fix(storage-mixin): do not monkey-patch prototypes in SpliceDiskIpcBackend (#3434)

It can not have been intentional that this overwrote the prototype
methods, rather than the methods of the specific instances, since
the previous code meant that using `SpliceDiskIpcBackend` would have
affected completely independent storage-mixing backends (even including
other instances of `SpliceDiskIpcBackend` itself).

This code has been broken like this since its inception. I did verify
that saving credentials works as it did before.

Author: addaleax

packages/storage-mixin/lib/backends/splice-disk-ipc.js

@@ -37,19 +37,17 @@ function SpliceDiskIpcBackend(options) {
 
   // patch the serialize methods in both backends
   var condition = options.secureCondition;
-  DiskBackend.prototype.serialize = function(model) {
+  this.diskBackend = new DiskBackend(options);
+  this.diskBackend.serialize = function(model) {
     debug('Serializing for disk backend with condition', condition);
-    var res = _.omitBy(model.serialize(), condition);
-    return res;
+    return _.omitBy(model.serialize(), condition);
   };
-  this.diskBackend = new DiskBackend(options);
 
-  SecureIpcBackend.prototype.serialize = function(model) {
+  this.secureBackend = new SecureIpcBackend(options);
+  this.secureBackend.serialize = function(model) {
     debug('Serializing for secure backend with condition', condition);
-    var res = _.pickBy(model.serialize(), condition);
-    return res;
+    return _.pickBy(model.serialize(), condition);
   };
-  this.secureBackend = new SecureIpcBackend(options);
 }
 
 inherits(SpliceDiskIpcBackend, BaseBackend);

packages/storage-mixin/lib/backends/splice-disk.js

@@ -27,19 +27,17 @@ function SpliceDiskBackend(options) {
 
   // patch the serialize methods in both backends
   var condition = options.secureCondition;
-  DiskBackend.prototype.serialize = function(model) {
+  this.diskBackend = new DiskBackend(options);
+  this.diskBackend.serialize = function(model) {
     debug('Serializing for disk backend with condition', condition);
-    var res = _.omitBy(model.serialize(), condition);
-    return res;
+    return _.omitBy(model.serialize(), condition);
   };
-  this.diskBackend = new DiskBackend(options);
 
-  SecureBackend.prototype.serialize = function(model) {
+  this.secureBackend = new SecureBackend(options);
+  this.secureBackend.serialize = function(model) {
     debug('Serializing for secure backend with condition', condition);
-    var res = _.pickBy(model.serialize(), condition);
-    return res;
+    return _.pickBy(model.serialize(), condition);
   };
-  this.secureBackend = new SecureBackend(options);
 }
 
 inherits(SpliceDiskBackend, BaseBackend);