Re: [PATCH v2] user-manual: add advanced topic "bisecting merges"

[Andreas Ericsson <ae@op5.se>]

Steffen Prohaska wrote:
> 
> On Nov 8, 2007, at 10:53 AM, Johannes Sixt wrote:
> 
>> Andreas Ericsson schrieb:
>>> Johannes Sixt wrote:
>>>> Steffen Prohaska schrieb:
>>>>>
>>>>> On Nov 8, 2007, at 8:19 AM, Johannes Sixt wrote:
>>>>>
>>>>>> Steffen Prohaska schrieb:
>>>>>>> +If you linearize the history by rebasing the lower branch on
>>>>>>> +top of the upper, instead of merging, the bug becomes much 
>>>>>>> easier to
>>>>>>> +find and understand.  Your history would instead be:
>>>>>>
>>>>>> At this point I'm missing the words
>>>>>>
>>>>>>     The solution is ...
>>>>>>
>>>>>> I.e.:
>>>>>>
>>>>>> The solution is to linearize the history by rebasing the lower 
>>>>>> branch on
>>>>>> top of the upper, instead of merging. Now the bug becomes much 
>>>>>> easier to
>>>>>> find and understand.  Your history would instead be:
>>>>>
>>>>> Hmm. It might be a solution if you did not publish history.
>>>>
>>>> This is about finding the commit that introduced a bug. Once you 
>>>> found it, better: you know how to fix the bug, you are expected to 
>>>> throw away the rebased branch, not to publish it! Maybe a note along 
>>>> these lines could be appended:
>>>>
>>>> Now that you know what caused the error (and how to fix it), throw 
>>>> away the rebased branch, and commit a fix on top of D.
>>>>
>>> Well, if rebasing becomes the standard for normal development, it's 
>>> hardly
>>> right to throw it away, is it? I like Steffen's suggestion better.
>>
>> There is a big misunderstanding. The text that the patch amends is 
>> about bisecting history that reveals that a merge commit breaks, which 
>> is not helpful, and then how to find where and what and why the 
>> breakage really was introduce.
>>
>> And the answer to "how to find" is to rebase and bisect in the rebased 
>> history.
> 
> Do you use rebase like this in real life?
> 
> I thought of the text as background information that might
> be helpful for users who want do decide wether to merge or
> to rebase. The problem described may be valuable information
> supporting a decision about a recommended workflow for a group
> of users.
> 
> My personal conclusion was: I'll accept the danger of complex
> merges that might be hard to bisect. I now understand this
> risk, but I nonetheless prefer the simplicity of a merge
> based workflow. This avoids the danger that published history
> gets rewritten.
> 
> But now I'm wondering if your suggestions of rebasing only for
> locating the evil commit is feasible in reality. You may need
> to solve a lot of merge conflicts if you rebase a larger part
> of the history. If you do not have them in your rerere cache
> this might be time consuming. ...
> 

It is no great chore to put one merge-parent on top of another
and then re-run bisect on the result. git-bisect could even be
taught to do that by itself.

> 
>> My initial complaint was that in the flow of reading the instructions 
>> the pointer to "the solution" was missing. Rather, at the point where 
>> the reader is supposed to think "ah, yes, that's how to do it", there 
>> is the conditional statement "If you linearize history". My suggestion 
>> is to put a big emphasis on the solution by using the words "The 
>> solution is".
>>
>> Now, the user can *always* rebase one of the branches on top of the 
>> other, even if both histories are already published. *But* if both 
>> were indeed published, then the rebased history must be thrown away, 
>> and the only thing you learnt from it was where and what and why the 
>> breakage really was introduced.
>>
>> Of course we could include a few "ifs" and "unlesses" (about published 
>> histories), before suggesting to throw away rebased history. But once 
>> the task is accomplished (find the bogus commit), throwing away the 
>> rebased history (and continuing at commit D) is always correct, but 
>> keeping it (and continuing at D*) is not.
> 
> ... So, again, the question for me is if someone does use
> rebase in reality in the way that you suggests. Do you?


I don't, but if I'd thought a bit further I would have on at least one
occasion in the past. Instead I spent two days manually auditing every
commit of several branches.

-- 
Andreas Ericsson                   andreas.ericsson@op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231