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

W dniu 27.09.2016 o 10:39, Jeff King pisze:

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

quoted

On 25 Sep 2016, at 13:26, Jakub Narębski [off-list ref] wrote:

quoted

W dniu 20.09.2016 o 21:02, larsxschneider@gmail.com pisze:

quoted

From: Lars Schneider <redacted>
...

+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.

True.

quoted

quoted

quoted

+	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.

There is *one* example: "fill_textconv is not remotely thread-safe;"
comment in grep.c, but not in diff.{c,h} where it is declared/defined.

Also, it is static function; we should know if it is thread-safe
or not.

I am thinking about supporting streaming in the future, and perhaps
also running different filter drivers (for different files) in parallel.
I guess that using "static __thread char packet_write_buffer[...]"
is out of question (still not reentrant)?

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.

The fact tha git code is undercommented and underdocumented does not
mean that we should not add comments and documentation.

quoted

quoted

quoted

+	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.

Right.  I should train myself to watch for overflows.

-- 
Jakub Narębski