Local caching behavior affected by network state

[Leo Famulari]

I've noticed confusing behavior regarding the memoized cache, which
depends on the state of the network connection.

I have a built package in my store and I am connected to the internet:

$ ./pre-inst-env guix build gss
substitute: updating list of substitutes from 'https://example.com'... 100.0%
/gnu/store/ywh8cz5j63s8iwr2w2lzj3vyhqwqlhcz-gss-1.0.3

Then, I disable my network interface and run the command again:

$ ./pre-inst-env guix build gss
@ substituter-started 
/gnu/store/y9akh452n3p4w2v631nj0injx7y0d68x-mpc-1.0.3.tar.gz 
/gnu/store/6z7d9fpaph1yl1csfnnkrdv6jzr4simn-guix-0.13.0-2.de9d8f0/libexec/guix/substitute
Downloading 
https://mirror.hydra.gnu.org/guix/nar/y9akh452n3p4w2v631nj0injx7y0d68x-mpc-1.0.3.tar.gz...
guix substitute: error: host name lookup error: Temporary failure in name 
resolution
@ substituter-failed 
/gnu/store/y9akh452n3p4w2v631nj0injx7y0d68x-mpc-1.0.3.tar.gz 256 fetching path 
`/gnu/store/y9akh452n3p4w2v631nj0injx7y0d68x-mpc-1.0.3.tar.gz' failed with exit 
code 1
@ build-started 
/gnu/store/y4r20fnm9ba129r2ry6s2k1ss975dknd-gcc-5.4.0.tar.xz.drv - x86_64-linux 
/var/log/guix/drvs/y4//r20fnm9ba129r2ry6s2k1ss975dknd-gcc-5.4.0.tar.xz.drv.bz2
guix build: error: build failed: some substitutes for the outputs of derivation 
`/gnu/store/ikdvc8qn0bi57hn3ci72l5xlqyss96ys-mpc-1.0.3.tar.gz.drv' failed 
(usually happens due to networking issues); try `--fallback' to build 
derivation from source
$ ./pre-inst-env guix build gss --fallback
/gnu/store/ywh8cz5j63s8iwr2w2lzj3vyhqwqlhcz-gss-1.0.3

So, when I am offline, I have to use --fallback for the local cache to
work.

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