Re: [PATCH v3 04/11] unpack-trees: introduce preserve_ignored to unpack_trees_options

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Fri, Oct 01 2021, Ævar Arnfjörð Bjarmason wrote:

> Aside about safety: One thing I'll sometimes do when I'm unsure about
> those sorts of fixes is to have my new INIT set a new "sentinel" field
> to "12345" or whatever, then just BUG() out in an entry point in the API
> that you can't avoid calling if it's not set like that, e.g. dir_clear()
> or whatever the setup/work function is.

For reference: Something like the below, which passes with my WIP
patches. Showing that no non-static entry point can reach the code in
unpack-trees.c without "foo" being 12345, which can only be the case if
callers have used the macro (and the code internal to unpack-trees.c is
easy enough to audit).

 unpack-trees.c | 25 +++++++++++++++++++++++++
 unpack-trees.h |  2 ++
 2 files changed, 27 insertions(+)

diff --git a/unpack-trees.c b/unpack-trees.c
index d40af221e1c..f2365ecf215 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -199,6 +199,8 @@ void clear_unpack_trees_porcelain(struct unpack_trees_options *opts)
 {
 	strvec_clear(&opts->msgs_to_free);
 	dir_clear(&opts->dir);
+	if (opts->foo != 12345)
+		BUG("noes");
 	memset(opts->msgs, 0, sizeof(opts->msgs));
 }
 
@@ -1702,6 +1704,9 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	struct pattern_list pl;
 	int free_pattern_list = 0;
 
+	if (o->foo != 12345)
+		BUG("noes");
+
 	if (len > MAX_UNPACK_TREES)
 		die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);
 
@@ -1903,6 +1908,9 @@ enum update_sparsity_result update_sparsity(struct unpack_trees_options *o)
 	unsigned old_show_all_errors;
 	int free_pattern_list = 0;
 
+	if (o->foo != 12345)
+		BUG("noes");
+
 	old_show_all_errors = o->show_all_errors;
 	o->show_all_errors = 1;
 
@@ -2033,6 +2041,8 @@ static int verify_uptodate_1(const struct cache_entry *ce,
 int verify_uptodate(const struct cache_entry *ce,
 		    struct unpack_trees_options *o)
 {
+	if (o->foo != 12345)
+		BUG("noes");
 	if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
 		return 0;
 	return verify_uptodate_1(ce, o, ERROR_NOT_UPTODATE_FILE);
@@ -2417,6 +2427,9 @@ int threeway_merge(const struct cache_entry * const *stages,
 	int no_anc_exists = 1;
 	int i;
 
+	if (o->foo != 12345)
+		BUG("noes");
+
 	for (i = 1; i < o->head_idx; i++) {
 		if (!stages[i] || stages[i] == o->df_conflict_entry)
 			any_anc_missing = 1;
@@ -2580,6 +2593,9 @@ int twoway_merge(const struct cache_entry * const *src,
 	const struct cache_entry *oldtree = src[1];
 	const struct cache_entry *newtree = src[2];
 
+	if (o->foo != 12345)
+		BUG("noes");
+
 	if (o->merge_size != 2)
 		return error("Cannot do a twoway merge of %d trees",
 			     o->merge_size);
@@ -2654,6 +2670,9 @@ int bind_merge(const struct cache_entry * const *src,
 	const struct cache_entry *old = src[0];
 	const struct cache_entry *a = src[1];
 
+	if (o->foo != 12345)
+		BUG("noes");
+
 	if (o->merge_size != 1)
 		return error("Cannot do a bind merge of %d trees",
 			     o->merge_size);
@@ -2680,6 +2699,9 @@ int oneway_merge(const struct cache_entry * const *src,
 	const struct cache_entry *old = src[0];
 	const struct cache_entry *a = src[1];
 
+	if (o->foo != 12345)
+		BUG("noes");
+
 	if (o->merge_size != 1)
 		return error("Cannot do a oneway merge of %d trees",
 			     o->merge_size);
@@ -2717,6 +2739,9 @@ int stash_worktree_untracked_merge(const struct cache_entry * const *src,
 	const struct cache_entry *worktree = src[1];
 	const struct cache_entry *untracked = src[2];
 
+	if (o->foo != 12345)
+		BUG("noes");
+
 	if (o->merge_size != 2)
 		BUG("invalid merge_size: %d", o->merge_size);
 
diff --git a/unpack-trees.h b/unpack-trees.h
index 75b67f90ccd..8dae0938ad1 100644
--- a/unpack-trees.h
+++ b/unpack-trees.h
@@ -90,10 +90,12 @@ struct unpack_trees_options {
 
 	struct pattern_list *pl; /* for internal use */
 	struct checkout_metadata meta;
+	int foo;
 };
 #define UNPACK_TREES_OPTIONS_INIT { \
 	.msgs_to_free = STRVEC_INIT, \
 	.dir = DIR_INIT, \
+	.foo = 12345, \
 }
 
 void unpack_trees_init(struct unpack_trees_options *options);