fix learner's wal is not empty in rare cases

src/kvstore/raftex/RaftPart.cpp

@@ -401,6 +401,15 @@ const char* RaftPart::roleStr(Role role) const {
 void RaftPart::start(std::vector<HostAddr>&& peers, bool asLearner) {
   std::lock_guard<std::mutex> g(raftLock_);
 
+  // There are some rare cases that the part start as learner, but wal is not empty. For example,
+  // the node is dead, and one partition is removed from raft group (majority still alive). However,
+  // the part is added back to raft group again as learner. So the wal may be not empty, what is
+  // worse, there could be case that commitLogId is 0, but wal's lastLogId is not 0, which is
+  // obviously not expected
+  if (asLearner) {
+    wal_->reset();
+  }
+
   lastLogId_ = wal_->lastLogId();
   lastLogTerm_ = wal_->lastLogTerm();
   term_ = lastLogTerm_;

src/kvstore/wal/FileBasedWal.cpp

@@ -631,6 +631,7 @@ bool FileBasedWal::reset() {
     unlink(absFn.c_str());
   }
   lastLogId_ = firstLogId_ = 0;
+  lastLogTerm_ = 0;
   return true;
 }