SERVER-34903 Allow killCursors within a transaction

4 files changed, 77 insertions, 0 deletions

diff --git a/jstests/core/txns/kill_cursors_in_transaction.js b/jstests/core/txns/kill_cursors_in_transaction.js
new file mode 100644
index 00000000000..e0acb9905dd
--- /dev/null
+++ b/jstests/core/txns/kill_cursors_in_transaction.js
@@ -0,0 +1,67 @@
+// Tests that the killCursors command is allowed in transactions.
+// @tags: [uses_transactions]
+(function() {
+    "use strict";
+
+    const dbName = "test";
+    const collName = "kill_cursors_in_transaction";
+    const testDB = db.getSiblingDB(dbName);
+    const session = db.getMongo().startSession({causalConsistency: false});
+    const sessionDb = session.getDatabase(dbName);
+    const sessionColl = sessionDb[collName];
+
+    sessionColl.drop();
+    for (let i = 0; i < 4; ++i) {
+        assert.commandWorked(sessionColl.insert({_id: i}));
+    }
+
+    jsTest.log("Test that the killCursors command is allowed in transactions.");
+
+    session.startTransaction();
+    let res = assert.commandWorked(sessionDb.runCommand({find: collName, batchSize: 2}));
+    assert(res.hasOwnProperty("cursor"), tojson(res));
+    assert(res.cursor.hasOwnProperty("id"), tojson(res));
+    assert.commandWorked(sessionDb.runCommand({killCursors: collName, cursors: [res.cursor.id]}));
+    session.commitTransaction();
+
+    jsTest.log("Test that the killCursors can be the first operation in a transaction.");
+    // Note that it is not a requirement to support this behavior. This test is present to ensure
+    // that the server does not crash or return an unhelpful error message.
+
+    res = assert.commandWorked(sessionDb.runCommand({find: collName, batchSize: 2}));
+    assert(res.hasOwnProperty("cursor"), tojson(res));
+    assert(res.cursor.hasOwnProperty("id"), tojson(res));
+    session.startTransaction();
+    assert.commandWorked(sessionDb.runCommand({killCursors: collName, cursors: [res.cursor.id]}));
+    session.commitTransaction();
+
+    jsTest.log("killCursors must not block on locks held by the transaction in which it is run.");
+
+    session.startTransaction();
+
+    // Open a cursor on the collection.
+    res = assert.commandWorked(sessionDb.runCommand({find: collName, batchSize: 2}));
+    assert(res.hasOwnProperty("cursor"), tojson(res));
+    assert(res.cursor.hasOwnProperty("id"), tojson(res));
+
+    // Start a drop, which will hang.
+    let awaitDrop = startParallelShell(function() {
+        db.getSiblingDB("test")["kill_cursors_in_transaction"].drop();
+    });
+
+    // Wait for the drop to have a pending MODE_X lock on the database.
+    assert.soon(function() {
+        return testDB.runCommand({find: collName, maxTimeMS: 100}).code ===
+            ErrorCodes.ExceededTimeLimit;
+    });
+
+    // killCursors does not block behind the pending MODE_X lock.
+    assert.commandWorked(sessionDb.runCommand({killCursors: collName, cursors: [res.cursor.id]}));
+
+    session.commitTransaction();
+
+    // Once the transaction has committed, the drop can proceed.
+    awaitDrop();
+
+    session.endSession();
+}());
diff --git a/src/mongo/db/commands/killcursors_cmd.cpp b/src/mongo/db/commands/killcursors_cmd.cpp
index e3dd7de3d35..ae6056c3289 100644
--- a/src/mongo/db/commands/killcursors_cmd.cpp
+++ b/src/mongo/db/commands/killcursors_cmd.cpp
@@ -47,6 +47,14 @@ class KillCursorsCmd final : public KillCursorsCmdBase {
 public:
     KillCursorsCmd() = default;
 
+    bool supportsReadConcern(const std::string& dbName,
+                             const BSONObj& cmdObj,
+                             repl::ReadConcernLevel level) const final {
+        // killCursors must support snapshot read concern in order to be run in transactions.
+        return level == repl::ReadConcernLevel::kLocalReadConcern ||
+            level == repl::ReadConcernLevel::kSnapshotReadConcern;
+    }
+
 private:
     Status _checkAuth(Client* client, const NamespaceString& nss, CursorId id) const final {
         auto opCtx = client->getOperationContext();
diff --git a/src/mongo/db/service_entry_point_common.cpp b/src/mongo/db/service_entry_point_common.cpp
index 5e29ff2d268..52c9ebab091 100644
--- a/src/mongo/db/service_entry_point_common.cpp
+++ b/src/mongo/db/service_entry_point_common.cpp
@@ -126,6 +126,7 @@ const StringMap<int> sessionCheckoutWhitelist = {{"abortTransaction", 1},
                                                  {"getMore", 1},
                                                  {"group", 1},
                                                  {"insert", 1},
+                                                 {"killCursors", 1},
                                                  {"mapReduce", 1},
                                                  {"parallelCollectionScan", 1},
                                                  {"prepareTransaction", 1},
diff --git a/src/mongo/db/session.cpp b/src/mongo/db/session.cpp
index ebdf3be0237..956b8c2c101 100644
--- a/src/mongo/db/session.cpp
+++ b/src/mongo/db/session.cpp
@@ -106,6 +106,7 @@ const StringMap<int> txnCmdWhitelist = {{"abortTransaction", 1},
                                         {"geoSearch", 1},
                                         {"getMore", 1},
                                         {"insert", 1},
+                                        {"killCursors", 1},
                                         {"prepareTransaction", 1},
                                         {"update", 1}};