SERVER-44397 two phase index builds with unique indexes do not fail over on stepdown

2 files changed, 80 insertions, 2 deletions

diff --git a/jstests/noPassthrough/index_stepdown_unique.js b/jstests/noPassthrough/index_stepdown_unique.js
new file mode 100644
index 00000000000..cbdbe6408fc
--- /dev/null
+++ b/jstests/noPassthrough/index_stepdown_unique.js
@@ -0,0 +1,62 @@
+/**
+ * Confirms that unique index builds on a primary are aborted when the node steps down during the
+ * collection scan phase. This applies to both two phase and single phase index builds.
+ * @tags: [requires_replication]
+ */
+(function() {
+"use strict";
+
+load('jstests/libs/check_log.js');
+load('jstests/noPassthrough/libs/index_build.js');
+
+const rst = new ReplSetTest({
+    nodes: [
+        {},
+        {
+            // Disallow elections on secondary.
+            rsConfig: {
+                priority: 0,
+                votes: 0,
+            },
+        },
+    ]
+});
+const nodes = rst.startSet();
+rst.initiate();
+
+const primary = rst.getPrimary();
+const testDB = primary.getDB('test');
+const coll = testDB.getCollection('test');
+
+assert.commandWorked(coll.insert({a: 1}));
+
+IndexBuildTest.pauseIndexBuilds(primary);
+
+const createIdx =
+    IndexBuildTest.startIndexBuild(primary, coll.getFullName(), {a: 1}, {unique: true});
+
+// When the index build starts, find its op id.
+const opId = IndexBuildTest.waitForIndexBuildToScanCollection(testDB, coll.getName(), 'a_1');
+
+IndexBuildTest.assertIndexBuildCurrentOpContents(testDB, opId);
+
+try {
+    // Step down the primary.
+    assert.commandWorked(primary.adminCommand({replSetStepDown: 60, force: true}));
+} finally {
+    IndexBuildTest.resumeIndexBuilds(primary);
+}
+
+// Wait for the index build to stop.
+IndexBuildTest.waitForIndexBuildToStop(testDB);
+
+const exitCode = createIdx({checkExitSuccess: false});
+assert.neq(0, exitCode, 'expected shell to exit abnormally due to index build being terminated');
+
+// Wait for the IndexBuildCoordinator thread, not the command thread, to report the index build
+// as failed.
+checkLog.contains(primary, '[IndexBuildsCoordinatorMongod-0] Index build failed: ');
+IndexBuildTest.assertIndexes(coll, 1, ['_id_']);
+
+rst.stopSet();
+})();
diff --git a/src/mongo/db/commands/create_indexes.cpp b/src/mongo/db/commands/create_indexes.cpp
index 68bcb6de670..94636ac0247 100644
--- a/src/mongo/db/commands/create_indexes.cpp
+++ b/src/mongo/db/commands/create_indexes.cpp
@@ -398,6 +398,20 @@ Collection* getOrCreateCollection(OperationContext* opCtx,
 }
 
 /**
+ * Returns true if index specs include any unique indexes. Due to uniqueness constraints set up at
+ * the start of the index build, we are not able to support failing over a two phase index build on
+ * a unique index to a new primary on stepdown.
+ */
+bool containsUniqueIndexes(const std::vector<BSONObj>& specs) {
+    for (const auto& spec : specs) {
+        if (spec["unique"].trueValue()) {
+            return true;
+        }
+    }
+    return false;
+}
+
+/**
  * Creates indexes using the given specs for the mobile storage engine.
  * TODO(SERVER-42513): Remove this function.
  */
@@ -741,7 +755,8 @@ bool runCreateIndexesWithCoordinator(OperationContext* opCtx,
             // If this node is no longer a primary, the index build will continue to run in the
             // background and will complete when this node receives a commitIndexBuild oplog entry
             // from the new primary.
-            if (indexBuildsCoord->supportsTwoPhaseIndexBuild() &&
+            // TODO(SERVER-44654): re-enable failover support for unique indexes.
+            if (indexBuildsCoord->supportsTwoPhaseIndexBuild() && !containsUniqueIndexes(specs) &&
                 ErrorCodes::InterruptedDueToReplStateChange == interruptionEx.code()) {
                 log() << "Index build continuing in background: " << buildUUID;
                 throw;
@@ -766,7 +781,8 @@ bool runCreateIndexesWithCoordinator(OperationContext* opCtx,
 
             // The index build will continue to run in the background and will complete when this
             // node receives a commitIndexBuild oplog entry from the new primary.
-            if (indexBuildsCoord->supportsTwoPhaseIndexBuild()) {
+            // TODO(SERVER-44654): re-enable failover support for unique indexes.
+            if (indexBuildsCoord->supportsTwoPhaseIndexBuild() && !containsUniqueIndexes(specs)) {
                 log() << "Index build continuing in background: " << buildUUID;
                 throw;
             }