Subject: Re: [PATCH 1/4] rebase -i: stop overwriting ORIG_HEAD buffer

From: Phillip Wood <[email-masked]>
Date: Sat, 31 Oct 2020 10:55:52 +0000

On 27/10/2020 21:10, Junio C Hamano wrote:
> "Phillip Wood via GitGitGadget" <[email-masked]> writes:
> 
>> From: Phillip Wood <[email-masked]>
> 
> My initial impression after seeing the recent report about ORIG_HEAD
> was "hmph, these days, rebasing is done on detached HEAD and the
> final step updates the target branch only once, so @{1} is much
> easier to use---perhaps it is time to deprecate use of ORIG_HEAD?".
> After all, ORIG_HEAD was invented way before we had reflog, and
> given that one of the goal of reflog was to give more general
> recovery mechanism than going back one-step like ORIG_HEAD allowed
> us to, and "rebase" were taught to work on detached HEAD to make
> @{1} more useful, it would not be too bad to eventually retire
> ORIG_HEAD in a distant future, I thought.

Thanks for filling in the history. As reset sets ORIG_HEAD as well as 
rebase it can be confusing so retiring it in the future maybe a good idea.

> But it is a good initiative anyway to make ORIG_HEAD again work as
> documented.  Thanks for working on it.
> 
>> After rebasing ORIG_HEAD is supposed to point to the old HEAD of the
> 
> A comma after "rebasing".

Sure

>> rebased branch. Unfortunately the buffer storing the oid was
>> overwritten with a new oid before ORIG_HEAD was created. The buffer is
>> also used when writing .git/rebase-merge/orig-head which is used by
>> `rebase --abort` to restore the previous head. Luckily that file is
>> written before the buffer is overwritten.  As we want the full oid
>> find_unique_abbrev() is replaced with oid_to_hex_r() rather than
>> find_unique_abbrev_r().
> 
> The above is hard to read and understand.  It is unclear where the
> observation of the current behaviour (which is often the explanation
> of the cause of the bug) ends and the description of new behaviour
> begins.
> 
>      ... old HEAD of the rebased branch.  The code used
>      find_unique_abbrev() to obtain the object name of the old HEAD
>      and wrote to both .git/rebase-merge/orig-head (used by `rebase
>      --abort` to go back to the previous state) and to ORIG_HEAD.
>      The buffer find_unique_abbrev() gives back is volatile,
>      unfortunately, and was overwritten after the former file is
>      written but before ORIG_FILE is written, leaving an incorrect
>      object name in it.
> 
> Up to that point is the observation of the current code, which
> explains where the bug comes from.  Please have a paragraph break
> after that, before explaining the solution, e.g.
> 
>      Avoid relying on the volatile buffer of find_unique_abbrev(),
>      and instead supply our own buffer to keep the object name.
>      Because we want to use the full object name, use oid_to_hex_r()
>      instead of find_unique_abbrev_r() to do so.

That's much clearer thanks. I wonder if swapping to oid_to_hex_r() 
rather than find_unique_abbev_r() is complicating this commit 
unnecessarily as the code that is changed here is deleted in patch 4, 
maybe we should do the switch there.

>> I think that all of the users of head_hash should actually be using
>> opts->orig_head instead as passing a string rather than a struct
>> object_id around is a hang over from the scripted implementation. This
>> patch just fixes the immediate bug and adds a regression test based on
>> Caspar's reproduction example. The users will be converted to use
>> struct object_id and head_hash removed in the next few commits.
> 
> Makes sense.
> 
>>
>> Reported-by: Caspar Duregger <[email-masked]>
>> Signed-off-by: Phillip Wood <[email-masked]>
>> ---
> 
> If you cite "Caspar's repro example" like that in the log message,
> it is incomplete to leave out a URL to the mail archive.

Good point I'll add a url

Thanks for your comments

Phillip

>>   builtin/rebase.c              | 10 +++++-----
>>   t/t3404-rebase-interactive.sh | 11 +++++++++++
>>   2 files changed, 16 insertions(+), 5 deletions(-)
>>
>> diff --git a/builtin/rebase.c b/builtin/rebase.c
>> index eeca53382f..6def28a533 100644
>> --- a/builtin/rebase.c
>> +++ b/builtin/rebase.c
>> @@ -270,15 +270,15 @@ static int edit_todo_file(unsigned flags)
>>   }
>>   
>>   static int get_revision_ranges(struct commit *upstream, struct commit *onto,
>> -			       struct object_id *orig_head, const char **head_hash,
>> +			       struct object_id *orig_head, char *head_hash,
>>   			       char **revisions, char **shortrevisions)
>>   {
>>   	struct commit *base_rev = upstream ? upstream : onto;
>>   	const char *shorthead;
>>   
>> -	*head_hash = find_unique_abbrev(orig_head, GIT_MAX_HEXSZ);
>> +	oid_to_hex_r(head_hash, orig_head);
>>   	*revisions = xstrfmt("%s...%s", oid_to_hex(&base_rev->object.oid),
>> -						   *head_hash);
>> +						   head_hash);
>>   
>>   	shorthead = find_unique_abbrev(orig_head, DEFAULT_ABBREV);
>>   
>> @@ -327,7 +327,7 @@ static void split_exec_commands(const char *cmd, struct string_list *commands)
>>   static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
>>   {
>>   	int ret;
>> -	const char *head_hash = NULL;
>> +	char head_hash[GIT_MAX_HEXSZ];
>>   	char *revisions = NULL, *shortrevisions = NULL;
>>   	struct strvec make_script_args = STRVEC_INIT;
>>   	struct todo_list todo_list = TODO_LIST_INIT;
>> @@ -335,7 +335,7 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
>>   	struct string_list commands = STRING_LIST_INIT_DUP;
>>   
>>   	if (get_revision_ranges(opts->upstream, opts->onto, &opts->orig_head,
>> -				&head_hash, &revisions, &shortrevisions))
>> +				head_hash, &revisions, &shortrevisions))
>>   		return -1;
>>   
>>   	if (init_basic_state(&replay,
>> diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
>> index 07a1617351..1e56696e4f 100755
>> --- a/t/t3404-rebase-interactive.sh
>> +++ b/t/t3404-rebase-interactive.sh
>> @@ -1797,6 +1797,17 @@ test_expect_success 'todo has correct onto hash' '
>>   	test_i18ngrep "^# Rebase ..* onto $onto" actual
>>   '
>>   
>> +test_expect_success 'ORIG_HEAD is updated correctly' '
>> +	test_when_finished "git checkout master && git branch -D test-orig-head" &&
>> +	git checkout -b test-orig-head A &&
>> +	git commit --allow-empty -m A1 &&
>> +	git commit --allow-empty -m A2 &&
>> +	git commit --allow-empty -m A3 &&
>> +	git commit --allow-empty -m A4 &&
>> +	git rebase master &&
>> +	test_cmp_rev ORIG_HEAD test-orig-head@{1}
>> +'
>> +
>>   # This must be the last test in this file
>>   test_expect_success '$EDITOR and friends are unchanged' '
>>   	test_editor_unchanged