Subject: Re: [PATCH 1/2] blob.c: remove buffer & size arguments to parse_blob_buffer()

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Sat, 10 Apr 2021 14:57:12 +0200

On Fri, Apr 09 2021, Jeff King wrote:

> On Fri, Apr 09, 2021 at 10:07:27AM +0200, Ævar Arnfjörð Bjarmason wrote:
>
>> As noted in the comment introduced in 837d395a5c0 (Replace
>> parse_blob() with an explanatory comment, 2010-01-18) the old
>> parse_blob() function and the current parse_blob_buffer() exist merely
>> to provide consistency in the API.
>> 
>> We're not going to parse blobs like we "parse" commits, trees or
>> tags. So let's not have the parse_blob_buffer() take arguments that
>> pretends that we do. Its only use is to set the "parsed" flag.
>> 
>> See bd2c39f58f9 ([PATCH] don't load and decompress objects twice with
>> parse_object(), 2005-05-06) for the introduction of parse_blob_buffer().
>
> OK. Calling it parse_blob_buffer() is a little silly since it doesn't
> even take a buffer anymore. But I guess parse_blob() might imply that it
> actually loads the contents from disk to check them (which the other
> parse_foo() functions do), so that's not a good name.
>
> So this might be the least bad thing. Given that there are only two
> callers, just setting blob->object.parsed might not be unreasonable,
> either. But I don't think it's worth spending too much time on.
>
>> @@ -266,7 +266,7 @@ struct object *parse_object(struct repository *r, const struct object_id *oid)
>>  			error(_("hash mismatch %s"), oid_to_hex(oid));
>>  			return NULL;
>>  		}
>> -		parse_blob_buffer(lookup_blob(r, oid), NULL, 0);
>> +		parse_blob_buffer(lookup_blob(r, oid));
>>  		return lookup_object(r, oid);
>
> Not new in your patch, but I wondered if this could cause a segfault
> when lookup_blob() returns NULL. I _think_ the answer is "no". We'd hit
> this code path when either:
>
>   - lookup_object() returns an object with type OBJ_BLOB, in which case
>     lookup_blob() would return that same object
>
>   - lookup_object() returned NULL, in which case lookup_blob() will call
>     it again, get NULL again, and then auto-create the blob and return
>     it
>
> So I think it is OK. But there are a bunch of duplicate hash lookups in
> this code. It would be clearer and more efficient as:
>
> diff --git a/object.c b/object.c
> index 2c32691dc4..2dfa038f13 100644
> --- a/object.c
> +++ b/object.c
> @@ -262,12 +262,14 @@ struct object *parse_object(struct repository *r, const struct object_id *oid)
>  	if ((obj && obj->type == OBJ_BLOB && repo_has_object_file(r, oid)) ||
>  	    (!obj && repo_has_object_file(r, oid) &&
>  	     oid_object_info(r, oid, NULL) == OBJ_BLOB)) {
> +		if (!obj)
> +			obj = create_object(r, oid, alloc_blob_node(r));
>  		if (check_object_signature(r, repl, NULL, 0, NULL) < 0) {
>  			error(_("hash mismatch %s"), oid_to_hex(oid));
>  			return NULL;
>  		}
> -		parse_blob_buffer(lookup_blob(r, oid), NULL, 0);
> -		return lookup_object(r, oid);
> +		parse_blob_buffer(obj, NULL, 0);
> +		return obj;
>  	}
>  
>  	buffer = repo_read_object_file(r, oid, &type, &size);
>
> but I doubt the efficiency matters much in practice. Those hash lookups
> will be lost in the noise of computing the hash of the blob contents.

I was trying to keep the changes smaller, but what about just doing this?:

diff --git a/blob.c b/blob.c
index 182718aba9..69293e7d8e 100644
--- a/blob.c
+++ b/blob.c
@@ -5,16 +5,16 @@
 
 const char *blob_type = "blob";
 
+struct blob *create_blob(struct repository *r, const struct object_id *oid)
+{
+	return create_object(r, oid, alloc_blob_node(r));
+}
+
 struct blob *lookup_blob(struct repository *r, const struct object_id *oid)
 {
 	struct object *obj = lookup_object(r, oid);
 	if (!obj)
-		return create_object(r, oid, alloc_blob_node(r));
-	return object_as_type(obj, OBJ_BLOB, 0);
-}
+		return create_blob(r, oid);
 
-int parse_blob_buffer(struct blob *item, void *buffer, unsigned long size)
-{
-	item->object.parsed = 1;
-	return 0;
+	return object_as_type(obj, OBJ_BLOB, 0);
 }
diff --git a/blob.h b/blob.h
index 1664872055..ad34f0e9cc 100644
--- a/blob.h
+++ b/blob.h
@@ -9,10 +9,9 @@ struct blob {
 	struct object object;
 };
 
+struct blob *create_blob(struct repository *r, const struct object_id *oid);
 struct blob *lookup_blob(struct repository *r, const struct object_id *oid);
 
-int parse_blob_buffer(struct blob *item, void *buffer, unsigned long size);
-
 /**
  * Blobs do not contain references to other objects and do not have
  * structured data that needs parsing. However, code may use the
diff --git a/object.c b/object.c
index 78343781ae..2699431404 100644
--- a/object.c
+++ b/object.c
@@ -195,8 +195,7 @@ struct object *parse_object_buffer(struct repository *r, const struct object_id
 	if (type == OBJ_BLOB) {
 		struct blob *blob = lookup_blob(r, oid);
 		if (blob) {
-			if (parse_blob_buffer(blob, buffer, size))
-				return NULL;
+			blob->object.parsed = 1;
 			obj = &blob->object;
 		}
 	} else if (type == OBJ_TREE) {
@@ -262,12 +261,16 @@ struct object *parse_object(struct repository *r, const struct object_id *oid)
 	if ((obj && obj->type == OBJ_BLOB && repo_has_object_file(r, oid)) ||
 	    (!obj && repo_has_object_file(r, oid) &&
 	     oid_object_info(r, oid, NULL) == OBJ_BLOB)) {
+		struct blob *blob;
+		if (!obj)
+			blob = create_blob(r, oid);
 		if (check_object_signature(r, repl, NULL, 0, NULL) < 0) {
 			error(_("hash mismatch %s"), oid_to_hex(oid));
 			return NULL;
 		}
-		parse_blob_buffer(lookup_blob(r, oid), NULL, 0);
-		return lookup_object(r, oid);
+		obj = &blob->object;
+		obj->parsed = 1;
+		return obj;
 	}
 
 	buffer = repo_read_object_file(r, oid, &type, &size);