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From: Ben Peart <peartben@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, asottile@umich.edu, benpeart@microsoft.com,
	pclouds@gmail.com
Subject: Re: [PATCH v2 0/2] Fix regression in checkout -b
Date: Tue, 22 Jan 2019 14:31:16 -0500	[thread overview]
Message-ID: <2a3ac803-133c-98fb-45e9-43f6e4a018d1@gmail.com> (raw)
In-Reply-To: <xmqq4la0h6am.fsf@gitster-ct.c.googlers.com>



On 1/22/2019 1:54 PM, Junio C Hamano wrote:
> Ben Peart <peartben@gmail.com> writes:
> 
>> diff --git a/builtin/checkout.c b/builtin/checkout.c
>> index af6b5c8336..9c6e94319e 100644
>> --- a/builtin/checkout.c
>> +++ b/builtin/checkout.c
>> @@ -517,12 +517,6 @@ static int skip_merge_working_tree(const struct checkout_opts *opts,
>>   	if (core_apply_sparse_checkout && !checkout_optimize_new_branch)
>>   		return 0;
>>   
>> -	/*
>> -	 * We must do the merge if this is the initial checkout
>> -	 */
>> -	if (is_cache_unborn())
>> -		return 0;
>> -
>>   	/*
>>   	 * We must do the merge if we are actually moving to a new commit.
>>   	 */
>> @@ -598,6 +592,13 @@ static int skip_merge_working_tree(const struct checkout_opts *opts,
>>   	 * Remaining variables are not checkout options but used to track state
>>   	 */
>>   
>> +	 /*
>> +	  * Do the merge if this is the initial checkout
>> +	  *
>> +	  */
>> +	if (!file_exists(get_index_file()))
>> +		return 0;
>> +
>>   	return 1;
>>   }
> 
> This is curious.  The location the new special case is added is
> different, and the way the new special case is detected is also
> different, between v1 and v2.  Are both of them significant?  IOW,
> if we moved the check down but kept using is_cache_unborn(), would
> it break?  Or if we did not move the check but switched to check the
> index file on the filesystem instead of calling is_cache_unborn(),
> would it break?
> 

I had to change the check to not use is_cache_unborn() because at this 
point, the index has not been loaded so cache_nr and timestamp.sec are 
always zero (thus defeating the entire optimization).  Since part of the 
optimization was to avoid loading the index when it isn't necessary, the 
only replacement I could think of was to check for the existence of the 
index file as if it is missing entirely, it is clearly unborn.  This 
solved the behavior change for the --no-checkout sequence reported.

The only reason I moved it lower in the function was a micro perf 
optimization.  Since file_exists() does file I/O, I thought I'd do all 
the in memory/flag checks first in case they drop out early and we can 
avoid the unnecessary file I/O.  As long as it is tested before the 
'return 1;' call, it is logically correct.

> There are three existing callers of is_{cache,index}_unborn(), all
> of which want to use it to decide if we are in this funny "unborn"
> state.  If this fixes the issue we saw in v1 of these two patches,
> does that mean these three existing callers also are buggy in the
> same way and we are better off rewriting is_index_unborn() to see if
> the index file is on the disk?
> 

It is just the fact that I needed to check for an unborn index _before_ 
it was loaded that makes me unable to use is_{cache,index}_unborn() 
here.  The other callers should still be fine.  I could add a comment in 
the code to clarify this if you think it will cause confusion later.

> I am *not* suggesting to make such a drastic change to the existing
> system.  I am wondering why they are working fine but only this new
> code has to avoid the existing is_index_unborn() logic and go
> directly to the filesystem.  Especially as this new exception added
> to "skip-merge-working-tree" is to allow the special case code in
> merge-working-tree that depends on is_cache_unborn() to trigger.
> 
> Thanks for working on this.
> 

  reply	other threads:[~2019-01-22 19:31 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-01 23:17 Regression `git checkout $rev -b branch` while in a `--no-checkout` clone does not check out files Anthony Sottile
2019-01-02 11:08 ` Duy Nguyen
2019-01-02 16:18   ` Anthony Sottile
2019-01-03 10:04     ` Duy Nguyen
2019-01-03 20:25       ` Junio C Hamano
2019-01-03 20:35         ` Anthony Sottile
2019-01-03 21:51           ` Junio C Hamano
2019-01-03 22:05             ` Anthony Sottile
2019-01-16 14:39               ` Ben Peart
2019-01-18 18:55 ` [PATCH v1 0/2] Fix regression in checkout -b Ben Peart
2019-01-18 18:55   ` [PATCH v1 1/2] checkout: add test to demonstrate regression with checkout -b on initial commit Ben Peart
2019-01-18 19:23     ` SZEDER Gábor
2019-01-18 18:55   ` [PATCH v1 2/2] checkout: fix regression in checkout -b on intitial checkout Ben Peart
2019-01-18 20:00     ` Junio C Hamano
2019-01-19  0:52     ` SZEDER Gábor
2019-01-19  1:26   ` [PATCH v1 0/2] Fix regression in checkout -b Junio C Hamano
2019-01-21 19:50   ` [PATCH v2 " Ben Peart
2019-01-21 19:50     ` [PATCH v2 1/2] checkout: add test to demonstrate regression with checkout -b on initial commit Ben Peart
2019-01-23 17:57       ` SZEDER Gábor
2019-01-21 19:50     ` [PATCH v2 2/2] checkout: fix regression in checkout -b on intitial checkout Ben Peart
2019-01-22 14:35       ` Johannes Schindelin
2019-01-22 18:42         ` Junio C Hamano
2019-01-22 18:49         ` Jeff King
2019-01-22 18:54     ` [PATCH v2 0/2] Fix regression in checkout -b Junio C Hamano
2019-01-22 19:31       ` Ben Peart [this message]
2019-01-23 19:14         ` Junio C Hamano
2019-01-23 20:01   ` [PATCH v3 " Ben Peart
2019-01-23 20:02     ` [PATCH v3 1/2] checkout: add test demonstrating regression with checkout -b on initial commit Ben Peart
2019-01-23 20:02     ` [PATCH v3 2/2] checkout: fix regression in checkout -b on intitial checkout Ben Peart
2019-01-23 21:23     ` [PATCH v3 0/2] Fix regression in checkout -b Junio C Hamano

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