threads / discuss / 56321

bugreport: git apply -3 confusing "lacks the necessary blob"

Subject: bugreport: git apply -3 confusing "lacks the necessary blob"

## tl;dr

7 messages between Aug 18, 2021 and Aug 27, 2021.

replies: 6people: 3as markdown or json

Jan Kratochvil· Aug 18, 2021, 18:17 UTC · lore

Thank you for filling out a Git bug report! Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue) git apply -3 b.diff

What did you expect to happen? (Expected behavior)
<<<<<<< ours
c
||||||| base
a
=======
b

>>>>>>> theirs

What happened instead? (Actual behavior) error: patch failed: x:1 error: repository lacks the necessary blob to fall back on 3-way merge. error: x: patch does not apply

What's different between what you expected and what actually happened? The conflicting patch has not been applied to resolve it manually.

Anything else you want to add: The problem is 'git apply -3' will print confusing "lacks the necessary blob" message despite the real error message should be: "b.diff is missing line starting with 'index', cannot apply by -3/--3way"

reproducer: (set -ex;: rm -rf gitgit;mkdir gitgit;cd gitgit;git init;echo a >x;git add x;git commit -am.;git checkout -b b;echo b >x;git commit -am.;git checkout master;echo c >x;git commit -am.;git diff master^..b|grep -v ^index >b.diff;git apply -3 b.diff || cat b.diff)

Please review the rest of the bug report below. You can delete any lines you don't wish to share.

[System Info] git version: git version 2.31.1 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.12.11-300.fc34.x86_64 #1 SMP Wed Jun 16 15:47:58 UTC 2021 x86_64 compiler info: gnuc: 11.0 libc info: glibc: 2.33 $SHELL (typically, interactive shell): /bin/bash

[Enabled Hooks] not run from a git repository - no hooks to show

Bagas Sanjaya· Aug 20, 2021, 09:19 UTC · re: Jan Kratochvil · lore

Re: bugreport: git apply -3 confusing "lacks the necessary blob"

On 19/08/21 01.17, Jan Kratochvil wrote:

> reproducer:
> (set -ex;: rm -rf gitgit;mkdir gitgit;cd gitgit;git init;echo a >x;git add x;git commit -am.;git checkout -b b;echo b >x;git commit -am.;git checkout master;echo c >x;git commit -am.;git diff master^..b|grep -v ^index >b.diff;git apply -3 b.diff || cat b.diff)

I can reproduce your issue on latest Git (2.33.0).

It seems like you remove `index` line, which **may** contain blob hash information required for three-way merge with git apply -3.

But if you don't remove it when generating patch that way, you will get expected conflict when git applying.

-- 
An old man doll... just what I always wanted! - Clara

Jan Kratochvil· Aug 20, 2021, 09:26 UTC · re: Bagas Sanjaya · lore

Re: bugreport: git apply -3 confusing "lacks the necessary blob"

On Fri, 20 Aug 2021 11:19:39 +0200, Bagas Sanjaya wrote:

Show 11 quoted lines

> On 19/08/21 01.17, Jan Kratochvil wrote:
> > reproducer:
> > (set -ex;: rm -rf gitgit;mkdir gitgit;cd gitgit;git init;echo a >x;git add x;git commit -am.;git checkout -b b;echo b >x;git commit -am.;git checkout master;echo c >x;git commit -am.;git diff master^..b|grep -v ^index >b.diff;git apply -3 b.diff || cat b.diff)
> 
> I can reproduce your issue on latest Git (2.33.0).
> 
> It seems like you remove `index` line, which **may** contain blob hash
> information required for three-way merge with git apply -3.
> 
> But if you don't remove it when generating patch that way, you will get
> expected conflict when git applying.

Yes.

The problem is when the 'index' line is already removed (as it was always being removed in my existing setup) it is difficult to find out that is the reason of this confusing message. So the error message should be different when the 'index' line is missing at all.

Jan

Junio C Hamano· Aug 20, 2021, 17:40 UTC · re: Bagas Sanjaya · lore

Re: bugreport: git apply -3 confusing "lacks the necessary blob"

Bagas Sanjaya <bagasdotme@gmail.com> writes:

Show 11 quoted lines

> On 19/08/21 01.17, Jan Kratochvil wrote:
>> reproducer:
>> (set -ex;: rm -rf gitgit;mkdir gitgit;cd gitgit;git init;echo a >x;git add x;git commit -am.;git checkout -b b;echo b >x;git commit -am.;git checkout master;echo c >x;git commit -am.;git diff master^..b|grep -v ^index >b.diff;git apply -3 b.diff || cat b.diff)
>
> I can reproduce your issue on latest Git (2.33.0).
>
> It seems like you remove `index` line, which **may** contain blob hash
> information required for three-way merge with git apply -3.
>
> But if you don't remove it when generating patch that way, you will
> get expected conflict when git applying.

