Fix error merging items with missing attachment files

Fixes #2453

chrome/content/zotero/xpcom/data/items.js

@@ -1195,7 +1195,19 @@ Zotero.Items = function() {
 	 * @return {Promise<String>}
 	 */
 	this._hashAttachmentText = async function (attachment) {
-		if ((await OS.File.stat(await attachment.getFilePathAsync())).size > 5e8) {
+		var fileInfo;
+		try {
+			fileInfo = await OS.File.stat(attachment.getFilePath());
+		}
+		catch (e) {
+			if (e instanceof OS.File.Error && e.becauseNoSuchFile) {
+				Zotero.debug('_hashAttachmentText: Attachment not found');
+				return null;
+			}
+			Zotero.logError(e);
+			return null;
+		}
+		if (fileInfo.size > 5e8) {
 			Zotero.debug('_hashAttachmentText: Attachment too large');
 			return null;
 		}

test/tests/itemsTest.js

@@ -517,6 +517,25 @@ describe("Zotero.Items", function () {
 			assert.isFalse(attachment2.deleted);
 		});
 		
+		it("should ignore attachment with missing file", async function () {
+			let item1 = await createDataObject('item');
+			let attachment1 = await importPDFAttachment(item1);
+			
+			let item2 = item1.clone();
+			await item2.saveTx();
+			let attachment2 = await importPDFAttachment(item2);
+			// Delete the attachment file
+			await OS.File.remove(await attachment2.getFilePathAsync());
+			
+			await Zotero.Items.merge(item1, [item2]);
+			
+			assert.isFalse(item1.deleted);
+			assert.isFalse(attachment1.deleted);
+			assert.equal(item1.numAttachments(true), 2);
+			assert.isTrue(item2.deleted);
+			assert.isFalse(attachment2.deleted);
+		});
+		
 		it("should allow small differences when hashing content", async function () {
 			let item1 = await createDataObject('item', { setTitle: true });
 			let attachment1 = await importFileAttachment('duplicatesMerge_JSTOR_1.pdf', { parentItemID: item1.id });