Fixes 3182 (#5598)

Issue 3182 "Can't paste two images from clipboard in markdown cell"
happens when the user tries to paste a second image into a text
cell and the image gets the same attachment key as the previous one.

This patch fixes that issue by adding an index to each picture that's
pasted after the previous one. Example:

* User pastes an image blob A. The text `![image.png](attachment:image.png)`
  is added to the text cell and the blob is saved as attachment `image.png`
* The user pastes a second image blob B. The same happens as with blob
  A, except the key is now `image-1.png` rather than `image.png`

Special case:

If the user pastes the exact same blob A twice, it will not be added to
the attachments, but the textcell will get a second entry of
`![image.png](attachment:image.png)`.

notebook/static/notebook/js/textcell.js

@@ -33,6 +33,58 @@ define([
     "use strict";
     function encodeURIandParens(uri){return encodeURI(uri).replace('(','%28').replace(')','%29')}
 
+    /**
+     * Given a file name and a list of existing file names, returns a new file name
+     * that is not in the existing list. If the file name already exists, a new one with
+     * an incremented index is returned instead.
+     *
+     * Example:
+     *  addIndexToFileName('attachment.png',
+     *                    ['attachment.png', 'attachment-3.png']) returns 'attachment-4.png'
+     *
+     * @param  {string} fileName  - original file name
+     * @param  {string} fileNames - other file names
+     * @return {string}             the original file name or one with a postfix
+     *                              index (before the extension, if one exists)
+     */
+    function addIndexToFileName(fileName, fileNames) {
+        if (fileNames === undefined) {
+            return fileName;
+        }
+        var lastDot = fileName.lastIndexOf('.');
+        var pre = fileName.substr(0, lastDot);
+        var optionalExt = fileName.substr(lastDot);
+
+        var indexMatch = '-(\\d+)';
+        // Make the index match optional so we can match both 'fileName.png' and 'fileName-2.png'
+        // The ?: makes it a non-capturing group.
+        var optionalIndexMatch = '(?:' + indexMatch + ')?';
+
+        var regex = new RegExp(pre + optionalIndexMatch + optionalExt);
+
+        var highestIndex = 0;
+        for (var existingFileName in fileNames) {
+            var match = existingFileName.match(regex);
+            var index = match[1];
+            if (index === undefined) {
+                index = 1;
+            }
+            else {
+                index = parseInt(index);
+            }
+            if (index > highestIndex) {
+                highestIndex = index;
+            }
+        }
+
+        if (highestIndex > 0) {
+            return pre + "-" + (highestIndex + 1) + optionalExt;
+        }
+        else {
+            return fileName;
+        }
+    };
+
     var Cell = cell.Cell;
 
     var TextCell = function (options) {
@@ -357,18 +409,37 @@ define([
         var key;
         if (blob.name !== undefined) {
             key = encodeURIandParens(blob.name);
+
+            // Add an index to the filename if we already have one with the same name
+            key = addIndexToFileName(key, that.attachments);
         } else {
             key = '_auto_' + Object.keys(that.attachments).length;
         }
 
         reader.onloadend = function() {
             var d = utils.parse_b64_data_uri(reader.result);
+            var blobData = d[1]
+
             if (blob.type != d[0]) {
                 // TODO(julienr): Not sure what we should do in this case
                 console.log('File type (' + blob.type + ') != data-uri ' +
                             'type (' + d[0] + ')');
             }
-            that.add_attachment(key, blob.type, d[1]);
+
+            // If we have the same attachment already under another key, we change the key to that.
+            // This ensures we don't create two attachments if pasting the same image twice.
+
+            for (var savedKey in that.attachments) {
+                var attachment = that.attachments[savedKey];
+                if (attachment === undefined) continue;
+
+                var savedBlob = attachment[blob.type];
+                if (savedBlob === blobData) {
+                    key = savedKey;
+                }
+            }
+
+            that.add_attachment(key, blob.type, blobData);
             var img_md = '![' + key + '](attachment:' + key + ')';
             that.code_mirror.replaceRange(img_md, pos);
         }
@@ -464,7 +535,7 @@ define([
         $(div).on('paste', function(evt) {
             var data = evt.originalEvent.clipboardData;
             var items = data.items;
-            if (data.items !== undefined) {
+            if (items !== undefined) {
                 for (var i = 0; i < items.length; ++i) {
                     var item = items[i];
                     if (item.kind == 'file' && attachment_regex.test(item.type)) {