SERVER-22767 mongos segfault when invoking .explain() on certain oper…

…ations

jstests/noPassthrough/server22767.js

@@ -0,0 +1,21 @@
+// test that the mongos doesn't segfault when it receives malformed BSON
+var st = new ShardingTest({shards:1});
+var testDB = st.getDB('test');
+testDB.test.insert({a:1});
+
+try {
+    testDB.test.find({key: {$regex: 'abcd\0xyz'}}).explain();
+} catch (e) {
+    /*
+     * if the mongos segfaults, the error is the msg:
+     * "Error: error doing query: failed: network error while attempting to run command 'explain' on host '127.0.0.1:20014'"
+     *
+     * if the mongos doesn't segfault, the error is the object:
+     * "Error: explain failed: {
+     *   "code" : 22,
+     *   "ok" : 0,
+     *   "errmsg" : "bson length doesn't match what we found in object with unknown _id"
+     * }"
+     */
+    assert.eq(22, e.code);
+}

src/mongo/client/parallel.cpp

@@ -849,8 +849,14 @@ void ParallelSortClusteredCursor::finishInit(OperationContext* txn) {
                 }
                 throw;
             } else {
-                warning() << "db exception when finishing on " << shardId
-                          << ", current connection state is " << mdata.toBSON() << causedBy(e);
+                // the InvalidBSON exception indicates that the BSON is malformed ->
+                // don't print/call "mdata.toBSON()" to avoid unexpected errors e.g. a segfault
+                if (e.getCode() == 22)
+                    warning() << "bson is malformed :: db exception when finishing on " << shardId
+                              << causedBy(e);
+                else
+                    warning() << "db exception when finishing on " << shardId
+                              << ", current connection state is " << mdata.toBSON() << causedBy(e);
                 mdata.errored = true;
                 throw;
             }