exactly one configuration, Meteor " +
+        "automatically creates a MongoDB index with a unique constraint on the " +
+        " meteor_accounts_loginServiceConfiguration collection. The " +
+        "_ensureIndex command which creates that index is failing.\n\n" +
+        "Meteor versions <= 1.0.3.1 did not create this index. If you recently " +
+        "upgraded and are seeing this error message for the first time, please " +
+        "check your meteor_accounts_loginServiceConfiguration collection for " +
+        "multiple configuration entries for the same service and delete " +
+        "configuration entries until there is no more than one configuration " +
+        "entry per service.\n\n" +
+        "If the meteor_accounts_loginServiceConfiguration collection looks " +
+        "fine, the _ensureIndex command is failing for some other reason.\n\n" +
+        "For more information on this history of this issue, please see " +
+        "https://github.com/meteor/meteor/pull/3514.\n"
+    );
+    throw err;
+}