Refactoring response handler to improve readability

3 files changed, 43 insertions, 23 deletions

diff --git a/bg/ExternalLicenses.js b/bg/ExternalLicenses.js
index 6eda008..6d35863 100644
--- a/bg/ExternalLicenses.js
+++ b/bg/ExternalLicenses.js
@@ -48,7 +48,17 @@ const ExternalLicenses = {
     cachedHrefs.delete(tabId);
   },
 
-  // Checks external script using web labels
+  /**
+   * Checks external script using web labels
+   * 
+   * If the script cannot be found in the web labels table, returns null.
+   * 
+   * If the script can be found in the web labels table, and at least
+   * one of its licenses can be found in our free license DB, returns
+   * "free".
+   *
+   * Otherwise returns "nonfree".
+   */
   async check(script) {
     const { url, tabId, frameId, documentUrl } = script;
     const tabCache = cachedHrefs.get(tabId);
diff --git a/bg/ResponseProcessor.js b/bg/ResponseProcessor.js
index 3c90d9c..429b778 100644
--- a/bg/ResponseProcessor.js
+++ b/bg/ResponseProcessor.js
@@ -29,6 +29,11 @@ const { ResponseMetaData } = require('./ResponseMetaData');
 
 const listeners = new WeakMap();
 const webRequestEvent = browser.webRequest.onHeadersReceived;
+// Some hardcoded return values for the onHeadersReceived callback.
+const BLOCKING_RESPONSES = {
+  ACCEPT: {}, // Do nothing
+  REJECT: { cancel: true }, // Cancel the request.  See docs of BlockingResponse
+};
 
 class ResponseProcessor {
 
@@ -53,13 +58,6 @@ class ResponseProcessor {
   }
 }
 
-Object.assign(ResponseProcessor, {
-  // control flow values to be returned by handler.pre() callbacks
-  ACCEPT: {},
-  REJECT: { cancel: true },
-  CONTINUE: null
-});
-
 class ResponseTextFilter {
   constructor(request) {
     this.request = request;
@@ -72,13 +70,16 @@ class ResponseTextFilter {
   }
 
   async process(handler) {
-    if (!this.canProcess) return ResponseProcessor.ACCEPT;
+    if (!this.canProcess) return BlockingResponses.ACCEPT;
     const { metaData, request } = this;
     const response = { request, metaData }; // we keep it around allowing callbacks to store state
 
     if (typeof handler.pre !== 'function' || typeof handler.post !== 'function') {
       throw new Error('handler should have a pre and post function.');
     }
+
+    // If can determine without checking and filtering response
+    // payload then return.
     const res = await handler.pre(response);
     if (res) return res;
 
@@ -132,8 +133,9 @@ class ResponseTextFilter {
       filter.close();
     }
 
-    return ResponseProcessor.ACCEPT;
+    // Accepting the filtered payload.
+    return BLOCKING_RESPONSES.ACCEPT;
   }
 }
 
-module.exports = { ResponseProcessor };
+module.exports = { ResponseProcessor, BLOCKING_RESPONSES };
diff --git a/main_background.js b/main_background.js
index 49815d5..40a8397 100644
--- a/main_background.js
+++ b/main_background.js
@@ -22,7 +22,7 @@
 */
 
 const checkLib = require('./common/checks.js');
-const { ResponseProcessor } = require('./bg/ResponseProcessor');
+const { ResponseProcessor, BLOCKING_RESPONSES } = require('./bg/ResponseProcessor');
 const { Storage, ListStore, hash } = require('./common/Storage');
 const { ListManager } = require('./bg/ListManager');
 const { ExternalLicenses } = require('./bg/ExternalLicenses');
@@ -470,15 +470,23 @@ async function blockBlacklistedScripts(request) {
 }
 
 /**
-*	This listener gets called as soon as we've got all the HTTP headers, can guess
-* content type and encoding, and therefore correctly parse HTML documents
-* and external script inclusions in search of non-free JavaScript
-*/
-
+ * An onHeadersReceived handler.  See bg/ResponseProcessor.js for how
+ * it is used.
+ * 
+ *	This listener gets called as soon as we've got all the HTTP
+ * headers, can guess content type and encoding, and therefore
+ * correctly parse HTML documents and external script inclusions in
+ * search of non-free JavaScript
+ */
 const ResponseHandler = {
   /**
-  *	Enforce white/black lists for url/site early (hashes will be handled later)
-  */
+   * Checks black/whitelists and web labels.  Returns a
+   * BlockingResponse (if we can determine) or null (if further work
+   * is needed).
+   * 
+   * Enforce white/black lists for url/site early (hashes will be
+   * handled later)
+   */
   async pre(response) {
     let { request } = response;
     let { url, type, tabId, frameId, documentUrl } = request;
@@ -494,7 +502,7 @@ const ResponseHandler = {
     if (blacklisted) {
       if (type === 'script') {
         // this shouldn't happen, because we intercept earlier in blockBlacklistedScripts()
-        return ResponseProcessor.REJECT;
+        return BLOCKING_RESPONSES.REJECT;
       }
       if (type === 'main_frame') { // we handle the page change here too, since we won't call edit_html()
         activityReports[tabId] = await createReport({ url: fullUrl, tabId });
@@ -520,12 +528,12 @@ const ResponseHandler = {
             url: topUrl,
             'whitelisted': [url, whitelistedSite ? `User whitelisted ${whitelistedSite}` : 'Whitelisted by user']
           });
-          return ResponseProcessor.ACCEPT;
+          return BLOCKING_RESPONSES.ACCEPT;
         } else {
           // Check for the weblabel method
           const scriptInfo = await ExternalLicenses.check({ url: fullUrl, tabId, frameId, documentUrl });
           if (scriptInfo) {
-            const [verdict, ret] = scriptInfo.free ? ['accepted', ResponseProcessor.ACCEPT] : ['blocked', ResponseProcessor.REJECT];
+            const [verdict, ret] = scriptInfo.free ? ['accepted', BLOCKING_RESPONSES.ACCEPT] : ['blocked', BLOCKING_RESPONSES.REJECT];
             const licenseIds = [...scriptInfo.licenses].map(l => l.identifier).sort().join(', ');
             const msg = licenseIds
               ? `Free license${scriptInfo.licenses.size > 1 ? 's' : ''} (${licenseIds})`
@@ -538,7 +546,7 @@ const ResponseHandler = {
     }
     // it's a page (it's too early to report) or an unknown script:
     //  let's keep processing
-    return ResponseProcessor.CONTINUE;
+    return null;
   },
 
   /**