Add automatic retry behavior in the event of a download error

This patch adds retry behavior designed to handle intermittent
ECONNRESET and other connection errors during download operations.

The retry will be limited to 3 attempts, and each subsequent retry
will have an increasing delay to give time for the problem to
resolve itself. After the third failure it will give up on the URL
and move on to the next.

For issue: #22

lib/cli.js

@@ -77,8 +77,15 @@ module.exports = {
         console.log('[' + index + '] Saved: ' + fileName);
         index--;
       });
-      dl.on('fail', function(fileName) {
+      dl.on('retry', function(fileName, retryCount, retryDelay, isFinalRetry) {
+        console.log('[' + index + '] Connection error while attempting to download file ' + fileName + '!');
+        console.log('    Waiting ' + retryDelay + ' seconds before ' + (isFinalRetry ? 'final attempt...' : 'retrying download...'));
+      });
+      dl.on('fail', function(fileName, err) {
         console.log('[' + index + '] Failed: ' + fileName);
+        if (err) {
+          console.log('    Error: [' + err.code + '] ' + err.message);
+        }
         index--;
       });
       dl.on('end', function() {

lib/downloader.js

@@ -18,6 +18,7 @@ var resPaths = require('./respaths.json');
 
 var USERAGENT = 'Mozilla/4.8 [en] (Windows NT 6.0; U)';
 var HOST = 'interfacelift.com';
+var RETRY_LIMIT = 3;
 
 module.exports = function(uriList, downloadPath, resolution) {
   var fileNameRegEx = new RegExp('\\d+_\\w+_' + resolution + '\\.jpg', 'i');
@@ -57,16 +58,16 @@ module.exports = function(uriList, downloadPath, resolution) {
     }
   }
 
-  function failFile(fileName) {
-    me.emit('fail', fileName);
+  function failFile(fileName, err) {
+    me.emit('fail', fileName, err);
     setImmediate(runNextUri);
   }
 
   this.start = function() {
     setImmediate(runNextUri);
   };
 
-  this.on('next', function(uri) {
+  this.on('next', function(uri, retryCount) {
     var fileName = fileNameRegEx.exec(uri);
     var filePath = path.join(downloadPath, fileName[0]);
     var requestConfig = {
@@ -78,6 +79,8 @@ module.exports = function(uriList, downloadPath, resolution) {
         Referer: 'https://' + HOST + resPaths[resolution]
       }
     };
+    var retryDelay = 0;
+    var isFinalRetry = false;
 
     // skip the file if it already exists
     if (fs.existsSync(filePath)) {
@@ -101,6 +104,24 @@ module.exports = function(uriList, downloadPath, resolution) {
           // 400 and 500 status codes
           failFile(fileName);
       }
+    }).on('error', function(err) {
+      // retry delay algorithm: 4^c seconds
+      // delay will be 4 seconds, then 16 seconds, then 64 seconds
+      retryCount = retryCount || 0;
+      retryCount++;
+      retryDelay = Math.pow(4, retryCount);
+      isFinalRetry = retryCount === RETRY_LIMIT;
+
+      if (retryCount <= RETRY_LIMIT) {
+        // wait for the specified time, then retry
+        me.emit('retry', fileName, retryCount, retryDelay, isFinalRetry);
+        setTimeout(function() {
+          me.emit('next', uri, retryCount);
+        }, retryDelay * 1000);
+      } else {
+        // give up
+        failFile(fileName, err);
+      }
     });
   });
 };