util: Explain why the path is cached

Summary: This is a backport of Core [[bitcoin/bitcoin#16300 | PR16300]]

Test Plan:
  make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5774

src/util/system.cpp

@@ -1,5 +1,5 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
-// Copyright (c) 2009-2016 The Bitcoin Core developers
+// Copyright (c) 2009-2019 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -803,15 +803,14 @@ fs::path GetDefaultDataDir() {
 static fs::path g_blocks_path_cache_net_specific;
 static fs::path pathCached;
 static fs::path pathCachedNetSpecific;
-static CCriticalSection csPathCached;
+static RecursiveMutex csPathCached;
 
 const fs::path &GetBlocksDir() {
     LOCK(csPathCached);
-
     fs::path &path = g_blocks_path_cache_net_specific;
 
-    // This can be called during exceptions by LogPrintf(), so we cache the
-    // value so we don't have to do memory allocations after that.
+    // Cache the path to avoid calling fs::create_directories on every call of
+    // this function
     if (!path.empty()) {
         return path;
     }
@@ -834,11 +833,10 @@ const fs::path &GetBlocksDir() {
 
 const fs::path &GetDataDir(bool fNetSpecific) {
     LOCK(csPathCached);
-
     fs::path &path = fNetSpecific ? pathCachedNetSpecific : pathCached;
 
-    // This can be called during exceptions by LogPrintf(), so we cache the
-    // value so we don't have to do memory allocations after that.
+    // Cache the path to avoid calling fs::create_directories on every call of
+    // this function
     if (!path.empty()) {
         return path;
     }

src/util/system.h

@@ -1,5 +1,5 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
-// Copyright (c) 2009-2016 The Bitcoin Core developers
+// Copyright (c) 2009-2019 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -26,13 +26,11 @@
 
 #include <boost/thread/condition_variable.hpp> // for boost::thread_interrupted
 
-#include <atomic>
 #include <cstdint>
 #include <exception>
 #include <map>
 #include <set>
 #include <string>
-#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -84,6 +82,7 @@ fs::path GetDefaultDataDir();
 // The blocks directory is always net specific.
 const fs::path &GetBlocksDir();
 const fs::path &GetDataDir(bool fNetSpecific = true);
+/** Tests only */
 void ClearDatadirCache();
 fs::path GetConfigFile(const std::string &confPath);
 #ifdef WIN32