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

From: Steffen Prohaska <prohaska@zib.de>
Date: Nov 8, 2007, 12:54 UTC
Message-ID: <97F64156-A457-4BC1-84BE-108369FFD18C@zib.de>
In-Reply-To: <4732DC98.70304@viscovery.net>

On Nov 8, 2007, at 10:53 AM, Johannes Sixt wrote:

Show 45 quoted lines

> 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. ...

Show 18 quoted lines

> 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? Does someone else?

	Steffen