You make it sound like an "it hurts when I do this, doctor. ---do not do it, then" exchange ;-).

But I think Jan is talking about the case where users get a patch that lacks the "index" information out of other people's "diff" implementation and try to "apply -3" without realizing that it is not Git's "diff" output.

Perhaps something like the attached patch would be a good start.

There is another place that relies on the index line (application of binary patch), but the code rejects any patch that does not have the full-length object name with "... without full index line". This message was originally meant to reject patches with abbreviated object names on the index line, but it would equally apply to one without the index line (perhaps by accident), so there is no need to touch that one.

 apply.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git i/apply.c w/apply.c
index 44bc31d6eb..9972ada57e 100644
--- i/apply.c
+++ w/apply.c
@@ -3566,6 +3566,8 @@ static int try_threeway(struct apply_state *state,
 	/* Preimage the patch was prepared for */
 	if (patch->is_new)
 		write_object_file("", 0, blob_type, &pre_oid);
+	else if (!*patch->old_oid_prefix && !*patch->new_oid_prefix)
+		return error(_("cannot 'apply -3' a patch without the index line"));
 	else if (get_oid(patch->old_oid_prefix, &pre_oid) ||
 		 read_blob_object(&buf, &pre_oid, patch->old_mode))
 		return error(_("repository lacks the necessary blob to perform 3-way merge."));

Jan Kratochvil· Aug 20, 2021, 18:30 UTC · re: Junio C Hamano · lore

Re: bugreport: git apply -3 confusing "lacks the necessary blob"

On Fri, 20 Aug 2021 19:40:12 +0200, Junio C Hamano wrote:

> But I think Jan is talking about the case where users get a patch
> that lacks the "index" information out of other people's "diff"
> implementation and try to "apply -3" without realizing that it is
> not Git's "diff" output.

Yes.

> Perhaps something like the attached patch would be a good start.

It does fix the problem for me, thanks!

Jan

Jan Kratochvil· Aug 27, 2021, 15:15 UTC · re: Junio C Hamano · lore

Re: bugreport: git apply -3 confusing "lacks the necessary blob"

On Fri, 20 Aug 2021 19:40:12 +0200, Junio C Hamano wrote:

> But I think Jan is talking about the case where users get a patch
> that lacks the "index" information out of other people's "diff"
> implementation and try to "apply -3" without realizing that it is
> not Git's "diff" output.

For example from Phabricator:
	https://reviews.llvm.org/file/data/3ceoc32b3yv43vk3nw4q/PHID-FILE-lfeeh2qu4vrngdcwwudo/D107456.diff

> Perhaps something like the attached patch would be a good start.

Do you plan to check it in?

Thanks, Jan Kratochvil

Show 13 quoted lines

> diff --git i/apply.c w/apply.c
> index 44bc31d6eb..9972ada57e 100644
> --- i/apply.c
> +++ w/apply.c
> @@ -3566,6 +3566,8 @@ static int try_threeway(struct apply_state *state,
>  	/* Preimage the patch was prepared for */
>  	if (patch->is_new)
>  		write_object_file("", 0, blob_type, &pre_oid);
> +	else if (!*patch->old_oid_prefix && !*patch->new_oid_prefix)
> +		return error(_("cannot 'apply -3' a patch without the index line"));
>  	else if (get_oid(patch->old_oid_prefix, &pre_oid) ||
>  		 read_blob_object(&buf, &pre_oid, patch->old_mode))
>  		return error(_("repository lacks the necessary blob to perform 3-way merge."));

Junio C Hamano· Aug 27, 2021, 17:14 UTC · re: Jan Kratochvil · lore

Re: bugreport: git apply -3 confusing "lacks the necessary blob"

Jan Kratochvil <jan@jankratochvil.net> writes:

Show 13 quoted lines

> On Fri, 20 Aug 2021 19:40:12 +0200, Junio C Hamano wrote:
>> But I think Jan is talking about the case where users get a patch
>> that lacks the "index" information out of other people's "diff"
>> implementation and try to "apply -3" without realizing that it is
>> not Git's "diff" output.
>
> For example from Phabricator:
> 	https://reviews.llvm.org/file/data/3ceoc32b3yv43vk3nw4q/PHID-FILE-lfeeh2qu4vrngdcwwudo/D107456.diff
>
>
>> Perhaps something like the attached patch would be a good start.
>
> Do you plan to check it in?

Maybe eventually but not yet. When I say "a good start", it is because it is known to be insufficient. I haven't had a chance to revisit the issue.