Introduce GetUniquePath(base) helper method to replace boost::filesystem::unique_path() which is not available in std::filesystem.

src/util/getuniquepath.cpp

@@ -0,0 +1,10 @@
+#include <random.h>
+#include <fs.h>
+#include <util/strencodings.h>
+
+fs::path GetUniquePath(const fs::path& base)
+{
+    FastRandomContext rnd;
+    fs::path tmpFile = base / HexStr(rnd.randbytes(8));
+    return tmpFile;
+}

src/util/getuniquepath.h

@@ -0,0 +1,19 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_GETUNIQUEPATH_H
+#define BITCOIN_UTIL_GETUNIQUEPATH_H
+
+#include <fs.h>
+
+/**
+ * Helper function for getting a unique path
+ *
+ * @param[in] base  Base path
+ * @returns base joined with a random 8-character long string.
+ * @post Returned path is unique with high probability.
+ */
+fs::path GetUniquePath(const fs::path& base);
+
+#endif // BITCOIN_UTIL_GETUNIQUEPATH_H

src/util/system.cpp

@@ -12,6 +12,7 @@
 #include <chainparamsbase.h>
 #include <sync.h>
 #include <util/check.h>
+#include <util/getuniquepath.h>
 #include <util/strencodings.h>
 #include <util/string.h>
 #include <util/translation.h>
@@ -124,7 +125,7 @@ void ReleaseDirectoryLocks()
 
 bool DirIsWritable(const fs::path& directory)
 {
-    fs::path tmpFile = directory / fs::unique_path();
+    fs::path tmpFile = GetUniquePath(directory);
 
     FILE* file = fsbridge::fopen(tmpFile, "a");
     if (!file) return false;