Add function to close all Db's and reload the databae environment

Adds a ReloadDbEnv function to BerkeleyEnvironment in order to close all Db
instances, closes the environment, resets it, and then reopens
the BerkeleyEnvironment.

Also adds a ReloadDbEnv function to BerkeleyDatabase that calls
BerkeleyEnvironment's ReloadDbEnv.

Github-Pull: bitcoin#12493
Rebased-From: 5d296ac

src/wallet/db.cpp

@@ -556,6 +556,7 @@ void BerkeleyBatch::Close()
         LOCK(cs_db);
         --env->mapFileUseCount[strFile];
     }
+    env->m_db_in_use.notify_all();
 }
 
 void BerkeleyEnvironment::CloseDb(const std::string& strFile)
@@ -572,6 +573,32 @@ void BerkeleyEnvironment::CloseDb(const std::string& strFile)
     }
 }
 
+void BerkeleyEnvironment::ReloadDbEnv()
+{
+    // Make sure that no Db's are in use
+    AssertLockNotHeld(cs_db);
+    std::unique_lock<CCriticalSection> lock(cs_db);
+    m_db_in_use.wait(lock, [this](){
+        for (auto& count : mapFileUseCount) {
+            if (count.second > 0) return false;
+        }
+        return true;
+    });
+
+    std::vector<std::string> filenames;
+    for (auto it : mapDb) {
+        filenames.push_back(it.first);
+    }
+    // Close the individual Db's
+    for (const std::string& filename : filenames) {
+        CloseDb(filename);
+    }
+    // Reset the environment
+    Flush(true); // This will flush and close the environment
+    Reset();
+    Open(true);
+}
+
 bool BerkeleyBatch::Rewrite(BerkeleyDatabase& database, const char* pszSkip)
 {
     if (database.IsDummy()) {
@@ -799,3 +826,10 @@ void BerkeleyDatabase::Flush(bool shutdown)
         if (shutdown) env = nullptr;
     }
 }
+
+void BerkeleyDatabase::ReloadDbEnv()
+{
+    if (!IsDummy()) {
+        env->ReloadDbEnv();
+    }
+}

src/wallet/db.h

@@ -38,6 +38,7 @@ class BerkeleyEnvironment
     std::unique_ptr<DbEnv> dbenv;
     std::map<std::string, int> mapFileUseCount;
     std::map<std::string, Db*> mapDb;
+    std::condition_variable_any m_db_in_use;
 
     BerkeleyEnvironment(const fs::path& env_directory);
     ~BerkeleyEnvironment();
@@ -75,6 +76,7 @@ class BerkeleyEnvironment
     void CheckpointLSN(const std::string& strFile);
 
     void CloseDb(const std::string& strFile);
+    void ReloadDbEnv();
 
     DbTxn* TxnBegin(int flags = DB_TXN_WRITE_NOSYNC)
     {
@@ -145,6 +147,8 @@ class BerkeleyDatabase
 
     void IncrementUpdateCounter();
 
+    void ReloadDbEnv();
+
     std::atomic<unsigned int> nUpdateCounter;
     unsigned int nLastSeen;
     unsigned int nLastFlushed;