Keep Downloads on an Unused Name Until They Complete

When the server supplied the name, the download marker stayed under the
URL-derived name, so the adopted name was not reserved against other
downloads. The final rename also replaced any file that took the name
while the download ran.

Move the marker to the adopted name before any data is written, and
check the name again right before the final rename, picking the next
free name if it is taken by then.

src/slic3r/GUI/DownloaderFileGet.cpp

@@ -213,6 +213,20 @@ void FileGet::priv::get_perform()
 					} catch (const boost::filesystem::filesystem_error&) {
 						unused.clear();
 					}
+					const boost::filesystem::path tmp_path = unused.empty() ? m_tmp_path : download_marker_path(m_dest_folder, unused);
+					if (tmp_path != m_tmp_path) {
+						// Move the marker to the adopted name so that other downloads see the name as taken.
+						// Only before anything is written, so that no downloaded data has to be carried over.
+						FILE* tmp_file = m_written == 0 ? fopen(wxString(tmp_path.wstring()).c_str(), "wb") : nullptr;
+						if (tmp_file != nullptr) {
+							fclose(file);
+							boost::system::error_code ec;
+							boost::filesystem::remove(m_tmp_path, ec);
+							file = tmp_file;
+							m_tmp_path = tmp_path;
+						} else
+							unused.clear();
+					}
 					if (!unused.empty())
 						m_filename = unused;
 					dest_path = m_dest_folder / m_filename;
@@ -320,6 +334,18 @@ void FileGet::priv::get_perform()
                     m_evt_handler->QueueEvent(evt);
                 }
 				fclose(file);
+				// Another file may have taken the name while downloading.
+				if (!dest_path.empty() && boost::filesystem::exists(dest_path)) {
+					std::string unused;
+					if (!find_unused_filename(m_dest_folder, m_filename, m_tmp_path, unused))
+						throw std::runtime_error("No unused file name.");
+					m_filename = unused;
+					dest_path = m_dest_folder / m_filename;
+					wxCommandEvent* evt = new wxCommandEvent(EVT_DWNLDR_FILE_NAME_CHANGE);
+					evt->SetString(boost::nowide::widen(m_filename));
+					evt->SetInt(m_id);
+					m_evt_handler->QueueEvent(evt);
+				}
 				boost::filesystem::rename(m_tmp_path, dest_path);
 			}
 			catch (const std::exception& /*e*/)