sha1-file: split OBJECT_INFO_FOR_PREFETCH

From: Derrick Stolee <[email protected]>

The OBJECT_INFO_FOR_PREFETCH bitflag was added to sha1-file.c in 0f4a4fb1 (sha1-file: support OBJECT_INFO_FOR_PREFETCH, 2019-03-29) and is used to prevent the fetch_objects() method when enabled.

However, there is a problem with the current use. The definition of OBJECT_INFO_FOR_PREFETCH is given by adding 32 to OBJECT_INFO_QUICK. This is clearly stated above the definition (in a comment) that this is so OBJECT_INFO_FOR_PREFETCH implies OBJECT_INFO_QUICK. The problem is that using "flag & OBJECT_INFO_FOR_PREFETCH" means that OBJECT_INFO_QUICK also implies OBJECT_INFO_FOR_PREFETCH.

Split out the single bit from OBJECT_INFO_FOR_PREFETCH into a new OBJECT_INFO_SKIP_FETCH_OBJECT as the single bit and keep OBJECT_INFO_FOR_PREFETCH as the union of two flags. This allows a clearer use of flag checking while also keeping the implication of OBJECT_INFO_QUICK.

Signed-off-by Derrick Stolee <[email protected]>

---
 object-store.h | 10 +++++++---
 sha1-file.c    |  2 +-
 2 files changed, 8 insertions(+), 4 deletions(-)

object-store.h +7 -3

index dd3f9b75f0..c90628d839 100644
@@ -282,10 +282,14 @@ struct object_info {
 #define OBJECT_INFO_IGNORE_LOOSE 16
 /*
  * Do not attempt to fetch the object if missing (even if fetch_is_missing is
- * nonzero). This is meant for bulk prefetching of missing blobs in a partial
- * clone. Implies OBJECT_INFO_QUICK.
+ * nonzero).
  */
-#define OBJECT_INFO_FOR_PREFETCH (32 + OBJECT_INFO_QUICK)
+#define OBJECT_INFO_SKIP_FETCH_OBJECT 32
+/*
+ * This is meant for bulk prefetching of missing blobs in a partial
+ * clone. Implies OBJECT_INFO_SKIP_FETCH_OBJECT and OBJECT_INFO_QUICK
+ */
+#define OBJECT_INFO_FOR_PREFETCH (OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK)
 
 int oid_object_info_extended(struct repository *r,
 			     const struct object_id *,

sha1-file.c +1 -1

index ad02649124..0299fdd516 100644
@@ -1371,7 +1371,7 @@ int oid_object_info_extended(struct repository *r, const struct object_id *oid,
 		/* Check if it is a missing object */
 		if (fetch_if_missing && repository_format_partial_clone &&
 		    !already_retried && r == the_repository &&
-		    !(flags & OBJECT_INFO_FOR_PREFETCH)) {
+		    !(flags & OBJECT_INFO_SKIP_FETCH_OBJECT)) {
 			/*
 			 * TODO Investigate having fetch_object() return
 			 * TODO error/success and stopping the music here.

From: Derrick Stolee <[email protected]>

The OBJECT_INFO_FOR_PREFETCH bitflag was added to sha1-file.c in 0f4a4fb1 (sha1-file: support OBJECT_INFO_FOR_PREFETCH, 2019-03-29) and is used to prevent the fetch_objects() method when enabled.

However, there is a problem with the current use. The definition of OBJECT_INFO_FOR_PREFETCH is given by adding 32 to OBJECT_INFO_QUICK. This is clearly stated above the definition (in a comment) that this is so OBJECT_INFO_FOR_PREFETCH implies OBJECT_INFO_QUICK. The problem is that using "flag & OBJECT_INFO_FOR_PREFETCH" means that OBJECT_INFO_QUICK also implies OBJECT_INFO_FOR_PREFETCH.

Split out the single bit from OBJECT_INFO_FOR_PREFETCH into a new OBJECT_INFO_SKIP_FETCH_OBJECT as the single bit and keep OBJECT_INFO_FOR_PREFETCH as the union of two flags. This allows a clearer use of flag checking while also keeping the implication of OBJECT_INFO_QUICK.

+ * This is meant for bulk prefetching of missing blobs in a partial
+ * clone. Implies OBJECT_INFO_SKIP_FETCH_OBJECT and OBJECT_INFO_QUICK
+ */
+#define OBJECT_INFO_FOR_PREFETCH (OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK)

Quoted 13 lines · Derrick Stolee via GitGitGadget

From: Derrick Stolee <[email protected]>

The OBJECT_INFO_FOR_PREFETCH bitflag was added to sha1-file.c in 0f4a4fb1 (sha1-file: support OBJECT_INFO_FOR_PREFETCH, 2019-03-29) and is used to prevent the fetch_objects() method when enabled.

However, there is a problem with the current use. The definition of OBJECT_INFO_FOR_PREFETCH is given by adding 32 to OBJECT_INFO_QUICK. This is clearly stated above the definition (in a comment) that this is so OBJECT_INFO_FOR_PREFETCH implies OBJECT_INFO_QUICK. The problem is that using "flag & OBJECT_INFO_FOR_PREFETCH" means that OBJECT_INFO_QUICK also implies OBJECT_INFO_FOR_PREFETCH.

Quoted 4 lines

Split out the single bit from OBJECT_INFO_FOR_PREFETCH into a new OBJECT_INFO_SKIP_FETCH_OBJECT as the single bit and keep OBJECT_INFO_FOR_PREFETCH as the union of two flags. This allows a clearer use of flag checking while also keeping the implication of OBJECT_INFO_QUICK.

Oof. I actually suggested splitting these up for review, but thought it was only a clarity/flexibility issue, and completely missed the correctness aspect of checking when the bit is set.

I agree with Junio's other response that using "==" would be the right way for a multi-bit check, in general. But I like the split here, because I think the result is more clear to read and harder to get wrong for future checks.

I'd even go so far as to say...

Quoted 4 lines

+ * This is meant for bulk prefetching of missing blobs in a partial
+ * clone. Implies OBJECT_INFO_SKIP_FETCH_OBJECT and OBJECT_INFO_QUICK
+ */
+#define OBJECT_INFO_FOR_PREFETCH (OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK)

we could dump this, and callers should just say what they mean (i.e., specify both flags).

There are only two of them, and I think both would be more readable with a helper more like:

int should_prefetch_object(struct repository *r,

                             const struct object_id *oid) {
	return !oid_object_info_extended(r, oid, NULL,
	                                 OBJECT_INFO_SKIP_FETCH_OBJECT |
					 OBJECT_INFO_QUICK);

}

but unless everybody is immediately on-board with "yes, that is much nicer", I don't want bikeshedding to hold up your important and obviously-correct fix.