Hello! I've noticed that when offloading, --fallback sometimes fails to actually fall back. Here is a shell transcript: --8<---------------cut here---------------start------------->8--- $ ./pre-inst-env guix build --substitute-urls="https://mirror.hydra.gnu.org" --fallback font-arabic-misc @ substituter-started /gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz /gnu/store/19wpps1nmlg4j1530knl7kdhvgcmj5fc-guix-0.13.0-8.357ab93/libexec/guix/substitute Downloading https://mirror.hydra.gnu.org/guix/nar/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz... guix substitute: error: download from 'https://mirror.hydra.gnu.org/guix/nar/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz' failed: 404, "Not Found" @ substituter-failed /gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz 256 fetching path `/gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz' failed with exit code 1 process 8395 acquired build slot '/var/guix/offload/192.168.2.11/0' process 8395 acquired build slot '/var/guix/offload/192.168.2.5/0' load on machine '192.168.2.11' is 0.08 (normalized: 0.02) @ build-started /gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv - x86_64-linux /var/log/guix/drvs/rq//gfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv.bz2 sending 0 store items to '192.168.2.11'... offloading '/gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv' to '192.168.2.11'... @ build-remote /gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv 192.168.2.11 @ substituter-started /gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz /gnu/store/19wpps1nmlg4j1530knl7kdhvgcmj5fc-guix-0.13.0-8.357ab93/libexec/guix/substitute Downloading https://mirror.hydra.gnu.org/guix/nar/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz... guix substitute: error: download from 'https://mirror.hydra.gnu.org/guix/nar/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz' failed: 404, "Not Found" @ substituter-failed /gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz 256 fetching path `/gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz' failed with exit code 1 derivation '/gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv' offloaded to '192.168.2.11' failed: some substitutes for the outputs of derivation `/gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv' failed (usually happens due to networking issues); try `--fallback' to build derivation from source @ build-failed /gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv - 1 builder for `/gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv' failed with exit code 100 cannot build derivation `/gnu/store/gna22xl7ycq4rzaj487axa5ijbyhqq1q-font-arabic-misc-1.0.3.drv': 1 dependencies couldn't be built guix build: error: build failed: build of `/gnu/store/gna22xl7ycq4rzaj487axa5ijbyhqq1q-font-arabic-misc-1.0.3.drv' failed --8<---------------cut here---------------end--------------->8--- Passing --no-build-hook makes it succeed: --8<---------------cut here---------------start------------->8--- $ ./pre-inst-env guix build --substitute-urls="https://mirror.hydra.gnu.org" --fallback --no-build-hook font-arabic-misc @ substituter-started /gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz /gnu/store/19wpps1nmlg4j1530knl7kdhvgcmj5fc-guix-0.13.0-8.357ab93/libexec/guix/substitute Downloading https://mirror.hydra.gnu.org/guix/nar/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz... guix substitute: error: download from 'https://mirror.hydra.gnu.org/guix/nar/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz' failed: 404, "Not Found" @ substituter-failed /gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz 256 fetching path `/gnu/store/26nk7rgc2rhi7v0d8xhqqxglpg532syh-font-arabic-misc-1.0.3.tar.xz' failed with exit code 1 @ build-started /gnu/store/rqgfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv - x86_64-linux /var/log/guix/drvs/rq//gfr39aamfzqq93fl87ai2y61b5s992-font-arabic-misc-1.0.3.tar.xz.drv.bz2 font-arabic-misc-1.0.3/ font-arabic-misc-1.0.3/arabic24.bdf font-arabic-misc-1.0.3/aclocal.m4 [...] --8<---------------cut here---------------end--------------->8--- Interestingly no download was done on the second try, so apparently the source was already in my store. It looks like the offload hook starts before the actual download (possibly because a substitute was advertised), and then fails when the substitute 404s.