Subject: Re: [PATCH] fetch: delay fetch_if_missing=0 until after config

From: Emily Shaffer <[email-masked]>
Date: Wed, 23 Oct 2019 14:03:24 -0700

On Mon, Oct 07, 2019 at 11:18:25AM -0700, Jonathan Tan wrote:
> When running "git fetch" in a partial clone with no blobs, for example,
> by:
> 
>   git clone --filter=blob:none --no-checkout \
>     https://kernel.googlesource.com/pub/scm/git/git
>   git -C git fetch
> 
> "git fetch" will fail to load the config blob object, printing "unable
> to load config blob object".

I'm having some trouble figuring out which object is actually missing.
Is this the .git/config object? (That doesn't make much sense to me...)
Is it .gitmodules?

> 
> This is because fetch_if_missing is set to 0 before the config is
> processed. Git must set fetch_if_missing to 0 before the fetch because
> as part of the fetch, packfile negotiation happens (and we do not want
> to fetch any missing objects when checking existence of objects), but we
> do not need to set it so early. Move the setting of fetch_if_missing to
> the earliest possible point in cmd_fetch(), right before any fetching
> happens.

Doubts aside about what's actually failing, I definitely agree with the
premise of not setting this until the last moment we need it. Plus, I
may be alone here, but it'd make it easier for me to understand the code
if I saw a note explaining *why* we don't want to fetch_if_missing in
this case.

By the way, I think I understand that this is OK to go in
unconditionally because:
 - In the full clone case, it's a no-op; we haven't got anything that's
   missing, so who cares.
 - In the filter case, it's as you said - we don't want to
   fetch_if_missing because that will turn someone's partial clone into
   a a full clone.
   - This probably applies to bare checkout, too.

Of course if I'm wrong I'd like to know, but that's how I understand it
at the moment.

> ---
> This is not a full solution, but this helps in the use case described in
> the commit message. The full solution probably will involve teaching the
> fetch mechanism to support arbitrary struct repository objects, and by
> moving fetch_if_missing into the repository object. (Alternatively, we
> could add the equivalent of OBJECT_INFO_SKIP_FETCH_OBJECT to functions
> like parse_commit() that are used by files like negotiator/default.c, or
> split up commit parsing into object reading - which already has that
> flag - and commit parsing.)

Ah, I remember this was listed as one of the potential intern projects -
I think we dismissed it as being too tech-debt-y for an intern to feel
good about. :(

> ---
>  builtin/fetch.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/builtin/fetch.c b/builtin/fetch.c
> index 24d382b2fb..865ae6677d 100644
> --- a/builtin/fetch.c
> +++ b/builtin/fetch.c
> @@ -1666,8 +1666,6 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
>  
>  	packet_trace_identity("fetch");
>  
> -	fetch_if_missing = 0;
> -
>  	/* Record the command line for the reflog */
>  	strbuf_addstr(&default_rla, "fetch");
>  	for (i = 1; i < argc; i++)
> @@ -1734,6 +1732,8 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
>  		}
>  	}
>  
> +	fetch_if_missing = 0;
> +
>  	if (remote) {
>  		if (filter_options.choice || has_promisor_remote())
>  			fetch_one_setup_partial(remote);
> -- 
> 2.23.0.581.g78d2f28ef7-goog
> 

The diff, though, looks fine for me.

 - Emily