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

[Lars Schneider <larsxschneider@gmail.com>]

> On 10 Aug 2016, at 15:28, Jeff King <peff@peff.net> wrote:
> 
> On Wed, Aug 10, 2016 at 03:04:00PM +0200, larsxschneider@gmail.com wrote:
> 
>> From: Lars Schneider <larsxschneider@gmail.com>
>> 
>> 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()"


>> 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!


>> +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