SERVER-45944 disallow downgrade to 4.2 if there is an index build in progress

2 files changed, 18 insertions, 1 deletions

diff --git a/jstests/noPassthrough/index_downgrade_fcv.js b/jstests/noPassthrough/index_downgrade_fcv.js
index bbc29a5664b..a999feac75a 100644
--- a/jstests/noPassthrough/index_downgrade_fcv.js
+++ b/jstests/noPassthrough/index_downgrade_fcv.js
@@ -36,7 +36,9 @@ IndexBuildTest.waitForIndexBuildToScanCollection(testDB, coll.getName(), 'a_1');
 
 // Downgrade the primary using the setFeatureCompatibilityVersion command.
 try {
-    assert.commandWorked(primary.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
+    assert.commandFailedWithCode(
+        primary.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}),
+        ErrorCodes.ConflictingOperationInProgress);
 } finally {
     IndexBuildTest.resumeIndexBuilds(primary);
 }
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index 6dc12d4af3f..7904e8db864 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -43,6 +43,7 @@
 #include "mongo/db/concurrency/d_concurrency.h"
 #include "mongo/db/db_raii.h"
 #include "mongo/db/dbdirectclient.h"
+#include "mongo/db/index_builds_coordinator.h"
 #include "mongo/db/namespace_string.h"
 #include "mongo/db/ops/write_ops.h"
 #include "mongo/db/read_write_concern_defaults.h"
@@ -289,6 +290,20 @@ public:
                     }
                 }
             }
+
+            // Two phase index builds are only supported in 4.4. If the user tries to downgrade the
+            // cluster to FCV42, they must first wait for all index builds to run to completion, or
+            // abort the index builds (using the dropIndexes command).
+            if (auto indexBuildsCoord = IndexBuildsCoordinator::get(opCtx)) {
+                auto numIndexBuilds = indexBuildsCoord->getActiveIndexBuildCount(opCtx);
+                uassert(
+                    ErrorCodes::ConflictingOperationInProgress,
+                    str::stream()
+                        << "Cannot downgrade the cluster when there are index builds in progress: "
+                        << numIndexBuilds,
+                    numIndexBuilds == 0U);
+            }
+
             FeatureCompatibilityVersion::setTargetDowngrade(opCtx);
 
             {