util: Fix multiple use of LockDirectory

This commit fixes problems with calling LockDirectory multiple times on
the same directory, or from multiple threads. It also fixes the build on
OpenBSD.

- Wrap the boost::interprocess::file_lock in a std::unique_ptr inside
  the map that keeps track of per-directory locks. This fixes a build
  issue with the clang 4.0.0+boost-1.58.0p8 version combo on OpenBSD
  6.2, and should have no observable effect otherwise.

- Protect the locks map using a mutex.

- Make sure that only locks that are successfully acquired are inserted
  in the map.

- Open the lock file for appending only if we know we don't have the
  lock yet - The `FILE* file = fsbridge::fopen(pathLockFile, "a");`
  wipes the 'we own this lock' administration, likely because it opens
  a new fd for the locked file then closes it.

src/util.cpp

@@ -375,18 +375,32 @@ int LogPrintStr(const std::string &str)
 
 bool LockDirectory(const fs::path& directory, const std::string lockfile_name, bool probe_only)
 {
+    // A map that contains all the currently held directory locks. After
+    // successful locking, these will be held here until the global
+    // destructor cleans them up and thus automatically unlocks them.
+    static std::map<std::string, std::unique_ptr<boost::interprocess::file_lock>> locks;
+    // Protect the map with a mutex
+    static std::mutex cs;
+    std::lock_guard<std::mutex> ulock(cs);
     fs::path pathLockFile = directory / lockfile_name;
-    FILE* file = fsbridge::fopen(pathLockFile, "a"); // empty lock file; created if it doesn't exist.
+
+    // If a lock for this directory already exists in the map, don't try to re-lock it
+    if (locks.count(pathLockFile.string())) {
+        return true;
+    }
+
+    // Create empty lock file if it doesn't exist.
+    FILE* file = fsbridge::fopen(pathLockFile, "a");
     if (file) fclose(file);
 
     try {
-        static std::map<std::string, boost::interprocess::file_lock> locks;
-        boost::interprocess::file_lock& lock = locks.emplace(pathLockFile.string(), pathLockFile.string().c_str()).first->second;
-        if (!lock.try_lock()) {
+        auto lock = MakeUnique<boost::interprocess::file_lock>(pathLockFile.string().c_str());
+        if (!lock->try_lock()) {
             return false;
         }
-        if (probe_only) {
-            lock.unlock();
+        if (!probe_only) {
+            // Lock successful and we're not just probing, put it into the map
+            locks.emplace(pathLockFile.string(), std::move(lock));
         }
     } catch (const boost::interprocess::interprocess_exception& e) {
         return error("Error while attempting to lock directory %s: %s", directory.string(), e.what());