Re: [PATCH v8 06/11] pkt-line: add packet_write_gently()

From: Jeff King <peff@peff.net>
Date: Sep 27, 2016, 08:39 UTC
Message-ID: <20160927083939.par2ea5nqjrtkkjs@sigill.intra.peff.net>
In-Reply-To: <4CF26590-7B88-4DB6-B6D4-A6CFE0FD6ABF@gmail.com>

On Mon, Sep 26, 2016 at 09:21:10PM +0200, Lars Schneider wrote:

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> On 25 Sep 2016, at 13:26, Jakub Narębski <jnareb@gmail.com> wrote:
> 
> > W dniu 20.09.2016 o 21:02, larsxschneider@gmail.com pisze:
> >> From: Lars Schneider <larsxschneider@gmail.com>
> >> ...
> >> 
> >> +static int packet_write_gently(const int fd_out, const char *buf, size_t size)
> > 
> > I'm not sure what naming convention the rest of Git uses, but isn't
> > it more like '*data' rather than '*buf' here?
> 
> pkt-line seems to use 'buf' or 'buffer' for everything else.

I do not think we have definite rules, but I would generally expect to see "data" as an opaque thing (e.g., passing "void *data" to callbacks). "buf" or "buffer" makes sense here, but I don't think it really matters that much either way.

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> >> +	static char packet_write_buffer[LARGE_PACKET_MAX];
> > 
> > I think there should be warning (as a comment before function
> > declaration, or before function definition), that packet_write_gently()
> > is not thread-safe (nor reentrant, but the latter does not matter here,
> > I think).
> > 
> > Thread-safe vs reentrant: http://stackoverflow.com/a/33445858/46058
> > 
> > This is not something terribly important; I guess git code has tons
> > of functions not marked as thread-unsafe...
> 
> I agree that the function is not thread-safe. However, I can't find 
> an example in the Git source that marks a function as not thread-safe.
> Unless is it explicitly stated in the coding guidelines I would prefer
> not to start way to mark functions.

I'd agree. A large number of functions in git are not reentrant, and I would not want to give the impression that those missing a warning are safe to use.

Show 8 quoted lines

> >> +	if (size > sizeof(packet_write_buffer) - 4) {
> > 
> > First, wouldn't the following be more readable:
> > 
> >  +	if (size + 4 > LARGE_PACKET_MAX) {
> 
> Peff suggested that here:
> http://public-inbox.org/git/20160810132814.gqnipsdwyzjmuqjy@sigill.intra.peff.net/

There is a good reason to do size checks as a subtraction from a known quantity: you can be sure that you are not introducing an overflow (e.g., Jakub's suggestion does the wrong thing when "size" is within 4 bytes of its maximum value). That's unlikely in this case, but then so is the size exceeding LARGE_PACKET_MAX in the first place (arguably this should be a die("BUG"), because it is the caller's responsibility to split their packets.

-Peff