SERVER-61427 Skip duplicate index key handler when exact key already exists in index

(cherry picked from commit 9f15b2b1eeb356f4e3d0c6e06302941ec685cc0a)
(cherry picked from commit cbfe08edee4a6aa41694b73be690b4e56de0bb3a)

1 files changed, 37 insertions, 0 deletions

diff --git a/jstests/noPassthrough/unique_index_insert_during_collection_scan.js b/jstests/noPassthrough/unique_index_insert_during_collection_scan.js
new file mode 100644
index 00000000000..947a9b234fe
--- /dev/null
+++ b/jstests/noPassthrough/unique_index_insert_during_collection_scan.js
@@ -0,0 +1,37 @@
+/**
+ * Tests inserting into collection while a unique index build is in the collection scan phase.
+ * Ensures that even though the insert is seen by both the collection scan and the side writes
+ * table, the index build does not need to resolve any duplicate keys.
+ */
+(function() {
+"use strict";
+
+load('jstests/libs/fail_point_util.js');
+load('jstests/libs/parallel_shell_helpers.js');
+
+const conn = MongoRunner.runMongod();
+const coll = conn.getDB('test')[jsTestName()];
+
+assert.commandWorked(coll.insert({_id: 0, a: 0}));
+
+const fp = configureFailPoint(conn, 'hangAfterInitializingIndexBuild');
+const awaitCreateIndex =
+    startParallelShell(funWithArgs(function(collName) {
+                           assert.commandWorked(db[collName].createIndex({a: 1}, {unique: true}));
+                       }, coll.getName()), conn.port);
+fp.wait();
+assert.commandWorked(coll.insert({_id: 1, a: 1}));
+fp.off();
+
+awaitCreateIndex();
+
+// Ensure that the second document was seen by both the collection scan and the side writes table.
+checkLog.containsJson(conn, 20685, {namespace: coll.getFullName(), index: 'a_1', keysInserted: 2});
+checkLog.containsJson(
+    conn, 20689, {namespace: coll.getFullName(), index: 'a_1', numApplied: 1, totalInserted: 1});
+
+// Ensure that there were no duplicates to resolve.
+assert(!checkLog.checkContainsOnceJson(conn, 20677, {indexName: 'a_1'}));
+
+MongoRunner.stopMongod(conn);
+})();