ContextResolver.js

/*
 * Copyright (c) 2019 Digital Bazaar, Inc. All rights reserved.
 */
'use strict';

const {
  isArray: _isArray,
  isObject: _isObject,
  isString: _isString,
} = require('./types');
const {
  asArray: _asArray
} = require('./util');
const {prependBase} = require('./url');
const JsonLdError = require('./JsonLdError');
const ResolvedContext = require('./ResolvedContext');

const MAX_CONTEXT_URLS = 10;

module.exports = class ContextResolver {
  /**
   * Creates a ContextResolver.
   *
   * @param sharedCache a shared LRU cache with `get` and `set` APIs.
   */
  constructor({sharedCache}) {
    this.perOpCache = new Map();
    this.sharedCache = sharedCache;
  }

  async resolve({
    activeCtx, context, documentLoader, base, cycles = new Set()
  }) {
    // process `@context`
    if(context && _isObject(context) && context['@context']) {
      context = context['@context'];
    }

    // context is one or more contexts
    context = _asArray(context);

    // resolve each context in the array
    const allResolved = [];
    for(const ctx of context) {
      if(_isString(ctx)) {
        // see if `ctx` has been resolved before...
        let resolved = this._get(ctx);
        if(!resolved) {
          // not resolved yet, resolve
          resolved = await this._resolveRemoteContext(
            {activeCtx, url: ctx, documentLoader, base, cycles});
        }

        // add to output and continue
        if(_isArray(resolved)) {
          allResolved.push(...resolved);
        } else {
          allResolved.push(resolved);
        }
        continue;
      }
      if(ctx === null) {
        // handle `null` context, nothing to cache
        allResolved.push(new ResolvedContext({document: null}));
        continue;
      }
      if(!_isObject(ctx)) {
        _throwInvalidLocalContext(context);
      }
      // context is an object, get/create `ResolvedContext` for it
      const key = JSON.stringify(ctx);
      let resolved = this._get(key);
      if(!resolved) {
        // create a new static `ResolvedContext` and cache it
        resolved = new ResolvedContext({document: ctx});
        this._cacheResolvedContext({key, resolved, tag: 'static'});
      }
      allResolved.push(resolved);
    }

    return allResolved;
  }

  _get(key) {
    // get key from per operation cache; no `tag` is used with this cache so
    // any retrieved context will always be the same during a single operation
    let resolved = this.perOpCache.get(key);
    if(!resolved) {
      // see if the shared cache has a `static` entry for this URL
      const tagMap = this.sharedCache.get(key);
      if(tagMap) {
        resolved = tagMap.get('static');
        if(resolved) {
          this.perOpCache.set(key, resolved);
        }
      }
    }
    return resolved;
  }

  _cacheResolvedContext({key, resolved, tag}) {
    this.perOpCache.set(key, resolved);
    if(tag !== undefined) {
      let tagMap = this.sharedCache.get(key);
      if(!tagMap) {
        tagMap = new Map();
        this.sharedCache.set(key, tagMap);
      }
      tagMap.set(tag, resolved);
    }
    return resolved;
  }

  async _resolveRemoteContext({activeCtx, url, documentLoader, base, cycles}) {
    // resolve relative URL and fetch context
    url = prependBase(base, url);
    const {context, remoteDoc} = await this._fetchContext(
      {activeCtx, url, documentLoader, cycles});

    // update base according to remote document and resolve any relative URLs
    base = remoteDoc.documentUrl || url;
    _resolveContextUrls({context, base});

    // resolve, cache, and return context
    const resolved = await this.resolve(
      {activeCtx, context, documentLoader, base, cycles});
    this._cacheResolvedContext({key: url, resolved, tag: remoteDoc.tag});
    return resolved;
  }

  async _fetchContext({activeCtx, url, documentLoader, cycles}) {
    // check for max context URLs fetched during a resolve operation
    if(cycles.size > MAX_CONTEXT_URLS) {
      throw new JsonLdError(
        'Maximum number of @context URLs exceeded.',
        'jsonld.ContextUrlError',
        {
          code: activeCtx.processingMode === 'json-ld-1.0' ?
            'loading remote context failed' :
            'context overflow',
          max: MAX_CONTEXT_URLS
        });
    }

    // check for context URL cycle
    // shortcut to avoid extra work that would eventually hit the max above
    if(cycles.has(url)) {
      throw new JsonLdError(
        'Cyclical @context URLs detected.',
        'jsonld.ContextUrlError',
        {
          code: activeCtx.processingMode === 'json-ld-1.0' ?
            'recursive context inclusion' :
            'context overflow',
          url
        });
    }

    // track cycles
    cycles.add(url);

    let context;
    let remoteDoc;

    try {
      remoteDoc = await documentLoader(url);
      context = remoteDoc.document || null;
      // parse string context as JSON
      if(_isString(context)) {
        context = JSON.parse(context);
      }
    } catch(e) {
      throw new JsonLdError(
        'Dereferencing a URL did not result in a valid JSON-LD object. ' +
        'Possible causes are an inaccessible URL perhaps due to ' +
        'a same-origin policy (ensure the server uses CORS if you are ' +
        'using client-side JavaScript), too many redirects, a ' +
        'non-JSON response, or more than one HTTP Link Header was ' +
        'provided for a remote context. ' +
        `URL: "${url}".`,
        'jsonld.InvalidUrl',
        {code: 'loading remote context failed', url, cause: e});
    }

    // ensure ctx is an object
    if(!_isObject(context)) {
      throw new JsonLdError(
        'Dereferencing a URL did not result in a JSON object. The ' +
        'response was valid JSON, but it was not a JSON object. ' +
        `URL: "${url}".`,
        'jsonld.InvalidUrl', {code: 'invalid remote context', url});
    }

    // use empty context if no @context key is present
    if(!('@context' in context)) {
      context = {'@context': {}};
    } else {
      context = {'@context': context['@context']};
    }

    // append @context URL to context if given
    if(remoteDoc.contextUrl) {
      if(!_isArray(context['@context'])) {
        context['@context'] = [context['@context']];
      }
      context['@context'].push(remoteDoc.contextUrl);
    }

    return {context, remoteDoc};
  }
};

function _throwInvalidLocalContext(ctx) {
  throw new JsonLdError(
    'Invalid JSON-LD syntax; @context must be an object.',
    'jsonld.SyntaxError', {
      code: 'invalid local context', context: ctx
    });
}

/**
 * Resolve all relative `@context` URLs in the given context by inline
 * replacing them with absolute URLs.
 *
 * @param context the context.
 * @param base the base IRI to use to resolve relative IRIs.
 */
function _resolveContextUrls({context, base}) {
  if(!context) {
    return;
  }

  const ctx = context['@context'];

  if(_isString(ctx)) {
    context['@context'] = prependBase(base, ctx);
    return;
  }

  if(_isArray(ctx)) {
    for(let i = 0; i < ctx.length; ++i) {
      const element = ctx[i];
      if(_isString(element)) {
        ctx[i] = prependBase(base, element);
        continue;
      }
      if(_isObject(element)) {
        _resolveContextUrls({context: {'@context': element}, base});
      }
    }
    return;
  }

  if(!_isObject(ctx)) {
    // no @context URLs can be found in non-object
    return;
  }

  // ctx is an object, resolve any context URLs in terms
  for(const term in ctx) {
    _resolveContextUrls({context: ctx[term], base});
  }
}