On Mon, Apr 05 2021, Derrick Stolee via GitGitGadget wrote:

This changes the names of the refs that would be fetched by the default
refspec. Instead of "refs/prefetch/<remote>/<branch>" they will now go
to "refs/prefetch/remotes/<remote>/<branch>". While this is a change, it
is not a seriously breaking one: these refs are intended to be hidden
and not used.

Not "a seriously breaking one" just because we'll assume nobody had a
remote they'd named "remotes" and they'll need to manually clean that
mess up (if needed), or ...?

[...]
 	objects from all registered remotes. For each remote, a `git fetch`
 	command is run. The refmap is custom to avoid updating local or remote
 	branches (those in `refs/heads` or `refs/remotes`). Instead, the
-	remote refs are stored in `refs/prefetch/<remote>/`. Also, tags are
-	not updated.
+	refs are stored in `refs/prefetch/`. Also, tags are not updated.

So, "tags are not updated", but:

 
-	strvec_pushf(&child.args, "+refs/heads/*:refs/prefetch/%s/*", remote->name);
+	for (i = 0; i < remote->fetch.nr; i++) {
+		struct refspec_item replace;
+		struct refspec_item *rsi = &remote->fetch.items[i];
+		struct strbuf new_dst = STRBUF_INIT;
+		size_t ignore_len = 0;
+
+		if (rsi->negative) {
+			strvec_push(&child.args, remote->fetch.raw[i]);
+			continue;
+		}
+
+		refspec_item_init(&replace, remote->fetch.raw[i], 1);
+
+		/*
+		 * If a refspec dst starts with "refs/" at the start,
+		 * then we will replace "refs/" with "refs/prefetch/".
+		 * Otherwise, we will prepend the dst string with
+		 * "refs/prefetch/".
+		 */
+		if (!strncmp(replace.dst, "refs/", 5))
+			ignore_len = 5;
+
+		strbuf_addstr(&new_dst, "refs/prefetch/");
+		strbuf_addstr(&new_dst, replace.dst + ignore_len);
+		free(replace.dst);
+		replace.dst = strbuf_detach(&new_dst, NULL);
+
+		strvec_push(&child.args, refspec_item_format(&replace));
+
+		refspec_item_clear(&replace);
+	}

Isn't a blanket replacement of refs/heads/* with refs/* going to change
that? I haven't tested this so maybe it still doesn't work, but:

 	GIT_TRACE2_EVENT="$(pwd)/run-prefetch.txt" git maintenance run --task=prefetch 2>/dev/null &&
 	fetchargs="--prune --no-tags --no-write-fetch-head --recurse-submodules=no --refmap= --quiet" &&
-	test_subcommand git fetch remote1 $fetchargs +refs/heads/*:refs/prefetch/remote1/* <run-prefetch.txt &&
-	test_subcommand git fetch remote2 $fetchargs +refs/heads/*:refs/prefetch/remote2/* <run-prefetch.txt &&
+	test_subcommand git fetch remote1 $fetchargs +refs/heads/*:refs/prefetch/remotes/remote1/* <run-prefetch.txt &&
+	test_subcommand git fetch remote2 $fetchargs +refs/heads/*:refs/prefetch/remotes/remote2/* <run-prefetch.txt &&
[...]

+	fetchargs="--prune --no-tags --no-write-fetch-head --recurse-submodules=no --refmap= --quiet" &&

It seems we should at least have a test for the case of having a refspec
that pulls down tags.

I suspect that we could document this as an absolute before, as
refs/heads/* is the only namespace that'll refuse tags, but now that we
fetch refs/<whatever> that'll no longer be the case.

I'd think that this new behavior (if I'm right) is a feature, but that
we need to update the docs/tests appropriately.