git-fetch without a hash

[Stephen Paul Weber]

Hello Guix!

It seem that url-fetch will work without a hash (that is, with (sha256 #f)) 
but git-fetch will not.

As near as I can tell this is because git-fetch uses a fixed derivation 
build going via nix/build.cc stuff which contains this line:

        if (i.second.hash == "") fixedOutput = false;

And this results in /etc/resolv.conf not getting written and DNS resolution 
failing.

Now, I *think* by my reading that it is intended for *only* fixed-output 
derivations to have network access, because otherwise the operation would be 
impure?

And then url-fetch is just not actually ending up in a build container and 
so it works even when it isn't fixed-output?

However, there's no real reason that git-fetch *needs* to be fixed-output in 
terms of having a hash pre-defined, at least for local development and other 
purposes.  So is there a way around this?  If having a way around it is not 
desirable should url-fetch consider this an error as well?

Finally, *if* git-fetch should not allow this, the current error message is 
beyond confusing (DNS resolution fails during git fetch and it tries to fall 
back to SWH).  So should git-fetch check for hash of #f and raise at that 
point with a better error message in that case?

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