Fix "c1 is undefined" sync CR error

chrome/content/zotero/xpcom/sync/syncLocal.js

@@ -1486,6 +1486,10 @@ Zotero.Sync.Data.Local = {
 		for (let i = 0; i < changeset1.length; i++) {
 			for (let j = 0; j < changeset2.length; j++) {
 				let c1 = changeset1[i];
+				// If we've removed all local changes, keep remaining remote changes
+				if (!c1) {
+					break;
+				}
 				let c2 = changeset2[j];
 				if (c1.field != c2.field) {
 					continue;
@@ -1507,6 +1511,9 @@ Zotero.Sync.Data.Local = {
 							if (c1.op == 'member-add' && c2.op == 'member-add'
 									&& c1.value.tag === c2.value.tag) {
 								changeset1.splice(i--, 1);
+								// We're in the inner loop without an incrementor for i, so don't go
+								// below 0
+								if (i < 0) i = 0;
 								changeset2.splice(j--, 1);
 								if (c1.value.type > 0) {
 									changeset2.push({

test/tests/syncLocalTest.js

@@ -2191,7 +2191,78 @@ describe("Zotero.Sync.Data.Local", function() {
 				);
 				assert.lengthOf(result.conflicts, 0);
 			})
-		})
+		});
+		
+		
+		describe("tags", function () {
+			// https://forums.zotero.org/discussion/79429/syncing-error-c1-is-undefined
+			it("should handle multiple local type 1 and remote type 0", async function () {
+				var cacheJSON = {
+					tags: []
+				};
+				var json1 = {
+					tags: [
+						{
+							tag: 'C',
+							type: 1
+						},
+						{
+							tag: 'D',
+							type: 1
+						}
+					]
+				};
+				var json2 = {
+					tags: [
+						{
+							tag: 'C'
+						},
+						{
+							tag: 'D'
+						}
+					]
+				};
+				var result = Zotero.Sync.Data.Local._reconcileChanges(
+					'tag', cacheJSON, json1, json2
+				);
+				assert.lengthOf(result.changes, 4);
+				assert.sameDeepMembers(
+					result.changes,
+					[
+						{
+							field: "tags",
+							op: "member-remove",
+							value: {
+								tag: "C",
+								type: 1
+							}
+						},
+						{
+							field: "tags",
+							op: "member-add",
+							value: {
+								tag: "C"
+							}
+						},
+						{
+							field: "tags",
+							op: "member-remove",
+							value: {
+								tag: "D",
+								type: 1
+							}
+						},
+						{
+							field: "tags",
+							op: "member-add",
+							value: {
+								tag: "D"
+							}
+						}
+					]
+				);
+			});
+		});
 	})