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Catch caching of transient flake evaluation error #12717

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Catch caching of transient flake evaluation error #12717

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20 changes: 20 additions & 0 deletions src/libexpr/primops.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -4653,6 +4653,16 @@ static RegisterPrimOp primop_splitVersion({
.fun = prim_splitVersion,
});

static void prim_testThrowErrorMaybe(EvalState & state, const PosIdx pos, Value * * args, Value & v)
{
if (getEnv("_NIX_TEST_DO_THROW_ERROR") == "1") {
state.error<EvalError>("this is a dummy error").atPos(pos).debugThrow();
} else {
state.forceValue(*args[0], pos);
v = *args[0];
}
}


/*************************************************************
* Primop registration
Expand Down Expand Up @@ -4880,6 +4890,16 @@ void EvalState::createBaseEnv()
.fun = settings.traceVerbose ? prim_trace : prim_second,
});

if (getEnv("_NIX_TEST_PRIMOPS") == "1") {
addPrimOp({
.name = "__testThrowErrorMaybe",
.args = { "e" },
.arity = 1,
.doc = "throw dummy error if _NIX_TEST_DO_THROW_ERROR == 1, return arg otherwise",
.fun = prim_testThrowErrorMaybe
});
}

/* Add a value containing the current Nix expression search path. */
auto list = buildList(lookupPath.elements.size());
for (const auto & [n, i] : enumerate(lookupPath.elements)) {
Expand Down
34 changes: 34 additions & 0 deletions tests/functional/flakes/eval-cache.sh
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Expand Up @@ -28,6 +28,8 @@ cat >"$flake1Dir/flake.nix" <<EOF
in
assert (f 100); self.drv;
ifd = assert (import self.drv); self.drv;
artificialTwoDifferentThrows = builtins.testThrowErrorMaybe (throw "real throw");
artificialThrowOrDrv = builtins.testThrowErrorMaybe self.drv;
};
}
EOF
Expand All @@ -48,3 +50,35 @@ nix build --no-link "$flake1Dir#stack-depth"
expect 1 nix build "$flake1Dir#ifd" --option allow-import-from-derivation false 2>&1 \
| grepQuiet 'error: cannot build .* during evaluation because the option '\''allow-import-from-derivation'\'' is disabled'
nix build --no-link "$flake1Dir#ifd"

# make sure this builtin does not reach production
nix eval --expr 'assert ! (builtins ? testThrowErrorMaybe); null'

# and that it does reach the testing environment
export _NIX_TEST_PRIMOPS=1
nix eval --expr 'assert (builtins.testThrowErrorMaybe true); null'

# and that it works
_NIX_TEST_DO_THROW_ERROR=1 expect 1 nix eval --expr 'builtins.testThrowErrorMaybe true' 2>&1 \
| grepQuiet 'this is a dummy error'


# make sure error details do not get cached
_NIX_TEST_DO_THROW_ERROR=1 expect 1 nix eval "$flake1Dir#artificialTwoDifferentThrows" 2>&1 \
| grepQuiet "this is a dummy error"
expect 1 nix eval "$flake1Dir#artificialTwoDifferentThrows" 2>&1 \
| grepQuiet "real throw"

# If a cached error cannot be reproduced, do not continue as usual.
# Instead, throw an error.
#
# To be clear, the following test case should never occur in production.
# But it might, due to an implementation error in Nix or plugins.

# create an artificial exception cache entry
_NIX_TEST_DO_THROW_ERROR=1 expect 1 nix build "$flake1Dir#artificialThrowOrDrv" 2>&1 \
| grepQuiet "this is a dummy error"

# Invoke again but without the artificial throwing of an exception
expect 1 nix build "$flake1Dir#artificialThrowOrDrv" 2>&1 \
| grepQuiet "unexpected evaluation success despite the existence of a cached error. This should not happen. It is a bug in the implementation of Nix or a plugin. A transient exception should not have been cached."
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