Subject: Re: [PATCH 6/6] send-pack: support push negotiation

From: Derrick Stolee <[email-masked]>
Date: Mon, 3 May 2021 11:35:02 -0400

On 4/8/21 9:10 PM, Jonathan Tan wrote:
> Teach Git the push.negotiate config variable.
...
> +push.negotiate::
> +	If set to "true", attempt to reduce the size of the packfile
> +	sent by rounds of negotiation in which the client and the
> +	server attempt to find commits in common. If "false", Git will
> +	rely solely on the server's ref advertisement to find commits
> +	in common.

Works for me.

> diff --git a/send-pack.c b/send-pack.c
> index 5f215b13c7..9cb9f71650 100644
> --- a/send-pack.c
> +++ b/send-pack.c
> @@ -56,7 +56,9 @@ static void feed_object(const struct object_id *oid, FILE *fh, int negative)
>  /*
>   * Make a pack stream and spit it out into file descriptor fd
>   */
> -static int pack_objects(int fd, struct ref *refs, struct oid_array *extra, struct send_pack_args *args)
> +static int pack_objects(int fd, struct ref *refs, struct oid_array *advertised,
> +			struct oid_array *negotiated,
> +			struct send_pack_args *args)

At the moment, I don't see why we need two oid_arrays here.
Instead, this 'extra' could instead be renamed to something
like 'server_objects' or 'base_objects' to make it clear
that we don't want those objects, and can even use them and
their reachable objects as delta bases, when appropriate.

Or, just don't touch it.

> +static void get_commons_through_negotiation(const char *url,
> +					    const struct ref *remote_refs,
> +					    struct oid_array *commons)
> +{
> +	struct child_process child = CHILD_PROCESS_INIT;
> +	const struct ref *ref;
> +	int len = the_hash_algo->hexsz + 1; /* hash + NL */
> +
> +	child.git_cmd = 1;
> +	child.no_stdin = 1;
> +	child.out = -1;
> +	strvec_pushl(&child.args, "fetch", "--negotiate-only", NULL);
> +	for (ref = remote_refs; ref; ref = ref->next)
> +		strvec_pushf(&child.args, "--negotiation-tip=%s", oid_to_hex(&ref->new_oid));
> +	strvec_push(&child.args, url);

Oh! We are using a 'git fetch --negotiate-only' subprocess here. You
can ignore my previous message about updating the docs for this to be
used only by tests.

> +
> +	if (start_command(&child))
> +		die(_("send-pack: unable to fork off fetch subprocess"));
> +
> +	do {
> +		char hex_hash[GIT_MAX_HEXSZ + 1];
> +		int read_len = read_in_full(child.out, hex_hash, len);
> +		struct object_id oid;
> +		const char *end;
> +
> +		if (!read_len)
> +			break;
> +		if (read_len != len)
> +			die("invalid length read %d", read_len);
> +		if (parse_oid_hex(hex_hash, &oid, &end) || *end != '\n')
> +			die("invalid hash");
> +		oid_array_append(commons, &oid);
This appends, so there is no reason why it needs to be empty before
the method. Is there a way we could feed the extra_have set when
calling this method? Or is it happening at a strange time?

> +	} while (1);
> +
> +	if (finish_command(&child)) {
> +		/*
> +		 * The information that push negotiation provides is useful but
> +		 * not mandatory.
> +		 */
> +		warning(_("push negotiation failed; proceeding anyway with push"));
> +	}
> +}
> +
>  int send_pack(struct send_pack_args *args,
>  	      int fd[], struct child_process *conn,
>  	      struct ref *remote_refs,
>  	      struct oid_array *extra_have)
>  {
> +	struct oid_array commons = OID_ARRAY_INIT;
>  	int in = fd[0];
>  	int out = fd[1];
>  	struct strbuf req_buf = STRBUF_INIT;
> @@ -426,6 +474,7 @@ int send_pack(struct send_pack_args *args,
>  	int quiet_supported = 0;
>  	int agent_supported = 0;
>  	int advertise_sid = 0;
> +	int push_negotiate = 0;
>  	int use_atomic = 0;
>  	int atomic_supported = 0;
>  	int use_push_options = 0;
> @@ -437,6 +486,10 @@ int send_pack(struct send_pack_args *args,
>  	const char *push_cert_nonce = NULL;
>  	struct packet_reader reader;
>  
> +	git_config_get_bool("push.negotiate", &push_negotiate);
> +	if (push_negotiate)
> +		get_commons_through_negotiation(args->url, remote_refs, &commons);
> +
>  	git_config_get_bool("transfer.advertisesid", &advertise_sid);
>  
>  	/* Does the other end support the reporting? */
> @@ -625,7 +678,7 @@ int send_pack(struct send_pack_args *args,
>  			   PACKET_READ_DIE_ON_ERR_PACKET);
>  
>  	if (need_pack_data && cmds_sent) {
> -		if (pack_objects(out, remote_refs, extra_have, args) < 0) {
> +		if (pack_objects(out, remote_refs, extra_have, &commons, args) < 0) {

Here, it would be nice if extra_have and commons were merged before calling
pack_objects(). I mentioned a way to perhaps make that easier above, but
the context might not make that be super-simple. Running a loop here to
scan 'commons' and append them to 'extra_have' might be a sufficient
approach.

Generally, this approach seems like it would work. I have not done any
local testing, yet.

Thanks,
-Stolee