From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 19 Aug 2016 20:31:27 GMT
Subject: Re: [PATCH v8 7/8] cache: add empty_tree_oid object
Message-ID: <xmqqfuq01oww.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20160819000031.24854-8-jacob.e.keller@intel.com>

Jacob Keller <jacob.e.keller@intel.com> writes:

> From: Jacob Keller <jacob.keller@gmail.com>
>
> Add an empty_tree_oid object which can be used in place of
> EMPTY_TREE_SHA1_BIN_LITERAL for code which is being converted to struct
> object_id.

How widely do you envision the users of this symbol would be spread
across the entire codebase?  I am debating myself if we need a
singleton in-core copy like this (if we end up referring to it from
everywhere), or we only need one level higher abstraction,
e.g. "is_empty_tree_oid()" helper (in which case I do not think we
even need a singleton; just imitate how is_empty_blob_sha1() is
implemented).

Even if we need such a singleton, I think we avoid ".field = value"
struct initializations in our code.

>
> Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
> ---
>  cache.h     | 2 ++
>  sha1_file.c | 3 +++
>  2 files changed, 5 insertions(+)
>
> diff --git a/cache.h b/cache.h
> index f30a4417efdf..da9f0be67d7b 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -964,6 +964,8 @@ static inline void oidclr(struct object_id *oid)
>  #define EMPTY_BLOB_SHA1_BIN \
>  	((const unsigned char *) EMPTY_BLOB_SHA1_BIN_LITERAL)
>  
> +extern const struct object_id empty_tree_oid;
> +
>  static inline int is_empty_blob_sha1(const unsigned char *sha1)
>  {
>  	return !hashcmp(sha1, EMPTY_BLOB_SHA1_BIN);
> diff --git a/sha1_file.c b/sha1_file.c
> index 1e23fc186a02..10883d56a600 100644
> --- a/sha1_file.c
> +++ b/sha1_file.c
> @@ -38,6 +38,9 @@ static inline uintmax_t sz_fmt(size_t s) { return s; }
>  
>  const unsigned char null_sha1[20];
>  const struct object_id null_oid;
> +const struct object_id empty_tree_oid = {
> +	.hash = EMPTY_TREE_SHA1_BIN_LITERAL
> +};
>  
>  /*
>   * This is meant to hold a *small* number of objects that you would