Subject: Re: [PATCH v2 01/30] object-file-convert: stubs for converting from one object format to another

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 15 Feb 2024 12:21:46 +0100

On Sun, Oct 01, 2023 at 09:40:05PM -0500, Eric W. Biederman wrote:
> From: "Eric W. Biederman" <[email-masked]>
> 
> Two basic functions are provided:
> - convert_object_file Takes an object file it's type and hash algorithm
>   and converts it into the equivalent object file that would
>   have been generated with hash algorithm "to".
> 
>   For blob objects there is no conversation to be done and it is an
>   error to use this function on them.
> 
>   For commit, tree, and tag objects embedded oids are replaced by the
>   oids of the objects they refer to with those objects and their
>   object ids reencoded in with the hash algorithm "to".  Signatures
>   are rearranged so that they remain valid after the object has
>   been reencoded.
> 
> - repo_oid_to_algop which takes an oid that refers to an object file
>   and returns the oid of the equivalent object file generated
>   with the target hash algorithm.
> 
> The pair of files object-file-convert.c and object-file-convert.h are
> introduced to hold as much of this logic as possible to keep this
> conversion logic cleanly separated from everything else and in the
> hopes that someday the code will be clean enough git can support
> compiling out support for sha1 and the various conversion functions.
> 
> Signed-off-by: "Eric W. Biederman" <[email-masked]>
> ---
>  Makefile              |  1 +
>  object-file-convert.c | 57 +++++++++++++++++++++++++++++++++++++++++++
>  object-file-convert.h | 24 ++++++++++++++++++
>  3 files changed, 82 insertions(+)
>  create mode 100644 object-file-convert.c
>  create mode 100644 object-file-convert.h
> 
> diff --git a/Makefile b/Makefile
> index 577630936535..f7e824f25cda 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1073,6 +1073,7 @@ LIB_OBJS += notes-cache.o
>  LIB_OBJS += notes-merge.o
>  LIB_OBJS += notes-utils.o
>  LIB_OBJS += notes.o
> +LIB_OBJS += object-file-convert.o
>  LIB_OBJS += object-file.o
>  LIB_OBJS += object-name.o
>  LIB_OBJS += object.o
> diff --git a/object-file-convert.c b/object-file-convert.c
> new file mode 100644
> index 000000000000..4777aba83636
> --- /dev/null
> +++ b/object-file-convert.c
> @@ -0,0 +1,57 @@
> +#include "git-compat-util.h"
> +#include "gettext.h"
> +#include "strbuf.h"
> +#include "repository.h"
> +#include "hash-ll.h"
> +#include "object.h"
> +#include "object-file-convert.h"
> +
> +int repo_oid_to_algop(struct repository *repo, const struct object_id *src,
> +		      const struct git_hash_algo *to, struct object_id *dest)
> +{
> +	/*
> +	 * If the source algorithm is not set, then we're using the
> +	 * default hash algorithm for that object.
> +	 */
> +	const struct git_hash_algo *from =
> +		src->algo ? &hash_algos[src->algo] : repo->hash_algo;
> +
> +	if (from == to) {
> +		if (src != dest)
> +			oidcpy(dest, src);
> +		return 0;
> +	}
> +	return -1;
> +}

In it's current form, `repo_oid_to_algop()` basically never does
anything except for copying over the object ID because we do not handle
the case where object hashes are different. I assume this is intended,
as we basically only provide stubs in this commit. But still, it would
help to document this in-code as well with a comment.

> +int convert_object_file(struct strbuf *outbuf,
> +			const struct git_hash_algo *from,
> +			const struct git_hash_algo *to,
> +			const void *buf, size_t len,
> +			enum object_type type,
> +			int gentle)
> +{
> +	int ret;
> +
> +	/* Don't call this function when no conversion is necessary */
> +	if ((from == to) || (type == OBJ_BLOB))
> +		BUG("Refusing noop object file conversion");

The extra braces around comparisons are unneeded and to the best of my
knowledge not customary in our code base. Also, error messages should
start with a lower-case letter.

> +	switch (type) {
> +	case OBJ_COMMIT:
> +	case OBJ_TREE:
> +	case OBJ_TAG:
> +	default:
> +		/* Not implemented yet, so fail. */
> +		ret = -1;
> +		break;
> +	}

It's a bit weird that we handle all object types except for blobs
separately, and then still have a `default` statement. I would've
thought that we should handle the object types specifically and set `ret
= -1` for all of them, and then the `default` case would instead call
`BUG()` due to an unknown object type.

> +	if (!ret)
> +		return 0;
> +	if (gentle) {
> +		strbuf_release(outbuf);
> +		return ret;
> +	}

Do you really intend to call `strbuf_release()` on the caller provided
buffer, or should this rather be `strbuf_reset()`? Memory management of
such an in/out parameter should typically be handled by the caller, not
the callee.

> +	die(_("Failed to convert object from %s to %s"),
> +		from->name, to->name);
> +}

The error message should start with a lower-case letter.

> diff --git a/object-file-convert.h b/object-file-convert.h
> new file mode 100644
> index 000000000000..a4f802aa8eea
> --- /dev/null
> +++ b/object-file-convert.h
> @@ -0,0 +1,24 @@
> +#ifndef OBJECT_CONVERT_H
> +#define OBJECT_CONVERT_H
> +
> +struct repository;
> +struct object_id;
> +struct git_hash_algo;
> +struct strbuf;
> +#include "object.h"
> +
> +int repo_oid_to_algop(struct repository *repo, const struct object_id *src,
> +		      const struct git_hash_algo *to, struct object_id *dest);
> +
> +/*
> + * Convert an object file from one hash algorithm to another algorithm.
> + * Return -1 on failure, 0 on success.
> + */
> +int convert_object_file(struct strbuf *outbuf,
> +			const struct git_hash_algo *from,
> +			const struct git_hash_algo *to,
> +			const void *buf, size_t len,
> +			enum object_type type,
> +			int gentle);

It would be nice to document what `gentle` does.

Patrick

> +#endif /* OBJECT_CONVERT_H */
> -- 
> 2.41.0
>