Don't throw when READ_WRITE mode is used; add .mcap file extension to…

… recorded files (ros2#14)

I may be missing something, but from a cursory glance at [this code](https://github.com/ros2/rosbag2/blob/342d8ed3c1c4ae0411a4a92b60e79a728b8974b8/rosbag2_storage/src/rosbag2_storage/impl/storage_factory_impl.hpp#L108-L135), it appears that the `APPEND` mode is never used. This means we need to support `READ_WRITE`.

This also adds a `.mcap` extension to recorded file names.

Signed-off-by: Jacob Bandes-Storch <jacob@foxglove.dev>

rosbag2_storage_mcap/src/mcap_storage.cpp

@@ -32,6 +32,8 @@
 
 namespace rosbag2_storage_plugins {
 
+static const char FILE_EXTENSION[] = ".mcap";
+
 /**
  * A storage implementation for the MCAP file format.
  */
@@ -120,11 +122,10 @@ MCAPStorage::~MCAPStorage() {
 /** BaseIOInterface **/
 void MCAPStorage::open(const rosbag2_storage::StorageOptions& storage_options,
                        rosbag2_storage::storage_interfaces::IOFlag io_flag) {
+  relative_path_ = storage_options.uri + FILE_EXTENSION;
+
   switch (io_flag) {
-    case rosbag2_storage::storage_interfaces::IOFlag::READ_WRITE:
-      throw std::runtime_error("MCAPStorage does not support READ_WRITE mode");
     case rosbag2_storage::storage_interfaces::IOFlag::READ_ONLY: {
-      relative_path_ = storage_options.uri;
       input_ = std::make_unique<std::ifstream>(relative_path_, std::ios::binary);
       data_source_ = std::make_unique<mcap::FileStreamReader>(*input_);
       mcap_reader_ = std::make_unique<mcap::McapReader>();
@@ -138,8 +139,11 @@ void MCAPStorage::open(const rosbag2_storage::StorageOptions& storage_options,
       linear_iterator_ = std::make_unique<mcap::LinearMessageView::Iterator>(linear_view_->begin());
       break;
     }
+    case rosbag2_storage::storage_interfaces::IOFlag::READ_WRITE:
     case rosbag2_storage::storage_interfaces::IOFlag::APPEND: {
-      relative_path_ = storage_options.uri;
+      // APPEND does not seem to be used; treat it the same as READ_WRITE
+      io_flag = rosbag2_storage::storage_interfaces::IOFlag::READ_WRITE;
+
       mcap_writer_ = std::make_unique<mcap::McapWriter>();
       mcap::McapWriterOptions options{"ros2"};
       // TODO(jhurliman): Use storage_options max_bagfile_duration / max_bagfile_size