bug#22423: git-fetch does not update checked out tree when commit hash changes

[ludo@gnu.org (Ludovic Courtès)]

Pjotr Prins <pjotr.public12@thebird.nl> skribis:

> On Thu, Jan 21, 2016 at 09:50:18AM +0100, Ludovic Courtès wrote:
>> This is expected: origins are fixed-output derivations, meaning that it
>> does not matter how we perform them (using Git, over HTTP, or thanks to
>> an avian carrier), as long as the result has the specified sha256.
>> 
>> Thus, when you change, say, the Git commit ID or origin ‘method’ without
>> changing the ‘sha256’ field, nothing happens: the daemon says “OK, I
>> already have a store item with that ‘sha256’, so I don’t do anything.”
>> 
>> Clearly, one has to be cautious with this, it’s easy to mistakenly use
>> the old source.
>
> Hmmm. I thought the sha256 was calculated over the derivation +
> sources

What you’re saying is true of the hash that appears in /gnu/store file
name, but I was referring to the ‘sha256’ field of origins, which is a
different thing.

Ludo’.