On Sun, Nov 18, 2018 at 03:04:33PM +0100, Björn Höfling wrote:
> I updated a package-definition and forgot to update the Checksum. When
> then updating, it just fails, and the new, nice, logging-reduced UI
> doesn't tell:

Yes, this is a classic "gotcha" of Guix package development.

In Guix, those "things" for which you provide a hash are called
"fixed-output derivations":

https://www.gnu.org/software/guix/manual/en/html_node/Derivations.html

Unlike regular derivations, we know in advance what the output of the
derivation will be. Therefore, it does not matter to us how it is built
(from source, downloaded over HTTP, downloaded with Git, found in
/gnu/store, etc).

When building fixed-output derivations, Guix first looks in /gnu/store
to see if they are already built. If so, it just uses what it finds
there.

So, if you give the wrong hash, as you did in your example, Guix will
use that wrong source code.

This failure mode is often discovered by people who used `guix download`
to calculate a source hash, but then put the wrong URI in their package
definition. Because the source is already in /gnu/store, the URI is not
tested and their package doesn't work for anybody else.

It may also happen when the source is only found on the
content-addressed mirrors provided by Nix; no file-name checking is
performed in that case.

> $ guix package -u guix-tools
> substitute: updating list of substitutes from 'https://berlin.guixsd.org'...   0
> [..]
> building /gnu/store/vy3s1y7bv1w6d8gmp5b10xppy9skbgkd-guix-tools-0.1.0-2.4f17b79-checkout.drv...
> build of /gnu/store/vy3s1y7bv1w6d8gmp5b10xppy9skbgkd-guix-tools-0.1.0-2.4f17b79-checkout.drv failed
> View build log at '/var/log/guix/drvs/vy/3s1y7bv1w6d8gmp5b10xppy9skbgkd-guix-tools-0.1.0-2.4f17b79-checkout.drv.bz2'.
> -guix package: error: build failed: build of `/gnu/store/iwma3gq778n32mqz6y7277g67nvx1abb-guix-tools-0.1.0-2.4f17b79.drv' failed

[...]

> Only when I explicitly built it, the error was shown:
> 
> $ guix build guix-tools
> 
> HEAD is now at 4f17b79 gdev: Add more package to environment.
> environment variable `PATH' unset
> output path `/gnu/store/s399g9f1k19v01rs992w5dl6aif87har-guix-tools-0.1.0-2.4f17b79-checkout' should have r:sha256 hash `1j4q43hk7jrys7zsmws37g6w7babzshfdb1s5myl7qwr3mcx6hnf', instead has `0b61q29915b4i6adidx3zixnx3m0zp58rjbfa9byqcz13szznc52'
> build of /gnu/store/vy3s1y7bv1w6d8gmp5b10xppy9skbgkd-guix-tools-0.1.0-2.4f17b79-checkout.drv failed
> View build log at '/var/log/guix/drvs/vy/3s1y7bv1w6d8gmp5b10xppy9skbgkd-guix-tools-0.1.0-2.4f17b79-checkout.drv.bz2'.
> cannot build derivation `/gnu/store/iwma3gq778n32mqz6y7277g67nvx1abb-guix-tools-0.1.0-2.4f17b79.drv': 1 dependencies couldn't be built
> guix build: error: build failed: build of `/gnu/store/iwma3gq778n32mqz6y7277g67nvx1abb-guix-tools-0.1.0-2.4f17b79.drv' failed
> 
> Is it intended that this build-failure detail is not shown? I suppose
> not. At least in a log it should be shown.

I agree that `guix package` should print the relevant error.