fix: Flush buffers before renaming cache file (#2927)

If clio shuts itself down due to exceeding graceful period when cache is
saved and renamed but the buffers are not flushed, we may end up with a
corrupted cache file. Clio will detect corruption and will not load
corrupted cache file, but we could avoid it by explicitly flushing
ofstream buffer.

src/data/impl/LedgerCacheFile.cpp

@@ -115,6 +115,11 @@ LedgerCacheFile::write(DataView dataView)
     auto const hash = file.hash();
     file.write(hash.data(), decltype(hash)::bytes);
 
+    // flush internal buffer explicitly before renaming
+    if (auto const expectedSuccess = file.close(); not expectedSuccess.has_value()) {
+        return expectedSuccess;
+    }
+
     try {
         std::filesystem::rename(newFilePath, path_);
     } catch (std::exception const& e) {

src/data/impl/OutputFile.cpp

@@ -23,6 +23,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <expected>
 #include <ios>
 #include <string>
 #include <utility>
@@ -59,4 +60,14 @@ OutputFile::hash() const
     return std::move(sum).finalize();
 }
 
+std::expected<void, std::string>
+OutputFile::close()
+{
+    file_.close();
+    if (not file_) {
+        return std::unexpected{"Error closing cache file"};
+    }
+    return {};
+}
+
 }  // namespace data::impl

src/data/impl/OutputFile.hpp

@@ -25,6 +25,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <expected>
 #include <fstream>
 #include <string>
 
@@ -60,6 +61,9 @@ public:
     ripple::uint256
     hash() const;
 
+    std::expected<void, std::string>
+    close();
+
 private:
     void
     writeToFile(char const* data, size_t size);