SERVER-39274 Mongos shouldn't upconvert txn read concern level to snapshot if none is provided

1 files changed, 16 insertions, 10 deletions

diff --git a/jstests/core/txns/concurrent_drops_and_creates.js b/jstests/core/txns/concurrent_drops_and_creates.js
index 8db02f1ae98..b458fbd41e5 100644
--- a/jstests/core/txns/concurrent_drops_and_creates.js
+++ b/jstests/core/txns/concurrent_drops_and_creates.js
@@ -8,8 +8,10 @@
     const dbName2 = "test2";
     const collNameA = "coll_A";
     const collNameB = "coll_B";
-    const testDB1 = db.getSiblingDB(dbName1);
-    const testDB2 = db.getSiblingDB(dbName2);
+
+    const sessionOutsideTxn = db.getMongo().startSession({causalConsistency: true});
+    const testDB1 = sessionOutsideTxn.getDatabase(dbName1);
+    const testDB2 = sessionOutsideTxn.getDatabase(dbName2);
     testDB1.runCommand({drop: collNameA, writeConcern: {w: "majority"}});
     testDB2.runCommand({drop: collNameB, writeConcern: {w: "majority"}});
 
@@ -20,8 +22,8 @@
     const sessionCollB = sessionDB2[collNameB];
 
     //
-    // A transaction cannot write to a collection that has been dropped since the transaction
-    // started.
+    // A transaction with snapshot read concern cannot write to a collection that has been dropped
+    // since the transaction started.
     //
 
     // Ensure collection A and collection B exist.
@@ -29,10 +31,12 @@
     assert.commandWorked(sessionCollB.insert({}));
 
     // Start the transaction with a write to collection A.
-    session.startTransaction();
+    session.startTransaction({readConcern: {level: "snapshot"}});
     assert.commandWorked(sessionCollA.insert({}));
 
-    // Drop collection B outside of the transaction.
+    // Drop collection B outside of the transaction. Advance the cluster time of the session
+    // performing the drop to ensure it happens at a later cluster time than the transaction began.
+    sessionOutsideTxn.advanceClusterTime(session.getClusterTime());
     assert.commandWorked(testDB2.runCommand({drop: collNameB, writeConcern: {w: "majority"}}));
 
     // We cannot write to collection B in the transaction, since it is illegal to implicitly create
@@ -45,8 +49,8 @@
                                  ErrorCodes.NoSuchTransaction);
 
     //
-    // A transaction cannot write to a collection that has been created since the transaction
-    // started.
+    // A transaction with snapshot read concern cannot write to a collection that has been created
+    // since the transaction started.
     //
 
     // Ensure collection A exists and collection B does not exist.
@@ -54,10 +58,12 @@
     testDB2.runCommand({drop: collNameB, writeConcern: {w: "majority"}});
 
     // Start the transaction with a write to collection A.
-    session.startTransaction();
+    session.startTransaction({readConcern: {level: "snapshot"}});
     assert.commandWorked(sessionCollA.insert({}));
 
-    // Create collection B outside of the transaction.
+    // Create collection B outside of the transaction. Advance the cluster time of the session
+    // performing the drop to ensure it happens at a later cluster time than the transaction began.
+    sessionOutsideTxn.advanceClusterTime(session.getClusterTime());
     assert.commandWorked(testDB2.runCommand({create: collNameB}));
 
     // We cannot write to collection B in the transaction, since it experienced catalog changes