SERVER-28830 check for closed conn in cursorHandleFromId

The old implementation of DBCommandCursor used when running the shell
with --readMode compatibility calls getConnection(which checks if the
connection has been closed) to construct a DBClientCursor which it
passes to setCursor. The new variant used with --readMode command calls
setCursorHandle with the cursor ID. Check that we aren't closed in the
new variant.

jstests/noPassthroughWithMongod/dbcommand_cursor_throws_on_closed_conn.js

@@ -0,0 +1,17 @@
+(function() {
+    "use strict";
+
+    var testDB = db.getSiblingDB('dbcommand_cursor_throws_on_closed_conn');
+    testDB.dropDatabase();
+    var coll = testDB.collection;
+    var conn = testDB.getMongo();
+    conn.forceReadMode("commands");
+    assert.commandWorked(coll.save({}));
+    var res = assert.commandWorked(testDB.runCommand({
+        find: coll.getName(),
+        batchSize: 0,
+    }));
+
+    conn.close();
+    assert.throws(() => new DBCommandCursor(testDB, res));
+}());

src/mongo/scripting/mozjs/mongo.cpp

@@ -530,6 +530,9 @@ void MongoBase::Functions::cursorHandleFromId::call(JSContext* cx, JS::CallArgs
         uasserted(ErrorCodes::BadValue, "2nd arg must be a NumberLong");
     }
 
+    // getConnectionRef verifies that the connection is still open
+    getConnectionRef(args);
+
     std::string ns = ValueWriter(cx, args.get(0)).toString();
     long long cursorId = NumberLongInfo::ToNumberLong(cx, args.get(1));