Re: [PATCHv2] attr: convert to new threadsafe API

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 12, 2016, 22:57 UTC
Message-ID: <xmqqlgxt18h0.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <CAGZ79kaBhXHLDEK0XMLjm3QofmtaGZspA3EEx5x4-qCYY--wZA@mail.gmail.com>

Stefan Beller <sbeller@google.com> writes:

Show 18 quoted lines

>> But many callers do not follow that; rather they do
>>
>>         loop to iterate over paths {
>>                 call a helper func to learn attr X for path
>>                 use the value of attr X
>>         }
>>
>> using a callchain that embeds a helper function deep inside, and
>> "check" is kept in the helper, check-attr function is called from
>> there, and "result" is not passed from the caller to the helper
>> (obviously, because it does not exist in the current API).  See the
>> callchain that leads down to convert.c::convert_attrs() for a
>> typical example.  When converted to the new API, it needs to have a
>> new "result" structure every time it is called, and cannot reuse the
>> one that was used in its previous call.
>
> Why would that be? i.e. I do not understand the reasoning/motivation
> as well as what you propose to change here.

The leaf function may look like

	check_eol(const char *path)
	{
                static struct git_attr_check *check;

		initl(&check, "eol", NULL);
		git_check_attr(&check, path, result);
		return nth_element_in_result(result, 0);
	}

and we want "result" to be thread-ready.

A naive and undesired way to put it on stack is like so:

	const char *check_eol(const char *path)
	{
                static struct git_attr_check *check;
		struct git_attr_result result = RESULT_INIT;
		const char *eol;

		initl(&check, "eol", NULL);
		init_result(&check, &result);
		git_check_attr(&check, path, &result);
		eol = nth_element_in_result(&result, 0);
		clear_result(&result);
		return eol;
	}

where your "struct git_attr_result" has a fixed size, and the actual result array is allocated via ALLOC_GROW() etc. That's overhead that we do not want. Instead can't we do this?

	const char *check_eol(const char *path)
	{
                static struct git_attr_check *check;
		/* we know we only want one */
		struct git_attr_result result[1];
		const char *eol;

		initl(&check, "eol", NULL);
		git_check_attr(&check, path, result);
		return result[0];
	}

That way, we won't be doing ALLOC_GROW() in init_result() or free in clear_result().

If you use a structure that has pointer to an array (i.e. the "naive and undesired way" above), you cannot amortize the alloc/free by making result "static" if you want to be thread-ready.