Assert valid linked list for write group (#7375)

Summary:
We've seen some segfaults in db_write_test, with at least one
suggesting corruption of a write group linked list. Adding an assertion
to have this fail in a more specific way if that is the broken
invariant.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7375

Test Plan: make check

Reviewed By: jay-zhuang

Differential Revision: D23638477

Pulled By: pdillinger

fbshipit-source-id: a76fd677cad60a3a516bd363947bfd9ce418edc1

db/write_thread.cc

@@ -438,6 +438,7 @@ size_t WriteThread::EnterAsBatchGroupLeader(Writer* leader,
   // (newest_writer) is inclusive. Iteration goes from old to new.
   Writer* w = leader;
   while (w != newest_writer) {
+    assert(w->link_newer);
     w = w->link_newer;
 
     if (w->sync && !leader->sync) {
@@ -512,6 +513,7 @@ void WriteThread::EnterAsMemTableWriter(Writer* leader,
 
     Writer* w = leader;
     while (w != newest_writer) {
+      assert(w->link_newer);
       w = w->link_newer;
 
       if (w->batch == nullptr) {
@@ -568,6 +570,7 @@ void WriteThread::ExitAsMemTableWriter(Writer* /*self*/,
     if (w == last_writer) {
       break;
     }
+    assert(next);
     w = next;
   }
   // Note that leader has to exit last, since it owns the write group.
@@ -721,6 +724,7 @@ void WriteThread::ExitAsBatchGroupLeader(WriteGroup& write_group,
     // leader now
 
     while (last_writer != leader) {
+      assert(last_writer);
       last_writer->status = status;
       // we need to read link_older before calling SetState, because as soon
       // as it is marked committed the other thread's Await may return and