Re: [PATCH v5 04/15] pkt-line: add packet_write_gently()

On Wed, Aug 10, 2016 at 03:04:00PM +0200, larsxschneider@gmail.com wrote:

From: Lars Schneider <redacted>

packet_write() has two shortcomings. First, it uses format_packet() which
lets the caller only send string data via "%s". That means it cannot be
used for arbitrary data that may contain NULs. Second, it will always
die on error.

Add packet_write_gently() which writes arbitrary data and returns `0` for
success and `-1` for an error.

So now we have packet_write() and packet_write_gently(), but they differ
in more than just whether they are gentle. That seems like a weird
interface.

Should we either be picking a new name (e.g., packet_write_mem() or
something), or migrating packet_write() to packet_write_fmt()?

Should this be static? I.e., are random other bits of the code allowed
to write into it (I guess not because it's not declared in pkt-line.h).

+int packet_write_gently(const int fd_out, const char *buf, size_t size)
+{
+	if (size > PKTLINE_DATA_MAXLEN)
+		return -1;
+	packet_trace(buf, size, 1);
+	memmove(packet_write_buffer + 4, buf, size);

It looks like this iteration drops the idea of callers using a
LARGE_PACKET_MAX buffer and only filling it at "buf+4" with
PKTLINE_DATA_MAXLEN bytes (which is fine).

I wonder if we still need PKTLINE_DATA_MAXLEN, or of it is just
obscuring things. The magic number "4" still appears separately here,
and it actually makes it harder to see that things are correct.  I.e.,
doing:

  if (size > sizeof(packet_write_buffer) - 4)
	return -1;
  memmove(packet_write_buffer + 4, buf, size);

is more obviously correct, because you do not have to wonder about the
relationship between the size of your buffer and the macro.

It might still be worth having PKTLINE_DATA_MAXLEN public, though, if
callers use it to size their input to packet_write_gently().

-Peff