Merge #21052: refactor: Replace fs::unique_path with GetUniquePath(path) calls

1bca2aa694 Introduce GetUniquePath(base) helper method to replace boost::filesystem::unique_path() which is not available in std::filesystem. (Kiminuo)

Pull request description:

  This PR makes it easier in #20744 to remove our dependency on the `boost::filesystem::unique_path()` function which does not have a direct equivalent in C++17.

  This PR attempts to re-implement `boost::filesystem::unique_path()` as `GetUniquePath(path)` but the implementations are not meant to be the same.

  Note:

  * Boost 1.75.0 implementation of `unique_path`: 9cab675b71/src/unique_path.cpp (L235)

  * In the previous implementation, I attempted to add:
      ```cpp
      fs::path GetUniquePath(const fs::path& base)
      {
          FastRandomContext rnd;
          fs::path tmpFile = base / HexStr(rnd.randbytes(8));
          return tmpFile;
      }
      ```

      to `fs.cpp` but this leads to a circular dependency: "fs -> random -> logging -> fs". That is why the modified implementation adds a new file.

ACKs for top commit:
  laanwj:
    Code review ACK 1bca2aa694
  ryanofsky:
    Code review ACK 1bca2aa694. It's a simple change and extra test coverage is nice

Tree-SHA512: f324bdf0e254160c616b5033c3ece33d87db23eb0135acee99346ade7b5cf0d30f3ceefe359a25a8e9b53ba8e4419f459c2bdd369e10fc0152ce95031d1f221c

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;