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eval: add NIX_VALIDATE_EVAL_NONDETERMINISM know for attrset comparison #8711

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eval: add NIX_VALIDATE_EVAL_NONDETERMINISM know for attrset comparison #8711

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21 changes: 18 additions & 3 deletions src/libexpr/eval.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -557,6 +557,8 @@ EvalState::EvalState(
{
countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";

validateNondeterminism = getEnv("NIX_VALIDATE_EVAL_NONDETERMINISM").value_or("0") != "0";

assert(gcInitialised);

static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
Expand Down Expand Up @@ -2464,11 +2466,24 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v

/* Otherwise, compare the attributes one by one. */
Bindings::iterator i, j;
for (i = v1.attrs->begin(), j = v2.attrs->begin(); i != v1.attrs->end(); ++i, ++j)
bool result = true;
for (i = v1.attrs->begin(), j = v2.attrs->begin(); i != v1.attrs->end(); ++i, ++j) {
if (i->name != j->name || !eqValues(*i->value, *j->value, pos, errorCtx))
return false;
result = false;
/* Do not short-cut comparison to catch attrset
* non-determinism caused by thrown exceptions
* when attribute sets are compared. Example:
* > { a = 1; b = assert false; 1; } == { a = 2; b = 1; }
* false
* > { b = 1; a = assert false; 1; } == { b = 2; a = 1; }
* error: assertion 'false' failed
*
* Exhaustive check should catch such non-determinism.
*/
if (!result && !validateNondeterminism) return result;
}

return true;
return result;
}

/* Functions are incomparable. */
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2 changes: 2 additions & 0 deletions src/libexpr/eval.hh
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Expand Up @@ -727,6 +727,8 @@ private:

bool countCalls;

bool validateNondeterminism;

typedef std::map<std::string, size_t> PrimOpCalls;
PrimOpCalls primOpCalls;

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4 changes: 4 additions & 0 deletions tests/misc.sh
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Expand Up @@ -30,3 +30,7 @@ expectStderr 1 nix-instantiate --eval -E '[]' -A 'x' | grepQuiet "should be a se
expectStderr 1 nix-instantiate --eval -E '{}' -A '1' | grepQuiet "should be a list"
expectStderr 1 nix-instantiate --eval -E '{}' -A '.' | grepQuiet "empty attribute name"
expectStderr 1 nix-instantiate --eval -E '[]' -A '1' | grepQuiet "out of range"

# Validate deterministic attrset comparison:
NIX_VALIDATE_EVAL_NONDETERMINISM=1 expectStderr 1 nix-instantiate --eval -E '{ a = 1; b = assert false; 1; } == { a = 2; b = 1; }' | grepQuiet "assertion 'false' failed"
NIX_VALIDATE_EVAL_NONDETERMINISM=1 expectStderr 1 nix-instantiate --eval -E '{ b = 1; a = assert false; 1; } == { b = 2; a = 1; }' | grepQuiet "assertion 'false' failed"