[PATCH] odb: do not use "blank" substitute for NULL

[Junio C Hamano <gitster@pobox.com>]

When various *object_info() functions are given an extended object
info structure as NULL by a caller that does not want any details,
the code uses a file-scope static blank_oi to pass it down to the
helper functions they use, to avoid handling NULL specifically.

The ps/object-read-stream topic graduated to 'master' recently
however had a bug that assumed that two identically named file-scope
static variables in two functions are the same, which of course is
not the case.  This made "git commit" take 0.38 seconds to 1508
seconds in some case, as reported by Aaron Plattner here:

  https://lore.kernel.org/git/f4ba7e89-4717-4b36-921f-56537131fd69@nvidia.com/

We _could_ move the blank_oi variable to a global scope in BSS to
fix this regression, but explicitly handling the NULL is a much
safer fix.  It would also reduce the chance of errors that somebody
accidentally writes into blank_oi, making its contents dirty, which
potentially will make subsequent calls into the callpath misbehave.

By explicitly handling NULL input, we no longer have to worry about
it.

Reported-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 object-file.c |  8 ++++----
 odb.c         | 29 +++++++++++++----------------
 packfile.c    |  3 +--
 3 files changed, 18 insertions(+), 22 deletions(-)

diff --git a/object-file.c b/object-file.c
index 12177a7dd7..e0cce3a62a 100644
--- a/object-file.c
+++ b/object-file.c
@@ -426,7 +426,7 @@ int odb_source_loose_read_object_info(struct odb_source *source,
 	unsigned long size_scratch;
 	enum object_type type_scratch;
 
-	if (oi->delta_base_oid)
+	if (oi && oi->delta_base_oid)
 		oidclr(oi->delta_base_oid, source->odb->repo->hash_algo);
 
 	/*
@@ -437,13 +437,13 @@ int odb_source_loose_read_object_info(struct odb_source *source,
 	 * return value implicitly indicates whether the
 	 * object even exists.
 	 */
-	if (!oi->typep && !oi->sizep && !oi->contentp) {
+	if (!oi || (!oi->typep && !oi->sizep && !oi->contentp)) {
 		struct stat st;
-		if (!oi->disk_sizep && (flags & OBJECT_INFO_QUICK))
+		if ((!oi || !oi->disk_sizep) && (flags & OBJECT_INFO_QUICK))
 			return quick_has_loose(source->loose, oid) ? 0 : -1;
 		if (stat_loose_object(source->loose, oid, &st, &path) < 0)
 			return -1;
-		if (oi->disk_sizep)
+		if (oi && oi->disk_sizep)
 			*oi->disk_sizep = st.st_size;
 		return 0;
 	}
diff --git a/odb.c b/odb.c
index f4cbee4b04..85dc21b104 100644
--- a/odb.c
+++ b/odb.c
@@ -664,34 +664,31 @@ static int do_oid_object_info_extended(struct object_database *odb,
 				       const struct object_id *oid,
 				       struct object_info *oi, unsigned flags)
 {
-	static struct object_info blank_oi = OBJECT_INFO_INIT;
 	const struct cached_object *co;
 	const struct object_id *real = oid;
 	int already_retried = 0;
 
-
 	if (flags & OBJECT_INFO_LOOKUP_REPLACE)
 		real = lookup_replace_object(odb->repo, oid);
 
 	if (is_null_oid(real))
 		return -1;
 
-	if (!oi)
-		oi = &blank_oi;
-
 	co = find_cached_object(odb, real);
 	if (co) {
-		if (oi->typep)
-			*(oi->typep) = co->type;
-		if (oi->sizep)
-			*(oi->sizep) = co->size;
-		if (oi->disk_sizep)
-			*(oi->disk_sizep) = 0;
-		if (oi->delta_base_oid)
-			oidclr(oi->delta_base_oid, odb->repo->hash_algo);
-		if (oi->contentp)
-			*oi->contentp = xmemdupz(co->buf, co->size);
-		oi->whence = OI_CACHED;
+		if (oi) {
+			if (oi->typep)
+				*(oi->typep) = co->type;
+			if (oi->sizep)
+				*(oi->sizep) = co->size;
+			if (oi->disk_sizep)
+				*(oi->disk_sizep) = 0;
+			if (oi->delta_base_oid)
+				oidclr(oi->delta_base_oid, odb->repo->hash_algo);
+			if (oi->contentp)
+				*oi->contentp = xmemdupz(co->buf, co->size);
+			oi->whence = OI_CACHED;
+		}
 		return 0;
 	}
 
diff --git a/packfile.c b/packfile.c
index 7a16aaa90d..2aa6135c3a 100644
--- a/packfile.c
+++ b/packfile.c
@@ -2095,7 +2095,6 @@ int packfile_store_read_object_info(struct packfile_store *store,
 				    struct object_info *oi,
 				    unsigned flags UNUSED)
 {
-	static struct object_info blank_oi = OBJECT_INFO_INIT;
 	struct pack_entry e;
 	int rtype;
 
@@ -2106,7 +2105,7 @@ int packfile_store_read_object_info(struct packfile_store *store,
 	 * We know that the caller doesn't actually need the
 	 * information below, so return early.
 	 */
-	if (oi == &blank_oi)
+	if (!oi)
 		return 0;
 
 	rtype = packed_object_info(store->odb->repo, e.p, e.offset, oi);
-- 
2.52.0-448-g904c30f108