Re: [PATCH 5/5] load_ref_decorations(): avoid parsing non-tag objects

On Tue, Jun 22 2021, Jeff King wrote:

[Enter Captain Hindsight]

If you use a slightly different coding style and leverage the information the compiler has to work with you'd get it to error for you, e.g. this on your original patch would catch it:

	diff --git a/log-tree.c b/log-tree.c
	index 8b700e9c142..7e3a011b533 100644
	--- a/log-tree.c
	+++ b/log-tree.c
	@@ -157,9 +157,12 @@ static int add_ref_decoration(const char *refname, const struct object_id *oid,
	 	}
	 
	 	objtype = oid_object_info(the_repository, oid, NULL);
	-	if (type < 0)
	+	switch (type) {
	+	case OBJ_BAD:
	 		return 0;
	-	obj = lookup_object_by_type(the_repository, oid, objtype);
	+	default:
	+		obj = lookup_object_by_type(the_repository, oid, objtype);
	+	}
	 
	 	if (starts_with(refname, "refs/heads/"))
	 		type = DECORATION_REF_LOCAL;

IMO the real problem is an over-reliance on C being so happy to treat
enums as ints (well, with them being ints). If you consistently use
labels you get the compiler to do the checking. For me with gcc and
clang with that on top:
	
	log-tree.c:161:2: error: case value ‘4294967295’ not in enumerated type ‘enum decoration_type’ [-Werror=switch]
	  case OBJ_BAD:
	  ^~~~
	log-tree.c:161:7: error: case value not in enumerated type 'enum decoration_type' [-Werror,-Wswitch]
	        case OBJ_BAD:
	             ^

I think we've disagreed on that exact point before recently, i.e. you think we shouldn't rely on OBJ_BAD in that way, and instead check for any negative value: https://lore.kernel.org/git/YHCZh5nLNVEHCWV2@coredump.intra.peff.net/

This sort of thing is a good reason to pick the opposite pattern. You get the same type checking you'd usually get with anything else in C.

Yes, it is more verbose e.g. in this case, and particularly (as noted downthread of what I linked to) because "enum object_type" contains so many uncommon things, and really should be split up.

In practice I don't think it's too verbose, because once you start
consistently using the pattern you'll usually not be doing conversions
all over the place, and would just do this sort of thing via a helper
that does the type checking, e.g. something like this (or anything else
where you don't lose the type & labels):
	
	diff --git a/log-tree.c b/log-tree.c
	index 8b700e9c142..a61fb01ba3f 100644
	--- a/log-tree.c
	+++ b/log-tree.c
	@@ -130,6 +130,30 @@ static int ref_filter_match(const char *refname,
	 	return 1;
	 }
	 
	+static enum object_type oid_object_info_ok(struct repository *repo,
	+					   struct object_id *oid,
	+					   enum object_type *typep,
	+					   unsigned long *sizep)
	+{
	+	enum object_type type = oid_object_info(repo, oid, sizep);
	+	*typep = type;
	+	switch (type) {
	+	case OBJ_BAD:
	+		return 0;
	+	case OBJ_COMMIT:
	+	case OBJ_TREE:
	+	case OBJ_BLOB:
	+	case OBJ_TAG:
	+		return 1;
	+	case OBJ_NONE:
	+	case OBJ_OFS_DELTA:
	+	case OBJ_REF_DELTA:
	+	case OBJ_ANY:
	+	case OBJ_MAX:
	+		BUG("the enum_object type is too large!");
	+	}
	+}
	+
	 static int add_ref_decoration(const char *refname, const struct object_id *oid,
	 			      int flags, void *cb_data)
	 {
	@@ -156,8 +180,7 @@ static int add_ref_decoration(const char *refname, const struct object_id *oid,
	 		return 0;
	 	}
	 
	-	objtype = oid_object_info(the_repository, oid, NULL);
	-	if (type < 0)
	+	if (!oid_object_info_ok(the_repository, oid, &type, NULL))
	 		return 0;
	 	obj = lookup_object_by_type(the_repository, oid, objtype);
	 

With that pattern GCC narrowlry pulls ahead with showing 4 warnings just about the loss of the type, with Clang at 3 :)