Code review for dns lookup code

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/780

- Handle DNS lookup failure

- Skip DNS lookup for non network-based URLs

- Benchmark code to be able to provide an estimate
  based on objective measurements regarding added
  overhead when DNS lookup is enabled
  (quick answer: a complete non-issue)

platform/chromium/vapi-background.js

@@ -1252,6 +1252,39 @@ vAPI.Net = class {
     canSuspend() {
         return false;
     }
+    async benchmark() {
+        if ( typeof µBlock !== 'object' ) { return; }
+        const requests = await µBlock.loadBenchmarkDataset();
+        if ( Array.isArray(requests) === false || requests.length === 0 ) {
+            console.info('No requests found to benchmark');
+            return;
+        }
+        console.info('vAPI.net.onBeforeSuspendableRequest()...');
+        const t0 = self.performance.now();
+        const promises = [];
+        for ( const request of requests ) {
+            const details = {
+                documentUrl: request.frameUrl,
+                tabId: Number.MAX_SAFE_INTEGER,
+                parentFrameId: -1,
+                frameId: 0,
+                type: request.cpt,
+                url: request.url,
+            };
+            promises.push(this.onBeforeSuspendableRequest(details));
+        }
+        return Promise.all(promises).then(results => {
+            let blockCount = 0;
+            for ( const r of results ) {
+                if ( r !== undefined ) { blockCount += 1; }
+            }
+            const t1 = self.performance.now();
+            const dur = t1 - t0;
+            console.info(`Evaluated ${requests.length} requests in ${dur.toFixed(0)} ms`);
+            console.info(`\tBlocked ${blockCount} requests`);
+            console.info(`\tAverage: ${(dur / requests.length).toFixed(3)} ms per request`);
+        });
+    }
 };
 
 /******************************************************************************/

platform/chromium/vapi-common.js

@@ -149,6 +149,14 @@ vAPI.webextFlavor = {
         return hostname;
     };
 
+    const reHostnameFromNetworkURL =
+        /^(?:http|ws|ftp)s?:\/\/([0-9a-z_][0-9a-z._-]*[0-9a-z])\//;
+
+    vAPI.hostnameFromNetworkURL = function(url) {
+        const matches = reHostnameFromNetworkURL.exec(url);
+        return matches !== null ? matches[1] : '';
+    };
+
     const psl = self.publicSuffixList;
     const reIPAddressNaive = /^\d+\.\d+\.\d+\.\d+$|^\[[\da-zA-Z:]+\]$/;
 

platform/firefox/vapi-webrequest.js

@@ -60,7 +60,7 @@
         constructor() {
             super();
             this.pendingRequests = [];
-            this.cnames = new Map();
+            this.cnames = new Map([ [ '', '' ] ]);
             this.cnameAliasList = null;
             this.cnameIgnoreList = null;
             this.url = new URL(vAPI.getURL('/'));
@@ -73,7 +73,7 @@
             this.cnameIgnoreList = this.regexFromStrList(options.cnameIgnoreList);
             this.cnameIgnore1stParty = options.cnameIgnore1stParty === true;
             this.cnameMaxTTL = options.cnameMaxTTL || 120;
-            this.cnames.clear();
+            this.cnames.clear(); this.cnames.set('', '');
         }
         normalizeDetails(details) {
             if ( mustPunycode && !reAsciiHostname.test(details.url) ) {
@@ -148,7 +148,6 @@
                 this.cnameIgnoreList !== null &&
                 this.cnameIgnoreList.test(cname)
             ) {
-
                 cname = '';
             }
             this.cnames.set(hn, cname);
@@ -156,7 +155,7 @@
                 this.cnameTimer = self.setTimeout(
                     ( ) => {
                         this.cnameTimer = undefined;
-                        this.cnames.clear();
+                        this.cnames.clear(); this.cnames.set('', '');
                     },
                     this.cnameMaxTTL * 60000
                 );
@@ -188,7 +187,7 @@
             let r = super.onBeforeSuspendableRequest(details);
             if ( r !== undefined ) { return r; }
             if ( this.cnameAliasList === null ) { return; }
-            const hn = vAPI.hostnameFromURI(details.url);
+            const hn = vAPI.hostnameFromNetworkURL(details.url);
             let cname = this.cnames.get(hn);
             if ( cname === '' ) { return; }
             if ( cname !== undefined ) {
@@ -202,7 +201,9 @@
                 const cname = this.recordCanonicalName(hn, rec);
                 if ( cname === '' ) { return; }
                 return this.processCanonicalName(cname, details);
-
+            }).catch(( ) => {
+                this.cnames.set(hn, '');
+            }).then(( ) => {
             });
         }
         suspendOneRequest(details) {

src/js/filtering-context.js

@@ -66,7 +66,7 @@
         this.realm = '';
         this.type = details.type;
         this.setURL(details.url);
-        this.cnameOf = details.cnameOf !== undefined ? details.cnameOf : '';
+        this.cnameOf = details.cnameOf || undefined;
         this.docId = details.type !== 'sub_frame'
             ? details.frameId
             : details.parentFrameId;