Single-threaded file watcher with overlapped I/O and filtering of multiple notifications

FileWatcher.cpp

	class FileWatcher
	{
	public:
		FileWatcher(const Path& path)
			: m_Path(path)
			, m_DirHandle(INVALID_HANDLE_VALUE)
			, m_BufferSize(CORE_KB(100))
			, m_Buffer(nullptr)
			, m_ChangedFiles(1024)
		{
			memset(&m_Overlapped, 0, sizeof(m_Overlapped));
			m_Buffer = new u8[m_BufferSize];
		}

		~FileWatcher()
		{
			// Abort any pending watches
			if (m_DirHandle != INVALID_HANDLE_VALUE)
			{
				CancelIo(m_DirHandle);
				DWORD nb_bytes;
				GetOverlappedResult(m_DirHandle, &m_Overlapped, &nb_bytes, TRUE);
			}

			if (m_DirHandle != INVALID_HANDLE_VALUE)
				CloseHandle(m_DirHandle);

			delete [] m_Buffer;
		}


		bool Init()
		{
			// Get a handle to the directory being watched
			m_DirHandle = CreateFileA(
				m_Path.c_str(),
				FILE_LIST_DIRECTORY,
				FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
				nullptr,
				OPEN_EXISTING,
				FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
				0);
			if (m_DirHandle == INVALID_HANDLE_VALUE)
				return false;

			// Create an event for async waiting
			// The even handle is unused when using completion routines
			// Use it to store the this pointer
			m_Overlapped.hEvent = this;

			// Kick-off the initial watch
			if (!Watch())
				return false;

			return true;
		}


		void Update(core::Vector<Path>& changed_files)
		{
			rmt_ScopedCPUSample(DirectoryWatcherUpdate);

			core::Milliseconds cur_time = core::GetLowResTimer();

			// Walk all changed files in the hash table
			core::Vector<u32> expired_changes;
			changed_files.clear_resize(0);
			for (core::HashTableIterator i(m_ChangedFiles); i.IsValid(); i.MoveNext())
			{
				ChangedFile& changed_file = *(ChangedFile*)i.GetPtr();

				// Pass filename back to the caller if not done before
				if (!changed_file.processed)
				{
					changed_files.push_back(changed_file.path);
					changed_file.processed = true;
				}

				// Track which changes are safe to discard
				if (cur_time - changed_file.first_change_time > core::Milliseconds(500))
					expired_changes.push_back(i.GetHash());
			}

			// Remove all expired file changes
			for (u32 i = 0; i < expired_changes.size(); i++)
				m_ChangedFiles.remove(expired_changes[i]);

			// Make the thread alertable so that IO events are processed
			MsgWaitForMultipleObjectsEx(0, NULL, 0, QS_ALLINPUT, MWMO_ALTERTABLE);
		}


private:
		void AddFile(const file::Path& path)
		{
			// Don't add if it's already been added
			ChangedFile change(path);
			if (m_ChangedFiles.find(change.hash))
				return;

			core::String512 text;
			text.setv("FILE WATCHER: Changed file %s", path.c_str());
			rmt_LogText(text.c_str());

			// Hash table owns the new entry
			m_ChangedFiles.insert(change.hash, new ChangedFile(change));
		}


		static void CALLBACK WatchCallback(DWORD dwErrorCode, DWORD dwNumberOfBytesTransferred, LPOVERLAPPED lpOverlapped)
		{
			if (dwNumberOfBytesTransferred == 0)
				return;
			if (dwErrorCode != 0)
				return;

			// Alias event in overlapped structure to get this pointer
			FileWatcher* watcher = (FileWatcher*)lpOverlapped->hEvent;
			if (watcher == nullptr)
				return;

			// Walk all notifications
			DWORD offset = 0;
			while (true)
			{
				FILE_NOTIFY_INFORMATION* notify = (FILE_NOTIFY_INFORMATION*)(watcher->m_Buffer + offset);

				// Hacky way to convert wchar path to ascii
				file::Path path;
				for (DWORD i = 0; i < notify->FileNameLength; i += 2)
					path.append(((char*)notify->FileName)[i]);

				watcher->AddFile(path);

				if (notify->NextEntryOffset == 0)
					break;
				offset += notify->NextEntryOffset;
			}

			// Kick-off the next watch
			watcher->Watch();
		}


		bool Watch()
		{
			return ReadDirectoryChangesW(
				m_DirHandle,
				m_Buffer,
				m_BufferSize,
				TRUE,
				FILE_NOTIFY_CHANGE_LAST_WRITE,
				NULL,
				&m_Overlapped,
				&WatchCallback) != 0;
		}


		OVERLAPPED m_Overlapped;

		Path m_Path;

		HANDLE m_DirHandle;

		const u32 m_BufferSize;
		u8* m_Buffer;

		// Hash table of recent changed files for filtering out double-notifications
		// from the file system
		core::HashTable m_ChangedFiles;
	};