[PATCH] object_as_type: initialize commit-graph-related fields of 'struct commit'

["SZEDER Gábor" <szeder.dev@gmail.com>]

When the commit graph and generation numbers were introduced in
commits 177722b344 (commit: integrate commit graph with commit
parsing, 2018-04-10) and 83073cc994 (commit: add generation number to
struct commit, 2018-04-25), they tried to make sure that the
corresponding 'graph_pos' and 'generation' fields of 'struct commit'
are initialized conservatively, as if the commit were not included in
the commit-graph file.

Alas, initializing those fields only in alloc_commit_node() missed the
case when an object that happens to be a commit is first looked up via
lookup_unknown_object(), and is then later converted to a 'struct
commit' via the object_as_type() helper function (either calling it
directly, or as part of a subsequent lookup_commit() call).
Consequently, both of those fields incorrectly remain set to zero,
which means e.g. that the commit is present in and is the first entry
of the commit-graph file.  This will result in wrong timestamp, parent
and root tree hashes, if such a 'struct commit' instance is later
filled from the commit-graph.

Extract the initialization of 'struct commit's fields from
alloc_commit_node() into a helper function, and call it from
object_as_type() as well, to make sure that it properly initializes
the two commit-graph-related fields, too.  With this helper function
it is hopefully less likely that any new fields added to 'struct
commit' in the future would remain uninitialized.

With this change alloc_commit_index() won't have any remaining callers
outside of 'alloc.c', so mark it as static.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---

So, it turns out that ec0c5798ee (revision: use commit graph in
get_reference(), 2018-12-04) is not the culprit after all, it merely
highlighted a bug that is as old as the commit-graph feature itself.
This patch fixes this and all other related issues I reported
upthread.

I couldn't find any other place where an object of unknown type is
turned into a 'struct commit', so this might have been the only place
that needed fixing.

Other object types seem to be fine, because they contain only fields
that should be zero initialized.  At least for now, because a similar
issue might arise in the future, if one of them gains a new field that
should not be initialized to zero...  but will they ever get such a
field?  So I'm not too keen on introducing similar init_tree_node(),
etc. helper funcions at the moment.

 alloc.c  | 11 ++++++++---
 alloc.h  |  2 +-
 object.c |  5 +++--
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/alloc.c b/alloc.c
index e7aa81b7aa..1c64c4dd16 100644
--- a/alloc.c
+++ b/alloc.c
@@ -99,18 +99,23 @@ void *alloc_object_node(struct repository *r)
 	return obj;
 }
 
-unsigned int alloc_commit_index(struct repository *r)
+static unsigned int alloc_commit_index(struct repository *r)
 {
 	return r->parsed_objects->commit_count++;
 }
 
-void *alloc_commit_node(struct repository *r)
+void init_commit_node(struct repository *r, struct commit *c)
 {
-	struct commit *c = alloc_node(r->parsed_objects->commit_state, sizeof(struct commit));
 	c->object.type = OBJ_COMMIT;
 	c->index = alloc_commit_index(r);
 	c->graph_pos = COMMIT_NOT_FROM_GRAPH;
 	c->generation = GENERATION_NUMBER_INFINITY;
+}
+
+void *alloc_commit_node(struct repository *r)
+{
+	struct commit *c = alloc_node(r->parsed_objects->commit_state, sizeof(struct commit));
+	init_commit_node(r, c);
 	return c;
 }
 
diff --git a/alloc.h b/alloc.h
index ba356ed847..ed1071c11e 100644
--- a/alloc.h
+++ b/alloc.h
@@ -9,11 +9,11 @@ struct repository;
 
 void *alloc_blob_node(struct repository *r);
 void *alloc_tree_node(struct repository *r);
+void init_commit_node(struct repository *r, struct commit *c);
 void *alloc_commit_node(struct repository *r);
 void *alloc_tag_node(struct repository *r);
 void *alloc_object_node(struct repository *r);
 void alloc_report(struct repository *r);
-unsigned int alloc_commit_index(struct repository *r);
 
 struct alloc_state *allocate_alloc_state(void);
 void clear_alloc_state(struct alloc_state *s);
diff --git a/object.c b/object.c
index c4170d2d0c..7bccfd5d8e 100644
--- a/object.c
+++ b/object.c
@@ -164,8 +164,9 @@ void *object_as_type(struct repository *r, struct object *obj, enum object_type
 		return obj;
 	else if (obj->type == OBJ_NONE) {
 		if (type == OBJ_COMMIT)
-			((struct commit *)obj)->index = alloc_commit_index(r);
-		obj->type = type;
+			init_commit_node(r, (struct commit *) obj);
+		else
+			obj->type = type;
 		return obj;
 	}
 	else {
-- 
2.20.1.642.gc55a771460