Ensure FLoC is opt-in by default

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/1553

This commit ensures FLoC is opt-in. The generic filter
`*##+js(no-floc)` in "uBlock filters -- Privacy" ensures
the feature is disabled when using default settings/lists.

Users can opt-in to FLoC by adding a generic exception
filter to their custom filters, `#@#+js(no-floc)`; or they
can opt-in only for a specific set of websites through a
more specific exception filter:

    example.com,shopping.example#@#+js(no-floc)

assets/resources/scriptlets.js

@@ -668,12 +668,18 @@
 /// no-floc.js
 //  https://github.com/uBlockOrigin/uBlock-issues/issues/1553
 (function() {
-    if ( document.interestCohort instanceof Function === false ) { return; }
-    document.interestCohort = new Proxy(document.interestCohort, {
+    if ( Document instanceof Object === false ) { return; }
+    if ( Document.prototype.interestCohort instanceof Function === false ) {
+        return;
+    }
+    Document.prototype.interestCohort = new Proxy(
+        Document.prototype.interestCohort,
+        {
             apply: function() {
                 return Promise.reject();
             }
-    });
+        }
+    );
 })();
 
 

src/js/scriptlet-filtering.js

@@ -382,6 +382,10 @@
         return out.join('\n');
     };
 
+    api.hasScriptlet = function(hostname, exceptionBit, scriptlet) {
+        return scriptletDB.hasStr(hostname, exceptionBit, scriptlet);
+    };
+
     api.injectNow = function(details) {
         if ( typeof details.frameId !== 'number' ) { return; }
         const request = {

src/js/static-ext-filtering.js

@@ -173,6 +173,35 @@
             }
         }
 
+        hasStr(hostname, exceptionBit, value) {
+            let found = false;
+            for (;;) {
+                let iHn = this.hostnameToSlotIdMap.get(hostname);
+                if ( iHn !== undefined ) {
+                    do {
+                        const strId = this.hostnameSlots[iHn+0];
+                        if ( this.strSlots[strId >>> this.nBits] === value ) {
+                            if ( (strId & exceptionBit) !== 0 ) {
+                                return false;
+                            }
+                            found = true;
+                        }
+                        iHn = this.hostnameSlots[iHn+1];
+                    } while ( iHn !== 0 );
+                }
+                if ( hostname === '' ) { break; }
+                const pos = hostname.indexOf('.');
+                if ( pos !== -1 ) {
+                    hostname = hostname.slice(pos + 1);
+                } else if ( hostname !== '*' ) {
+                    hostname = '*';
+                } else {
+                    hostname = '';
+                }
+            }
+            return found;
+        }
+
         toSelfie() {
             return {
                 hostnameToSlotIdMap: Array.from(this.hostnameToSlotIdMap),

src/js/traffic.js

@@ -53,6 +53,9 @@ window.addEventListener('webextFlavor', function() {
         vAPI.webextFlavor.major < 59;
 }, { once: true });
 
+// https://github.com/uBlockOrigin/uBlock-issues/issues/1553
+const supportsFloc = document.interestCohort instanceof Function;
+
 /******************************************************************************/
 
 // Intercept and filter web requests.
@@ -551,6 +554,9 @@ const onHeadersReceived = function(details) {
     if ( injectCSP(fctxt, pageStore, responseHeaders) === true ) {
         modifiedHeaders = true;
     }
+    if ( supportsFloc && foilFloc(fctxt, responseHeaders) ) {
+        modifiedHeaders = true;
+    }
 
     // https://bugzilla.mozilla.org/show_bug.cgi?id=1376932
     //   Prevent document from being cached by the browser if we modified it,
@@ -1012,6 +1018,23 @@ const injectCSP = function(fctxt, pageStore, responseHeaders) {
 
 /******************************************************************************/
 
+// https://github.com/uBlockOrigin/uBlock-issues/issues/1553
+// https://github.com/WICG/floc#opting-out-of-computation
+
+const foilFloc = function(fctxt, responseHeaders) {
+    const hn = fctxt.getHostname();
+    if ( µBlock.scriptletFilteringEngine.hasScriptlet(hn, 1, 'no-floc') === false ) {
+        return false;
+    }
+    responseHeaders.push({
+        name: 'Permissions-Policy',
+        value: 'interest-cohort=()' }
+    );
+    return true;
+};
+
+/******************************************************************************/
+
 // https://github.com/gorhill/uBlock/issues/1163
 //   "Block elements by size".
 // https://github.com/gorhill/uBlock/issues/1390#issuecomment-187310719