wallet2: make keys unlocker reentrant

protects against having your keys mangled

src/wallet/wallet2.cpp

@@ -1038,10 +1038,15 @@ uint64_t gamma_picker::pick()
   return first_rct + crypto::rand_idx(n_rct);
 };
 
+boost::mutex wallet_keys_unlocker::lockers_lock;
+unsigned int wallet_keys_unlocker::lockers = 0;
 wallet_keys_unlocker::wallet_keys_unlocker(wallet2 &w, const boost::optional<tools::password_container> &password):
   w(w),
   locked(password != boost::none)
 {
+  boost::lock_guard<boost::mutex> lock(lockers_lock);
+  if (lockers++ > 0)
+    locked = false;
   if (!locked || w.is_unattended() || w.ask_password() != tools::wallet2::AskPasswordToDecrypt || w.watch_only())
   {
     locked = false;
@@ -1056,6 +1061,9 @@ wallet_keys_unlocker::wallet_keys_unlocker(wallet2 &w, bool locked, const epee::
   w(w),
   locked(locked)
 {
+  boost::lock_guard<boost::mutex> lock(lockers_lock);
+  if (lockers++ > 0)
+    locked = false;
   if (!locked)
     return;
   w.generate_chacha_key_from_password(password, key);
@@ -1064,9 +1072,19 @@ wallet_keys_unlocker::wallet_keys_unlocker(wallet2 &w, bool locked, const epee::
 
 wallet_keys_unlocker::~wallet_keys_unlocker()
 {
+  try
+  {
+    boost::lock_guard<boost::mutex> lock(lockers_lock);
+    if (lockers == 0)
+    {
+      MERROR("There are no lockers in wallet_keys_unlocker dtor");
+      return;
+    }
+    --lockers;
     if (!locked)
       return;
-  try { w.encrypt_keys(key); }
+    w.encrypt_keys(key);
+  }
   catch (...)
   {
     MERROR("Failed to re-encrypt wallet keys");

src/wallet/wallet2.h

@@ -122,6 +122,8 @@ private:
     wallet2 &w;
     bool locked;
     crypto::chacha_key key;
+    static boost::mutex lockers_lock;
+    static unsigned int lockers;
   };
 
   class i_wallet2_callback