missing fsync in atomic create

[photoszzt]

I think this code is not atomic.

gg/src/util/path.cc

Lines 188 to 209 in febdf22

	void atomic_create( const string & contents, const path & dst,
	const bool set_mode, const mode_t target_mode )
	{
	string tmp_file_name;
	{
	UniqueFile tmp_file { dst.string() };
	tmp_file_name = tmp_file.name();
	if ( contents.size() > 0 ) {
	tmp_file.fd().write( contents );
	}
	if ( set_mode ) {
	CheckSystemCall( "fchmod", fchmod( tmp_file.fd().fd_num(), target_mode ) );
	}
	/* allow block to end so the UniqueFile gets closed() before rename. */
	/* not 100% sure readers will see fully-written file appear atomically otherwise */
	}
	rename( tmp_file_name, dst.string() );
	}

Two fsync is missing. You need to fsync the temp file and fsync the containing directory after the rename. See: https://lwn.net/Articles/457667/

[JustinAzoff]

I don't thing consistency in the face of a kernel panic or power failure is a requirement based on how that code is used. Adding 2 fsyncs to every file creation would just slow builds down.

[photoszzt]

@JustinAzoff The problem is if I don't put these two fsyncs, the program fails. It complaints that the file is missing when the file is created but the directory is not fsync.