Re: [PATCH v2 3/9] protocol: introduce protocol extention mechanisms

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 27, 2017, 05:17 UTC
Message-ID: <xmqq4lro7mab.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20170926235627.79606-4-bmwill@google.com>

Brandon Williams <bmwill@google.com> writes:

> +`GIT_PROTOCOL`::
> +	For internal use only.  Used in handshaking the wire protocol.
> +	Contains a colon ':' separated list of keys with optional values
> +	'key[=value]'.  Presence of unknown keys must be tolerated.

Is this meant to be used only on the "server" end? Am I correct to interpret "handshaking" to mean the initial connection acceptor (e.g. "git daemon") uses it to pass what it decided to the programs that implement the service (e.g. "git receive-pack")?

Show 6 quoted lines

> +/*
> + * Environment variable used in handshaking the wire protocol.
> + * Contains a colon ':' separated list of keys with optional values
> + * 'key[=value]'.  Presence of unknown keys must be tolerated.
> + */
> +#define GIT_PROTOCOL_ENVIRONMENT "GIT_PROTOCOL"

"Must be tolerated" feels a bit strange. When somebody asks you to use "version 3" or "version 1 variant 2", when you only know "version 0" or "version 1" and you are not yet even aware of the concept of "variant", we simply ignore "variant=2" as if it wasn't there, even though "version=3" will be rejected (because we know of "version"; it's just that we don't know "version=3").

Show 26 quoted lines

> +enum protocol_version determine_protocol_version_server(void)
> +{
> +	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
> +	enum protocol_version version = protocol_v0;
> +
> +	if (git_protocol) {
> +		struct string_list list = STRING_LIST_INIT_DUP;
> +		const struct string_list_item *item;
> +		string_list_split(&list, git_protocol, ':', -1);
> +
> +		for_each_string_list_item(item, &list) {
> +			const char *value;
> +			enum protocol_version v;
> +
> +			if (skip_prefix(item->string, "version=", &value)) {
> +				v = parse_protocol_version(value);
> +				if (v > version)
> +					version = v;
> +			}
> +		}
> +
> +		string_list_clear(&list, 0);
> +	}
> +
> +	return version;
> +}

This implements "the largest one wins", not "the last one wins". Is there a particular reason why the former is chosen?