fix: resolve relative paths for atomic write target (#1564)

Fixes #1383

This solves an edge case where the target path of an atomic write is
a relative path without a directory part.
In this case, the temporary file was written to the system's tempdir
before, which may then fail the atomic rename as the tempdir often
is on a different device such as tmpfs.

packager/file/file.cc

@@ -74,7 +74,19 @@ bool DeleteLocalFile(const char* file_name) {
 
 bool WriteLocalFileAtomically(const char* file_name,
                               const std::string& contents) {
-  const auto file_path = std::filesystem::u8path(file_name);
+  std::error_code ec;
+  // To atomically move the temporary file to the target location after write,
+  // they must be on the same device. For relative paths without a directory
+  // part, the parent path is empty, which fails TempFilePath's empty directory
+  // path check, thus falling back to /tmp that is on a potentially different
+  // device. We prevent this by resolving the absolute file path.
+  const auto file_path =
+      std::filesystem::absolute(std::filesystem::u8path(file_name), ec);
+  if (ec) {
+    LOG(ERROR) << "Failed to resolve file path '" << file_name
+               << "', error: " << ec;
+    return false;
+  }
   const auto dir_path = file_path.parent_path();
 
   std::string temp_file_name;
@@ -83,7 +95,6 @@ bool WriteLocalFileAtomically(const char* file_name,
   if (!File::WriteStringToFile(temp_file_name.c_str(), contents))
     return false;
 
-  std::error_code ec;
   auto temp_file_path = std::filesystem::u8path(temp_file_name);
   std::filesystem::rename(temp_file_path, file_name, ec);
   if (ec) {