Better support for using a processed context for null and caching @import.

Author: gkellogg

lib/context.js

@@ -111,6 +111,14 @@ api.process = async ({
         } else if(protectedMode === 'warn') {
           // FIXME: remove logging and use a handler
           console.warn('WARNING: invalid context nullification');
+
+          // get processed context from cache if available
+          const processed = resolvedContext.getProcessed(activeCtx);
+          if(processed) {
+            rval = activeCtx = processed;
+            continue;
+          }
+
           const oldActiveCtx = activeCtx;
           // copy all protected term definitions to fresh initial context
           rval = activeCtx = api.getInitialContext(options).clone();
@@ -323,19 +331,33 @@ api.process = async ({
           'jsonld.SyntaxError',
           {code: 'invalid remote context', context: localCtx});
       }
-      const importCtx = resolvedImport[0].document;
-      if('@import' in importCtx) {
-        throw new JsonLdError(
-          'Invalid JSON-LD syntax; imported context must not include @import.',
-          'jsonld.SyntaxError',
-          {code: 'invalid context entry', context: localCtx});
-      }
+      const processedImport = resolvedImport[0].getProcessed(activeCtx);
+      if(processedImport) {
+        // Note: if the same context were used in this active context
+        // as a reference context, then processed_input might not
+        // be a dict.
+        ctx = processedImport;
+      } else {
+        const importCtx = resolvedImport[0].document;
+        if('@import' in importCtx) {
+          throw new JsonLdError(
+            'Invalid JSON-LD syntax; imported context must not include @import.',
+            'jsonld.SyntaxError',
+            {code: 'invalid context entry', context: localCtx});
+        }
 
-      // merge ctx into importCtx and replace rval with the result
-      for(const key in importCtx) {
-        if(!ctx.hasOwnProperty(key)) {
-          ctx[key] = importCtx[key];
+        // merge ctx into importCtx and replace rval with the result
+        for(const key in importCtx) {
+          if(!ctx.hasOwnProperty(key)) {
+            ctx[key] = importCtx[key];
+          }
         }
+
+        // Note: this could potenially conflict if the import
+        // were used in the same active context as a referenced
+        // context and an import. In this case, we
+        // could override the cached result, but seems unlikely.
+        resolvedImport[0].setProcessed(activeCtx, ctx);
       }
 
       defined.set('@import', true);