pw_transfer: Fix File Transfer Handler

The transfer failed with two separate issues
that this CL fixes:
1. Renaming/Copy fails when the full path
 to the target file does not exist.
2. Rename fails when tmp file and target file
are not on the same file system.

Change-Id: I964cb3a221ec0aed59534fc23b53651f1babac57
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/109811
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Moussa Traore <traorem@google.com>

pw_transfer/atomic_file_transfer_handler.cc

@@ -23,6 +23,59 @@
 
 namespace pw::transfer {
 
+namespace {
+// Linux Error code for Cross-Device Link Error.
+constexpr auto CROSS_DEVICE_LINK_ERROR = 18;
+
+pw::Status EnsureDirectoryExists(std::string_view filepath) {
+  const auto target_directory = std::filesystem::path{filepath}.parent_path();
+  return std::filesystem::exists(target_directory) ||
+                 std::filesystem::create_directories(target_directory)
+             ? pw::OkStatus()
+             : pw::Status::Internal();
+}
+
+// Copy file and remove on succes.
+// If the copy fails, the file `input_target` is not removed.
+pw::Status CopyFile(const std::string_view input_target,
+                    const std::string_view output_target) {
+  auto err = std::error_code{};
+  std::filesystem::copy(input_target,
+                        output_target,
+                        std::filesystem::copy_options::overwrite_existing,
+                        err);
+  if (err) {
+    PW_LOG_ERROR("Error with status code: %d (%s) during copy of file %s",
+                 err.value(),
+                 err.message().c_str(),
+                 input_target.data());
+    return pw::Status::Internal();
+  }
+  PW_LOG_INFO("Successfully copied the file.");
+  if (!std::filesystem::remove(input_target)) {
+    PW_LOG_WARN("Failed to remove tmp file %s", input_target.data());
+  }
+  return pw::OkStatus();
+}
+
+// Uses the same approach as unix `mv` command. First try to rename. If we get
+// a cross-device link error, copies then deletes input_target.
+pw::Status RenameFile(const std::string_view input_target,
+                      const std::string_view output_target) {
+  auto err = std::error_code{};
+  std::filesystem::rename(input_target, output_target, err);
+  if (err && err.value() == CROSS_DEVICE_LINK_ERROR) {
+    PW_LOG_INFO("%s[%d] during rename of file %s. Trying Copy/Remove.",
+                err.message().c_str(),
+                err.value(),
+                input_target.data());
+    return CopyFile(input_target, output_target);
+  }
+  return err ? pw::Status::Internal() : pw::OkStatus();
+}
+
+}  // namespace
+
 Status AtomicFileTransferHandler::PrepareRead() {
   auto file_path = path_.c_str();
   PW_LOG_DEBUG("Preparing read for file %s", file_path);
@@ -47,23 +100,29 @@ Status AtomicFileTransferHandler::PrepareWrite() {
 
 Status AtomicFileTransferHandler::FinalizeWrite(Status status) {
   std::get<stream::StdFileWriter>(stream_).Close();
-  const auto tmp_file = GetTempFilePath(path_);
-  auto temp_file_path = tmp_file.c_str();
-  auto file_path = path_.c_str();
-  if (!status.ok() || !std::filesystem::exists(temp_file_path) ||
-      std::filesystem::is_empty(temp_file_path)) {
-    PW_LOG_ERROR("Status not ok, remove file %s", temp_file_path);
+  auto tmp_file = GetTempFilePath(path_);
+  if (!status.ok() || !std::filesystem::exists(tmp_file) ||
+      std::filesystem::is_empty(tmp_file)) {
+    PW_LOG_ERROR("Transfer unsuccesful, attempt to remove temp file %s",
+                 tmp_file.c_str());
     // Remove temp file if transfer fails.
     return std::filesystem::remove(tmp_file) ? status : Status::Aborted();
   }
-  PW_LOG_DEBUG("Renaming file from: %s, to: %s", temp_file_path, file_path);
-  std::error_code err;
-  std::filesystem::rename(temp_file_path, file_path, err);
-  if (err) {
-    PW_LOG_ERROR("Error during renaming of file %s", temp_file_path);
-    return std::filesystem::remove(tmp_file) ? Status::Internal()
-                                             : Status::Aborted();
+
+  const auto directory_exists_status = EnsureDirectoryExists(path_);
+  if (!directory_exists_status.ok()) {
+    std::filesystem::remove(tmp_file);
+    return directory_exists_status;
+  }
+
+  PW_LOG_DEBUG(
+      "Copying file from: %s, to: %s", tmp_file.c_str(), path_.c_str());
+  const auto rename_status = RenameFile(tmp_file, path_);
+  if (!rename_status.ok()) {
+    std::filesystem::remove(tmp_file);
+    return rename_status;
   }
+
   PW_LOG_INFO("File transfer was successful.");
   return OkStatus();
 }