Use CDN URLs as fall back URLs

Related feedback:
- https://github.com/uBlockOrigin/uBlock-issues/issues/1566#issuecomment-826473517

Additionally, add more CDN URLs to default filter lists.

assets/assets.json

@@ -22,6 +22,11 @@
 		"contentURL": [
 			"https://raw.githubusercontent.com/uBlockOrigin/uAssets/master/filters/badlists.txt",
 			"assets/ublock/badlists.txt"
+		],
+		"cdnURLs": [
+			"https://gitcdn.xyz/repo/uBlockOrigin/uAssets/master/filters/badlists.txt",
+			"https://cdn.jsdelivr.net/gh/uBlockOrigin/uAssets@master/filters/badlists.txt",
+			"https://cdn.statically.io/gh/uBlockOrigin/uAssets/master/filters/badlists.txt"
 		]
 	},
 	"ublock-filters": {
@@ -47,6 +52,11 @@
 			"https://raw.githubusercontent.com/uBlockOrigin/uAssets/master/filters/badware.txt",
 			"assets/ublock/badware.txt"
 		],
+		"cdnURLs": [
+			"https://gitcdn.xyz/repo/uBlockOrigin/uAssets/master/filters/badware.txt",
+			"https://cdn.jsdelivr.net/gh/uBlockOrigin/uAssets@master/filters/badware.txt",
+			"https://cdn.statically.io/gh/uBlockOrigin/uAssets/master/filters/badware.txt"
+		],
 		"supportURL": "https://github.com/gorhill/uBlock/wiki/Badware-risks",
 		"instructionURL": "https://github.com/gorhill/uBlock/wiki/Badware-risks"
 	},
@@ -57,6 +67,11 @@
 		"contentURL": [
 			"https://raw.githubusercontent.com/uBlockOrigin/uAssets/master/filters/privacy.txt",
 			"assets/ublock/privacy.txt"
+		],
+		"cdnURLs": [
+			"https://gitcdn.xyz/repo/uBlockOrigin/uAssets/master/filters/privacy.txt",
+			"https://cdn.jsdelivr.net/gh/uBlockOrigin/uAssets@master/filters/privacy.txt",
+			"https://cdn.statically.io/gh/uBlockOrigin/uAssets/master/filters/privacy.txt"
 		]
 	},
 	"ublock-abuse": {
@@ -66,6 +81,11 @@
 		"contentURL": [
 			"https://raw.githubusercontent.com/uBlockOrigin/uAssets/master/filters/resource-abuse.txt",
 			"assets/ublock/resource-abuse.txt"
+		],
+		"cdnURLs": [
+			"https://gitcdn.xyz/repo/uBlockOrigin/uAssets/master/filters/resource-abuse.txt",
+			"https://cdn.jsdelivr.net/gh/uBlockOrigin/uAssets@master/filters/resource-abuse.txt",
+			"https://cdn.statically.io/gh/uBlockOrigin/uAssets/master/filters/resource-abuse.txt"
 		]
 	},
 	"ublock-unbreak": {

src/js/assets.js

@@ -730,14 +730,20 @@ api.get = async function(assetKey, options = {}) {
 
     const assetRegistry = await getAssetSourceRegistry();
     assetDetails = assetRegistry[assetKey] || {};
-    let contentURLs = [];
+    const contentURLs = [];
     if ( typeof assetDetails.contentURL === 'string' ) {
-        contentURLs = [ assetDetails.contentURL ];
+        contentURLs.push(assetDetails.contentURL);
     } else if ( Array.isArray(assetDetails.contentURL) ) {
-        contentURLs = assetDetails.contentURL.slice(0);
+        contentURLs.push(...assetDetails.contentURL);
     } else if ( reIsExternalPath.test(assetKey) ) {
         assetDetails.content = 'filters';
-        contentURLs = [ assetKey ];
+        contentURLs.push(assetKey);
+    }
+
+    // https://github.com/uBlockOrigin/uBlock-issues/issues/1566#issuecomment-826473517
+    //   Use CDN URLs as fall back URLs.
+    if ( Array.isArray(assetDetails.cdnURLs) ) {
+        contentURLs.push(...assetDetails.cdnURLs);
     }
 
     for ( const contentURL of contentURLs ) {
@@ -776,24 +782,31 @@ const getRemote = async function(assetKey) {
         return details;
     };
 
-    let contentURLs = [];
+    const contentURLs = [];
     if ( typeof assetDetails.contentURL === 'string' ) {
-        contentURLs = [ assetDetails.contentURL ];
+        contentURLs.push(assetDetails.contentURL);
     } else if ( Array.isArray(assetDetails.contentURL) ) {
-        contentURLs = assetDetails.contentURL.slice(0);
+        contentURLs.push(...assetDetails.contentURL);
     }
 
     // If asked to be gentle on remote servers, favour using dedicated CDN
     // servers. If more than one CDN server is present, randomly shuffle the
     // set of servers so as to spread the bandwidth burden.
-    if ( remoteServerFriendly && Array.isArray(assetDetails.cdnURLs) ) {
+    //
+    // https://github.com/uBlockOrigin/uBlock-issues/issues/1566#issuecomment-826473517
+    //   In case of manual update, use CDNs URLs as fall back URLs.
+    if ( Array.isArray(assetDetails.cdnURLs) ) {
         const cdnURLs = assetDetails.cdnURLs.slice();
         for ( let i = 0, n = cdnURLs.length; i < n; i++ ) {
             const j = Math.floor(Math.random() * n);
             if ( j === i ) { continue; }
             [ cdnURLs[j], cdnURLs[i] ] = [ cdnURLs[i], cdnURLs[j] ];
         }
+        if ( remoteServerFriendly ) {
             contentURLs.unshift(...cdnURLs);
+        } else {
+            contentURLs.push(...cdnURLs);
+        }
     }
 
     for ( const contentURL of contentURLs ) {