fix: good errors for failures caused by allowSubstitutes

This caused an absolute saga which I would not like anyone else to have
to experience. Let's put in a laser targeted error message that
diagnoses this exact problem.

Fixes: https://git.lix.systems/lix-project/lix/issues/484
Change-Id: I2a79f04aeb4a1b67c10115e5e39501d958836298

2 files changed, 42 insertions, 0 deletions

diff --git a/tests/functional/meson.build b/tests/functional/meson.build
index fb8d77a57..bb706ce9e 100644
--- a/tests/functional/meson.build
+++ b/tests/functional/meson.build
@@ -189,6 +189,7 @@ functional_tests_scripts = [
   'test-libstoreconsumer.sh',
   'extra-sandbox-profile.sh',
   'substitute-truncated-nar.sh',
+  'regression-484.sh',
 ]
 
 # Plugin tests require shared libraries support.
diff --git a/tests/functional/regression-484.sh b/tests/functional/regression-484.sh
new file mode 100644
index 000000000..1aa039ac2
--- /dev/null
+++ b/tests/functional/regression-484.sh
@@ -0,0 +1,41 @@
+source common.sh
+
+# Ensures that nix build will deliver a usable error message when it encounters
+# a build failure potentially caused by allowSubstitutes.
+
+clearStore
+
+cd $TEST_HOME
+
+putDrv() {
+    cat > "$1" <<EOF
+builtins.derivation {
+  name = "meow";
+  builder = "/bin/sh";
+  args = [];
+  system = "unicornsandrainbows-linux";
+  allowSubstitutes = ${2};
+  preferLocalBuild = true;
+}
+EOF
+}
+
+putDrv drv-disallow-substitutes.nix false
+putDrv drv-allow-substitutes.nix true
+
+expect 1 nix build --substitute --substituters '' --offline -f drv-disallow-substitutes.nix 2> output.txt
+
+# error: a 'unicornsandrainbows-linux' with features {} is required to build '$TMPDIR/regression-484/store/...-meow.drv', but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}
+#
+#        Hint: the failing derivation has allowSubstitutes set to false, forcing it to be built rather than substituted.
+#        Passing --always-allow-substitutes to force substitution may resolve this failure if the path is available in a substituter.
+cat output.txt
+grepQuiet unicornsandrainbows-linux output.txt
+grepQuiet always-allow-substitutes output.txt
+grepQuiet allowSubstitutes output.txt
+
+expect 1 nix build --substitute --substituters '' --offline -f drv-allow-substitutes.nix 2> output.txt
+
+cat output.txt
+grepQuiet unicornsandrainbows-linux output.txt
+grepQuiet -v allowSubstitutes output.txt