patchreplace: fix replacing object with itself

> When object is replaced with itself git shows unhelpful messages like(git log):
>     "fatal: replace depth too high for object <SHA1>"
>
> Prevents user from replacing object with itself(with test for checking
> this case).
>
> Signed-off-by: Manzur Mukhitdinov <manzurmm@gmail.com>
> ---

>  builtin/replace.c  |  8 +++-----
>  t/t6050-replace.sh | 11 +++++++++--
>  2 files changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/builtin/replace.c b/builtin/replace.c
> index 294b61b..628377a 100644
> --- a/builtin/replace.c
> +++ b/builtin/replace.c
> @@ -157,6 +157,9 @@ static int replace_object_sha1(const char *object_ref,
>  	char ref[PATH_MAX];
>  	struct ref_lock *lock;
>  
> +	if (!hashcmp(object, repl))
> +		return error("new object is the same as the old one: '%s'", sha1_to_hex(object));
> +
>  	obj_type = sha1_object_info(object, NULL);
>  	repl_type = sha1_object_info(repl, NULL);
>  	if (!force && obj_type != repl_type)
> @@ -295,9 +298,6 @@ static int edit_and_replace(const char *object_ref, int force, int raw)
>  
>  	free(tmpfile);
>  
> -	if (!hashcmp(old, new))
> -		return error("new object is the same as the old one: '%s'", sha1_to_hex(old));
> -
>  	return replace_object_sha1(object_ref, old, "replacement", new, force);
>  }
>  
> @@ -406,8 +406,6 @@ static int create_graft(int argc, const char **argv, int force)
>  
>  	strbuf_release(&buf);
>  
> -	if (!hashcmp(old, new))
> -		return error("new commit is the same as the old one: '%s'", sha1_to_hex(old));
>  
>  	return replace_object_sha1(old_ref, old, "replacement", new, force);
>  }
> diff --git a/t/t6050-replace.sh b/t/t6050-replace.sh
> index 4d5a25e..5f96374 100755
> --- a/t/t6050-replace.sh
> +++ b/t/t6050-replace.sh
> @@ -369,9 +369,8 @@ test_expect_success '--edit with and without already replaced object' '
>  	git cat-file commit "$PARA3" | grep "A fake Thor"
>  '
>  
> -test_expect_success '--edit and change nothing or command failed' '
> +test_expect_success '--edit with failed editor' '
>  	git replace -d "$PARA3" &&
> -	test_must_fail env GIT_EDITOR=true git replace --edit "$PARA3" &&
>  	test_must_fail env GIT_EDITOR="./fakeeditor;false" git replace --edit "$PARA3" &&
>  	GIT_EDITOR=./fakeeditor git replace --edit "$PARA3" &&
>  	git replace -l | grep "$PARA3" &&
> @@ -440,4 +439,12 @@ test_expect_success GPG '--graft on a commit with a mergetag' '
>  	git replace -d $HASH10
>  '
>  
> +test_expect_success 'replacing object with itself must fail' '
> +    test_must_fail git replace $HASH1 $HASH1 &&
> +    HASH8=$(git rev-parse --verify HEAD) &&
> +    test_must_fail git replace HEAD $HASH8 &&
> +    test_must_fail git replace --graft HEAD HEAD^ &&
> +    test_must_fail env GIT_EDITOR=true git replace --edit HEAD
> +'
> +
>  test_done

> Manzur Mukhitdinov <manzurmm@gmail.com> writes:
>
>> When object is replaced with itself git shows unhelpful messages like(git log):
>>     "fatal: replace depth too high for object <SHA1>"
>>
>> Prevents user from replacing object with itself(with test for checking
>> this case).
>>
>> Signed-off-by: Manzur Mukhitdinov <manzurmm@gmail.com>
>> ---
>
> The patch is not wrong per-se, but I wonder how useful this "do not
> replace itself but all other forms of loops are not checked at all"
> would be in practice.  If your user did this:
>
>         git replace A B ;# pretend as if what is in B is in A
>         git replace B C ;# pretend as if what is in C is in B
>         git replace C A ;# pretend as if we have loop
>         git log C
>
> she would not be helped with this patch at all, no?

> We have the "replace depth" thing, which is a poor-man's substitute
> for loop detection, primarily because we do not want to incur high
> cost of loop detection at runtime.  Shouldn't we be doing at least
> the same amount of loop-avoidance check, if we really want to avoid
> triggering the "replace depth" check at runtime?

>> The patch is not wrong per-se, but I wonder how useful this "do not
>> replace itself but all other forms of loops are not checked at all"
>> would be in practice.  If your user did this:
>>
>>         git replace A B ;# pretend as if what is in B is in A
>>         git replace B C ;# pretend as if what is in C is in B
>>         git replace C A ;# pretend as if we have loop
>>         git log C
>>
>> she would not be helped with this patch at all, no?
>
> Yeah, but such loops are much less likely mistakes and are more
> difficult to detect, so I think this patch is at least a good first
> step.