[PATCH v2] fetch: delay fetch_if_missing=0 until after config

[Jonathan Tan <jonathantanmy@google.com>]

Suppose, from a repository that has ".gitmodules", we clone with
--filter=blob:none:

  git clone --filter=blob:none --no-checkout \
    https://kernel.googlesource.com/pub/scm/git/git

Then we fetch:

  git -C git fetch

This will cause a "unable to load config blob object", because the
fetch_config_from_gitmodules() invocation in cmd_fetch() will attempt to
load ".gitmodules" (which Git knows to exist because the client has the
tree of HEAD) while fetch_if_missing is set to 0.

fetch_if_missing is set to 0 too early - ".gitmodules" here should be
lazily fetched.  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.
---
No changes from v1 except that I improved the commit message.

Thanks, Emily, for taking a look.

> 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?

Yes, it is indeed .gitmodules. I improved the commit message to further
explain what is going on.

> 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.

Yes, that is correct. What do you mean by bare checkout? If you mean the
checkout that happens after clone (that we can suppress with
--no-checkout), that indeed happens after fetch_if_missing=0, so we
shouldn't have a problem there.

 builtin/fetch.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/builtin/fetch.c b/builtin/fetch.c
index 0c345b5dfe..863c858fde 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1755,8 +1755,6 @@ 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++)
@@ -1824,6 +1822,8 @@ 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);
-- 
2.23.0.866.gb869b98d4c-goog