Support rel=alternate.

Note XHR and Node loaders don't have their own tests.

Author: gkellogg

lib/constants.js

@@ -7,7 +7,7 @@ const RDF = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#';
 const XSD = 'http://www.w3.org/2001/XMLSchema#';
 
 module.exports = {
-  LINK_HEADER_REL: 'http://www.w3.org/ns/json-ld#context',
+  LINK_HEADER_CONTEXT: 'http://www.w3.org/ns/json-ld#context',
 
   RDF,
   RDF_LIST: RDF + 'List',

lib/documentLoaders/node.js

@@ -4,9 +4,10 @@
 'use strict';
 
 const {parseLinkHeader, buildHeaders} = require('../util');
-const {LINK_HEADER_REL} = require('../constants');
+const {LINK_HEADER_CONTEXT} = require('../constants');
 const JsonLdError = require('../JsonLdError');
 const RequestQueue = require('../RequestQueue');
+const {prependBase} = require('../url');
 
 /**
  * Creates a built-in node document loader.
@@ -61,6 +62,7 @@ module.exports = ({
     }
 
     let result;
+    let alternate = null;
     try {
       result = await _request(request, {
         url,
@@ -95,21 +97,30 @@ module.exports = ({
     if(res.headers.link &&
       res.headers['content-type'] !== 'application/ld+json') {
       // only 1 related link header permitted
-      const linkHeader = parseLinkHeader(res.headers.link)[LINK_HEADER_REL];
-      if(Array.isArray(linkHeader)) {
+      const linkHeaders = parseLinkHeader(res.headers.link);
+      const linkedContext = linkHeaders[LINK_HEADER_CONTEXT];
+      if(Array.isArray(linkedContext)) {
         throw new JsonLdError(
           'URL could not be dereferenced, it has more than one associated ' +
           'HTTP Link Header.',
           'jsonld.InvalidUrl',
           {code: 'multiple context link headers', url});
       }
-      if(linkHeader) {
-        doc.contextUrl = linkHeader.target;
+      if(linkedContext) {
+        doc.contextUrl = linkedContext.target;
+      }
+
+      // "alternate" link header is a redirect
+      alternate = linkHeaders['alternate'];
+      if(alternate &&
+        alternate.type == 'application/ld+json' &&
+        !(res.headers['content-type'] || '').match(/^application\/(\w*\+)?json$/)) {
+        res.headers.location = prependBase(url, alternate.target);
       }
     }
 
     // handle redirect
-    if(res.statusCode >= 300 && res.statusCode < 400 && res.headers.location) {
+    if((alternate || res.statusCode >= 300 && res.statusCode < 400) && res.headers.location) {
       if(redirects.length === maxRedirects) {
         throw new JsonLdError(
           'URL could not be dereferenced; there were too many redirects.',

lib/documentLoaders/xhr.js

@@ -4,9 +4,10 @@
 'use strict';
 
 const {parseLinkHeader, buildHeaders} = require('../util');
-const {LINK_HEADER_REL} = require('../constants');
+const {LINK_HEADER_CONTEXT} = require('../constants');
 const JsonLdError = require('../JsonLdError');
 const RequestQueue = require('../RequestQueue');
+const {prependBase} = require('../url');
 
 const REGEX_LINK_HEADER = /(^|(\r\n))link:/i;
 
@@ -65,6 +66,7 @@ module.exports = ({
     }
 
     const doc = {contextUrl: null, documentUrl: url, document: req.response};
+    let alternate = null;
 
     // handle Link Header (avoid unsafe header warning by existence testing)
     const contentType = req.getResponseHeader('Content-Type');
@@ -74,16 +76,25 @@ module.exports = ({
     }
     if(linkHeader && contentType !== 'application/ld+json') {
       // only 1 related link header permitted
-      linkHeader = parseLinkHeader(linkHeader)[LINK_HEADER_REL];
-      if(Array.isArray(linkHeader)) {
+      const linkHeaders = parseLinkHeader(res.headers.link);
+      const linkedContext = linkHeaders[LINK_HEADER_CONTEXT];
+      if(Array.isArray(linkedContext)) {
         throw new JsonLdError(
           'URL could not be dereferenced, it has more than one ' +
           'associated HTTP Link Header.',
           'jsonld.InvalidUrl',
           {code: 'multiple context link headers', url});
       }
-      if(linkHeader) {
-        doc.contextUrl = linkHeader.target;
+      if(linkedContext) {
+        doc.contextUrl = linkedContext.target;
+      }
+
+      // "alternate" link header is a redirect
+      alternate = linkHeaders['alternate'];
+      if(alternate &&
+        alternate.type == 'application/ld+json' &&
+        !(contentType || '').match(/^application\/(\w*\+)?json$/)) {
+        doc = await loader(prependBase(url, alternate.target));
       }
     }
 

tests/test-common.js

@@ -6,6 +6,7 @@ const EarlReport = require('./earl-report');
 const benchmark = require('benchmark');
 const join = require('join-path-js');
 const rdfCanonize = require('rdf-canonize');
+const {prependBase} = require('../lib/url');
 
 module.exports = function(options) {
 
@@ -102,8 +103,6 @@ const TEST_TYPES = {
         /expand-manifest.jsonld#thc05$/,
         // remote
         /remote-doc-manifest.jsonld#t0013$/, // HTML
-        /remote-doc-manifest.jsonld#tla01$/,
-        /remote-doc-manifest.jsonld#tla05$/,
         // @import
         /expand-manifest.jsonld#tso01$/,
         /expand-manifest.jsonld#tso02$/,
@@ -1120,17 +1119,28 @@ function createDocumentLoader(test) {
         if(!contentType && url.indexOf('.jsonld', url.length - 7) !== -1) {
           contentType = 'application/ld+json';
         }
+        if(!contentType && url.indexOf('.json', url.length - 5) !== -1) {
+          contentType = 'application/json';
+        }
         let linkHeader = options.httpLink;
         if(Array.isArray(linkHeader)) {
           linkHeader = linkHeader.join(',');
         }
-        linkHeader = jsonld.parseLinkHeader(
-          linkHeader)['http://www.w3.org/ns/json-ld#context'];
-        if(linkHeader && contentType !== 'application/ld+json') {
-          if(Array.isArray(linkHeader)) {
+        const linkHeaders = jsonld.parseLinkHeader(
+          linkHeader)
+        const linkedContext = linkHeaders['http://www.w3.org/ns/json-ld#context'];
+        if(linkedContext && contentType !== 'application/ld+json') {
+          if(Array.isArray(linkedContext)) {
             throw {name: 'multiple context link headers'};
           }
-          doc.contextUrl = linkHeader.target;
+          doc.contextUrl = linkedContext.target;
+        }
+
+        // If not JSON-LD, alternate may point there
+        if(linkHeaders['alternate'] &&
+          linkHeaders['alternate'].type == 'application/ld+json' &&
+          !(contentType || '').match(/^application\/(\w*\+)?json$/)) {
+          doc.documentUrl = prependBase(url, linkHeaders['alternate'].target);
         }
       }
     }