Support context sealing.

Author: cwebber

lib/context.js

@@ -45,7 +45,8 @@ api.cache = new ActiveContextCache();
  *
  * @return the new active context.
  */
-api.process = ({activeCtx, localCtx, options}) => {
+api.process = ({activeCtx, localCtx, options,
+                propertyTermDefinition = false}) => {
   // normalize local context to an array of @context objects
   if(_isObject(localCtx) && '@context' in localCtx &&
     _isArray(localCtx['@context'])) {
@@ -66,6 +67,16 @@ api.process = ({activeCtx, localCtx, options}) => {
 
     // reset to initial context
     if(ctx === null) {
+      // We can't nullify if there are sealed terms and we're
+      // not in a term definition
+      if(!propertyTermDefinition &&
+         Object.keys(activeCtx.sealed).length !== 0) {
+        throw new JsonLdError(
+          'Tried to nullify a context with sealed terms outside of ' +
+          'a term definition.',
+          'jsonld.SyntaxError',
+          {code: 'invalid context nullification'});
+      }
       rval = activeCtx = api.getInitialContext(options);
       continue;
     }
@@ -182,6 +193,11 @@ api.process = ({activeCtx, localCtx, options}) => {
       defined['@language'] = true;
     }
 
+    // handle @sealed; determine whether this sub-context is declaring
+    // all its terms to be "sealed" (exceptions can be made on a
+    // per-definition basis)
+    defined['@sealed'] = ctx['@sealed'] || false;
+
     // process all other keys
     for(const key in ctx) {
       api.createTermDefinition(rval, ctx, key, defined);
@@ -235,6 +251,15 @@ api.createTermDefinition = (activeCtx, localCtx, term, defined) => {
       {code: 'invalid term definition', context: localCtx});
   }
 
+  // Is this term currently sealed?
+  // Then redefining it here is an error!
+  if(activeCtx.sealed[term]) {
+    throw new JsonLdError(
+      'Invalid JSON-LD syntax; tried to redefine a sealed term.',
+      'jsonld.SyntaxError',
+      {code: 'sealed term redefinition', context: localCtx, term: term});
+  }
+
   // remove old mapping
   if(activeCtx.mappings[term]) {
     delete activeCtx.mappings[term];
@@ -274,7 +299,7 @@ api.createTermDefinition = (activeCtx, localCtx, term, defined) => {
 
   // JSON-LD 1.1 support
   if(api.processingMode(activeCtx, 1.1)) {
-    validKeys.push('@context', '@nest', '@prefix');
+    validKeys.push('@context', '@nest', '@prefix', '@sealed');
   }
 
   for(const kw in value) {
@@ -379,6 +404,17 @@ api.createTermDefinition = (activeCtx, localCtx, term, defined) => {
     }
   }
 
+  // Handle term sealing
+  if('@sealed' in value) {
+    if(value['@sealed']) {
+      activeCtx.sealed[term] = true;
+    }
+  } else if ('@sealed' in defined) {
+    if(defined['@sealed']) {
+      activeCtx.sealed[term] = true;
+    }
+  }
+
   // IRI mapping now defined
   defined[term] = true;
 
@@ -650,7 +686,8 @@ api.getInitialContext = options => {
     mappings: {},
     inverse: null,
     getInverse: _createInverseContext,
-    clone: _cloneActiveContext
+    clone: _cloneActiveContext,
+    sealed: {}
   };
   // TODO: consider using LRU cache instead
   if(INITIAL_CONTEXT_CACHE.size === INITIAL_CONTEXT_CACHE_MAX_SIZE) {
@@ -826,6 +863,7 @@ api.getInitialContext = options => {
     child.clone = this.clone;
     child.inverse = null;
     child.getInverse = this.getInverse;
+    child.sealed = util.clone(this.sealed);
     if('@language' in this) {
       child['@language'] = this['@language'];
     }
@@ -937,6 +975,7 @@ api.isKeyword = v => {
     case '@preserve':
     case '@requireAll':
     case '@reverse':
+    case '@sealed':
     case '@set':
     case '@type':
     case '@value':

lib/expand.js

@@ -556,7 +556,8 @@ function _expandObject({
     let termCtx = activeCtx;
     const ctx = _getContextValue(activeCtx, key, '@context');
     if(ctx) {
-      termCtx = _processContext({activeCtx, localCtx: ctx, options});
+      termCtx = _processContext({activeCtx, localCtx: ctx, options,
+                                 propertyTermDefinition: true});
     }
 
     const container = _getContextValue(termCtx, key, '@container') || [];