Re: [PATCH v2 1/2] read-cache: skip index SHA verification

[Jeff Hostetler <git@jeffhostetler.com>]

On 3/27/2017 6:44 PM, Jeff King wrote:
> On Mon, Mar 27, 2017 at 09:09:38PM +0000, git@jeffhostetler.com wrote:
>
>> From: Jeff Hostetler <jeffhost@microsoft.com>
>>
>> Teach git to skip verification of the index SHA in read_index().
>>
>> This is a performance optimization.  The index file SHA verification
>> can be considered an ancient relic from the early days of git and only
>> useful for detecting disk corruption.  For small repositories, this
>> SHA calculation is not that significant, but for gigantic repositories
>> this calculation adds significant time to every command.
>>
>> I added a global "skip_verify_index" variable to control this and
>> allow it to be tested.
>>
>> I did not create a config setting for this because of chicken-n-egg
>> problems with the loading the config and the index.
>
> Hrm, there shouldn't be any dependency of the config on the index (and
> there are a handful of options which impact the index already). Did you
> try it and run into problems?

Yeah, I tried adding a new "core.verifyindex" property and the
corresponding global variable.  But read_index() and verify_hdr()
was being called BEFORE the config was loaded.  And it wasn't clear
how best to solve that.

The issue was in "git status" where cmd_status() called
status_init_config() which called gitmodules_config() before
git_config().  but gitmodules_config() called read_index(),
so my settings weren't loaded yet in verify_hdr().

I tried switching the order in status_init_config(), but
that caused errors in t7508 with submodules not being handled
properly.

https://github.com/jeffhostetler/git/commits/upstream/core_verify_index

At this point I decided that it wasn't that important to have
this config setting, since we'll probably default it to be faster
and be done with it.

>
> In general, I'd much rather see us either:
>
>   1. Rip the code out entirely if it is not meant to be configurable,
>      and cannot be triggered by the actual git binary.
>
> or
>
>   2. Make it configurable, even if most people wouldn't use it. And then
>      have a test to exercise it using a git command (unlike the one-off
>      test helper, which isn't run at all).
>
> -Peff
>

I'm OK with (1) if everyone else is.

Jeff