Re: [PATCH 03/10] merge-ort: implement unique_path() helper

[Derrick Stolee <stolee@gmail.com>]

On 12/30/2020 10:10 AM, Elijah Newren wrote:
> On Wed, Dec 30, 2020 at 6:16 AM Derrick Stolee <stolee@gmail.com> wrote:
>> On 12/18/2020 12:51 AM, Elijah Newren via GitGitGadget wrote:
>>> From: Elijah Newren <newren@gmail.com>
>>>
>>> Implement unique_path(), based on the one from merge-recursive.c.  It is
>>> simplified, however, due to: (1) using strmaps, and (2) the fact that
>>> merge-ort lets the checkout codepath handle possible collisions with the
>>> working tree means that other code locations don't have to.
>>>
>>> Signed-off-by: Elijah Newren <newren@gmail.com>
>>> ---
>>>  merge-ort.c | 25 ++++++++++++++++++++++++-
>>>  1 file changed, 24 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/merge-ort.c b/merge-ort.c
>>> index d300a02810e..1adc27a11bc 100644
>>> --- a/merge-ort.c
>>> +++ b/merge-ort.c
>>> @@ -343,11 +343,34 @@ static void path_msg(struct merge_options *opt,
>>>       strbuf_addch(sb, '\n');
>>>  }
>>>
>>> +/* add a string to a strbuf, but converting "/" to "_" */
>>> +static void add_flattened_path(struct strbuf *out, const char *s)
>>> +{
>>> +     size_t i = out->len;
>>> +     strbuf_addstr(out, s);
>>> +     for (; i < out->len; i++)
>>> +             if (out->buf[i] == '/')
>>> +                     out->buf[i] = '_';
>>> +}
>>> +
>>
>> Thank you for pointing out that you based your code on merge-recursive.c.
>> I see that this implementation is identical to the one there. I question
>> whether this causes collisions in a problematic way, when "a/b/c" and
>> "a_b_c" both exist in a tree.
>>
>> To avoid such a problem, we'd likely need to also expand "_" to "__" or
>> similar. This might never actually affect any users because of the
>> strange filename matches _and_ we need to be in a directory/file conflict.
>>
>> And maybe it's not a hole at all? If it is, we can consider patching or
>> at least documenting the problem.
> 
> add_flattened_path() can certainly result in a collision, regardless
> of whether the char *s parameter has any '/' characters in it.  For
> example, if you are trying to get a unique path associated with
> builtin/commit-graph.c due to changes from the 'next' branch side of
> the merge, and builtin/commit-graph.c~next already exists, then you
> have a collision.  It's actually pretty rare that the parameter would
> have any '/' characters at all, since it's pretty rare for me to see
> folks (other than Junio) use hierarchical branch names.  But if the
> branch name were ds/line-log-on-bloom, then the provisional filename
> would be builtin/commit-graph.c~ds_line-log-on-bloom.  The '/' to '_'
> conversion exists just to make sure our new file remains in the same
> directory as where the conflict that caused us to need a new unique
> path occurred.

Ah, I am misinterpreting which '/' characters are getting squashed.
Thank you for fixing my confusion.

> But unique_path() does NOT end immediately after calling
> add_flattened_path() and there is no collision possible in the return
> from unique_path(), because it ends with a
>     while (strmap_contains(existing_paths, newpath.buf)) {
> loop that modifies the resulting path until it finds one that doesn't
> collide with an existing path.

And this extra care here is helpful. Thanks!

-Stolee