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

On 10 Aug 2016, at 15:28, Jeff King [off-list ref] wrote:

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

quoted

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()?

Done in "[PATCH v5 08/15] pkt-line: rename packet_write() to packet_write_fmt()"

quoted

diff --git a/pkt-line.c b/pkt-line.c
index e6b8410..4f25748 100644
--- a/pkt-line.c
+++ b/pkt-line.c

@@ -3,6 +3,7 @@

#include "run-command.h"

char packet_buffer[LARGE_PACKET_MAX];
+char packet_write_buffer[LARGE_PACKET_MAX];

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).

static is better!

quoted

+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().

I agree. In a later patch I am using PKTLINE_DATA_MAXLEN inside pkt-line.c,
too. I will change it to your suggestion.

For now I would remove PKTLINE_DATA_MAXLEN because it should be an implementation
detail of pkt-line.c (plus it is not used by anyone).

Thanks,
Lars