Throw when array and object deletion values do not match current version

This change adds validation to array and object deletion. If the values
provided in either `ld` or `od` do not match the current value, then
`apply` will `throw`. It will also throw if `oi` overwrites an existing
value without providing `od`.

The primary motivation of this change is to ensure that all submitted
ops remain reversible. At the moment, the following series of ops is
possible:

  - start with `{ foo: 'bar' }`
  - `apply` this op: `{ p: ['foo'], oi: 'baz' }`
  - ...resulting in `{ foo: 'baz' }`
  - `invert` the previous op: `{ p: ['foo'], od: 'baz' }`
  - and `apply` the inverted op: `{}`

When I apply, invert and apply, I should always wind up where I started,
but in this case I clearly do not.

By ensuring that the `od` matches the current value, we make sure that
all ops remain reversible.

Deep equality adds a dependency on [`fast-deep-equal`][1].

[1]: https://github.com/epoberezkin/fast-deep-equal

lib/json0.js

@@ -1,3 +1,5 @@
+var deepEqual = require('fast-deep-equal');
+
 /*
  This is the implementation of the JSON OT type.
 
@@ -190,7 +192,8 @@ json.apply = function(snapshot, op) {
     // List replace
     else if (c.li !== void 0 && c.ld !== void 0) {
       json.checkList(elem);
-      // Should check the list element matches c.ld
+      if (!deepEqual(elem[key], c.ld))
+        throw new Error('List deletion value does not match current value')
       elem[key] = c.li;
     }
 
@@ -203,7 +206,8 @@ json.apply = function(snapshot, op) {
     // List delete
     else if (c.ld !== void 0) {
       json.checkList(elem);
-      // Should check the list element matches c.ld here too.
+      if (!deepEqual(elem[key], c.ld))
+        throw new Error('List deletion value does not match current value')
       elem.splice(key,1);
     }
 
@@ -226,15 +230,17 @@ json.apply = function(snapshot, op) {
     else if (c.oi !== void 0) {
       json.checkObj(elem);
 
-      // Should check that elem[key] == c.od
+      if (!deepEqual(elem[key], c.od))
+        throw new Error('Object deletion value does not match current value')
       elem[key] = c.oi;
     }
 
     // Object delete
     else if (c.od !== void 0) {
       json.checkObj(elem);
 
-      // Should check that elem[key] == c.od
+      if (!deepEqual(elem[key], c.od))
+        throw new Error('Object deletion value does not match current value')
       delete elem[key];
     }
 

package.json

@@ -6,7 +6,9 @@
   "directories": {
     "test": "test"
   },
-  "dependencies": {},
+  "dependencies": {
+    "fast-deep-equal": "^2.0.1"
+  },
   "devDependencies": {
     "coffee-script": "^1.7.1",
     "mocha": "^9.0.2",

test/json0.coffee

@@ -72,10 +72,14 @@ genTests = (type) ->
 
   # Strings should be handled internally by the text type. We'll just do some basic sanity checks here.
   describe 'string', ->
-    describe '#apply()', -> it 'works', ->
-      assert.deepEqual 'abc', type.apply 'a', [{p:[1], si:'bc'}]
-      assert.deepEqual 'bc', type.apply 'abc', [{p:[0], sd:'a'}]
-      assert.deepEqual {x:'abc'}, type.apply {x:'a'}, [{p:['x', 1], si:'bc'}]
+    describe '#apply()', ->
+      it 'works', ->
+        assert.deepEqual 'abc', type.apply 'a', [{p:[1], si:'bc'}]
+        assert.deepEqual 'bc', type.apply 'abc', [{p:[0], sd:'a'}]
+        assert.deepEqual {x:'abc'}, type.apply {x:'a'}, [{p:['x', 1], si:'bc'}]
+
+      it 'throws when the deletion target does not match', ->
+        assert.throws -> type.apply 'abc', [{p:[0], sd:'x'}]
 
       it 'throws when the target is not a string', ->
         assert.throws -> type.apply [1], [{p: [0], si: 'a'}]
@@ -138,6 +142,10 @@ genTests = (type) ->
         assert.deepEqual ['a', 'b', 'c'], type.apply ['b', 'a', 'c'], [{p:[1], lm:0}]
         assert.deepEqual ['a', 'b', 'c'], type.apply ['b', 'a', 'c'], [{p:[0], lm:1}]
 
+      it 'throws when the deletion target does not match', ->
+        assert.throws -> type.apply ['a', 'b', 'c'], [{p:[0], ld: 'x'}]
+        assert.throws -> type.apply ['a', 'b', 'c'], [{p:[0], li: 'd', ld: 'x'}]
+
       it 'throws when keying a list replacement with a string', ->
         assert.throws -> type.apply ['a', 'b', 'c'], [{p: ['0'], li: 'x', ld: 'a'}]
 
@@ -418,6 +426,11 @@ genTests = (type) ->
       assert.deepEqual [], type.transform [{p:['k'], od:'x'}], [{p:['k'], od:'x'}], 'left'
       assert.deepEqual [], type.transform [{p:['k'], od:'x'}], [{p:['k'], od:'x'}], 'right'
 
+    it 'throws when the deletion target does not match', ->
+      assert.throws -> type.apply {x:'a'}, [{p:['x'], od: 'b'}]
+      assert.throws -> type.apply {x:'a'}, [{p:['x'], oi: 'c', od: 'b'}]
+      assert.throws -> type.apply {x:'a'}, [{p:['x'], oi: 'b'}]
+
     it 'throws when the insertion key is a number', ->
       assert.throws -> type.apply {'1':'a'}, [{p:[2], oi: 'a'}]