Re: [PATCH 2/9] maintenance: add prefetch task

On 8/12/2020 7:10 PM, Emily Shaffer wrote:

On Thu, Aug 06, 2020 at 04:30:17PM +0000, Derrick Stolee via GitGitGadget wrote:

quoted

3. By adding a new refspec "+refs/heads/*:refs/prefetch/<remote>/*"
   we can ensure that we actually load the new values somewhere in
   our refspace while not updating refs/heads or refs/remotes. By
   storing these refs here, the commit-graph job will update the
   commit-graph with the commits from these hidden refs.

[emily] How does the content of refs/prefetch/* get used?
[jrnieder] How does the content of refs/prefetch/* get cleaned up?
[jonathantan] refs/prefetch/* will get used in negotiation so it is
useful to keep these.

quoted

4. --prune will delete the refs/prefetch/<remote> refs that no
   longer appear on the remote.

[jrnieder] this is what cleans up refs/prefetch/* later :)

quoted

+static int fetch_remote(const char *remote, struct maintenance_opts *opts)
+{
+	struct child_process child = CHILD_PROCESS_INIT;
+
+	child.git_cmd = 1;
+	strvec_pushl(&child.args, "fetch", remote, "--prune", "--no-tags",
+		     "--no-write-fetch-head", "--refmap=", NULL);

[jonathantan] It would be good to pass --recurse-submodules=no.
[jrnieder] Since we are specifying the refmap here, if we were to
recurse into submodules, we'd have to be careful about making sure
refmap gets propagated correctly.

Good point.

quoted

+static int fill_each_remote(struct remote *remote, void *cbdata)

[jrnieder] Since this is a callback that happens for each remote, maybe
this should be named to indicate it only fills one remote at a time
instead. Nit :)

Sure. append_remote() is better.

quoted

+{
+	struct string_list *remotes = (struct string_list *)cbdata;
+
+	string_list_append(remotes, remote->name);
+	return 0;
+}
+
+static int maintenance_task_prefetch(struct maintenance_opts *opts)
+{
+	int result = 0;
+	struct string_list_item *item;
+	struct string_list remotes = STRING_LIST_INIT_DUP;
+
+	if (for_each_remote(fill_each_remote, &remotes)) {
+		error(_("failed to fill remotes"));
+		result = 1;
+		goto cleanup;
+	}
+
+	for (item = remotes.items;
+	     item && item < remotes.items + remotes.nr;
+	     item++)

Is there a reason not to use for_each_string_list_item() instead? This
would be more brief and also easier to read (less thinking about what
the loop is doing).

I just keep forgetting about that macro. Thanks.

quoted

+		result |= fetch_remote(item->string, opts);
+
+cleanup:
+	string_list_clear(&remotes, 0);
+	return result;
+}
+

quoted

 enum maintenance_task_label {
+	TASK_PREFETCH,

[jrnieder] Nit: Is there a sort order for these? Should we establish an order
early on (e.g. alphabetical)?

There _is_ an order, and maybe that should be up for debate.
This one is really about fetching new refs before (possibly)
deleting unreachable data or doing a full repack in the 'gc'
task. The rest of the tasks are after the 'gc' task (in an
unimportant order) because they are likely to be no-ops if the
'gc' task does work.

Of course, users can customize this order using the
--task=<task> options.

quoted

 	TASK_GC,
 	TASK_COMMIT_GRAPH,

quoted

+test_expect_success 'prefetch multiple remotes' '
+	git clone . clone1 &&
+	git clone . clone2 &&
+	git remote add remote1 "file://$(pwd)/clone1" &&
+	git remote add remote2 "file://$(pwd)/clone2" &&
+	git -C clone1 switch -c one &&
+	git -C clone2 switch -c two &&
+	test_commit -C clone1 one &&
+	test_commit -C clone2 two &&
+	GIT_TRACE2_EVENT="$(pwd)/run-prefetch.txt" git maintenance run --task=prefetch 2>/dev/null &&
+	fetchargs="--prune --no-tags --no-write-fetch-head --refmap= --quiet" &&
+	test_subcommand git fetch remote1 $fetchargs +refs/heads/\\*:refs/prefetch/remote1/\\* <run-prefetch.txt &&

[jrnieder] In practice, why were all the \\ needed? Trying to figure out
where Git is using a shell that would need the * escaped and finding it
hard to reason about.

It's the 'grep' way down inside test_subcommand that will
fail if it interprets these '*' incorrectly.

quoted

+	test_subcommand git fetch remote2 $fetchargs +refs/heads/\\*:refs/prefetch/remote2/\\* <run-prefetch.txt &&
+	test_path_is_missing .git/refs/remotes &&
+	test_cmp clone1/.git/refs/heads/one .git/refs/prefetch/remote1/one &&
+	test_cmp clone2/.git/refs/heads/two .git/refs/prefetch/remote2/two &&

[jrnieder] Should we use test_cmp_rev instead to make compatible with
packed-refs?

While that would be good, these are refs across two .git directories.

I suppose that I could fetch normally and check refs/remotes/remote1/one
versus refs/prefetch/remote1/one.

quoted

+	git log prefetch/remote1/one &&
+	git log prefetch/remote2/two

[jonathantan] Why do we use 'git log' to check? I'm a little confused
about what's going on; if you just want to check that the refs are
present you could use 'git rev-parse' instead?

The main point is to demonstrate that we have the actual objects
in the current repository, not just refs storing object IDs that
don't actually exist in the object database. Does rev-parse check
that the object exists? (A quick check in my local repository seems
to say no.)

Reorganizing the test to work with test_cmp_rev results in the
following:

test_expect_success 'prefetch multiple remotes' '
	git clone . clone1 &&
	git clone . clone2 &&
	git remote add remote1 "file://$(pwd)/clone1" &&
	git remote add remote2 "file://$(pwd)/clone2" &&
	git -C clone1 switch -c one &&
	git -C clone2 switch -c two &&
	test_commit -C clone1 one &&
	test_commit -C clone2 two &&
	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_path_is_missing .git/refs/remotes &&
	git log prefetch/remote1/one &&
	git log prefetch/remote2/two &&
	git fetch --all &&
	test_cmp_rev refs/remotes/remote1/one refs/prefetch/remote1/one &&
	test_cmp_rev refs/remotes/remote2/two refs/prefetch/remote2/two
'

Thanks,
-Stolee