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

1bca2aa694cd85984c09699ae28daec313077462 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`: https://github.com/boostorg/filesystem/blob/9cab675b71e98706886a87afe7c19eb9da568961/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 1bca2aa694cd85984c09699ae28daec313077462
  ryanofsky:
    Code review ACK 1bca2aa694cd85984c09699ae28daec313077462. It's a simple change and extra test coverage is nice

Tree-SHA512: f324bdf0e254160c616b5033c3ece33d87db23eb0135acee99346ade7b5cf0d30f3ceefe359a25a8e9b53ba8e4419f459c2bdd369e10fc0152ce95031d1f221c

6 files changed, 52 insertions, 3 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 2616eb8638..20f1316302 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -233,6 +233,7 @@ BITCOIN_CORE_H = \
   util/check.h \
   util/error.h \
   util/fees.h \
+  util/getuniquepath.h \
   util/golombrice.h \
   util/hasher.h \
   util/macros.h \
@@ -556,6 +557,7 @@ libbitcoin_util_a_SOURCES = \
   util/bytevectorhash.cpp \
   util/error.cpp \
   util/fees.cpp \
+  util/getuniquepath.cpp \
   util/hasher.cpp \
   util/system.cpp \
   util/message.cpp \
diff --git a/src/test/fs_tests.cpp b/src/test/fs_tests.cpp
index ec487aa3ff..e52cd5230c 100644
--- a/src/test/fs_tests.cpp
+++ b/src/test/fs_tests.cpp
@@ -5,6 +5,7 @@
 #include <fs.h>
 #include <test/util/setup_common.h>
 #include <util/system.h>
+#include <util/getuniquepath.h>
 
 #include <boost/test/unit_test.hpp>
 
@@ -69,6 +70,21 @@ BOOST_AUTO_TEST_CASE(fsbridge_fstream)
         BOOST_CHECK_EQUAL(tmpfile1, fsbridge::AbsPathJoin(tmpfile1, ""));
         BOOST_CHECK_EQUAL(tmpfile1, fsbridge::AbsPathJoin(tmpfile1, {}));
     }
+    {
+        fs::path p1 = GetUniquePath(tmpfolder);
+        fs::path p2 = GetUniquePath(tmpfolder);
+        fs::path p3 = GetUniquePath(tmpfolder);
+
+        // Ensure that the parent path is always the same.
+        BOOST_CHECK_EQUAL(tmpfolder, p1.parent_path());
+        BOOST_CHECK_EQUAL(tmpfolder, p2.parent_path());
+        BOOST_CHECK_EQUAL(tmpfolder, p3.parent_path());
+
+        // Ensure that generated paths are actually different.
+        BOOST_CHECK(p1 != p2);
+        BOOST_CHECK(p2 != p3);
+        BOOST_CHECK(p1 != p3);
+    }
 }
 
-BOOST_AUTO_TEST_SUITE_END()
+BOOST_AUTO_TEST_SUITE_END()
\ No newline at end of file
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index 4133f2623b..dccbf00d6f 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -13,6 +13,7 @@
 #include <test/util/setup_common.h>
 #include <test/util/str.h>
 #include <uint256.h>
+#include <util/getuniquepath.h>
 #include <util/message.h> // For MessageSign(), MessageVerify(), MESSAGE_MAGIC
 #include <util/moneystr.h>
 #include <util/spanparsing.h>
@@ -1816,7 +1817,7 @@ BOOST_AUTO_TEST_CASE(test_DirIsWritable)
     BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), true);
 
     // Should not be able to write to a non-existent dir.
-    tmpdirname = tmpdirname / fs::unique_path();
+    tmpdirname = GetUniquePath(tmpdirname);
     BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), false);
 
     fs::create_directory(tmpdirname);
diff --git a/src/util/getuniquepath.cpp b/src/util/getuniquepath.cpp
new file mode 100644
index 0000000000..9839d2f624
--- /dev/null
+++ b/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;
+}
\ No newline at end of file
diff --git a/src/util/getuniquepath.h b/src/util/getuniquepath.h
new file mode 100644
index 0000000000..e0c6147876
--- /dev/null
+++ b/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
\ No newline at end of file
diff --git a/src/util/system.cpp b/src/util/system.cpp
index 9fe9d67411..9a2e719bbc 100644
--- a/src/util/system.cpp
+++ b/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;