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C API: Fix nix_c_primop_wrapper for strict initializers

Browse source 
https://github.com/NixOS/nix/pull/10555 added a check requiring
that output parameters always have an uninitialized Value as argument.

Unfortunately the output parameter of the primop callback received
a thunk instead.

See the comment for implementation considerations.
This commit is contained in:
Robert Hensing 2024-05-23 18:07:33 +02:00
parent 2c42e7b8d9
commit a942a34469
2 changed files with 56 additions and 3 deletions
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24
src/libexpr-c/nix_api_value.cc
View file

@ -73,10 +73,28 @@ static void nix_c_primop_wrapper(
PrimOpFun f, void * userdata, nix::EvalState & state, const nix::PosIdx pos, nix::Value ** args, nix::Value & v)
{
nix_c_context ctx;
f(userdata, &ctx, (EvalState *) &state, (Value **) args, (Value *) &v);
/* TODO: In the future, this should throw different errors depending on the error code */
if (ctx.last_err_code != NIX_OK)
// v currently has a thunk, but the C API initializers require an uninitialized value.
//
// We can't destroy the thunk, because that makes it impossible to retry,
// which is needed for tryEval and for evaluation drivers that evaluate more
// than one value (e.g. an attrset with two derivations, both of which
// reference v).
//
// Instead we create a temporary value, and then assign the result to v.
// This does not give the primop definition access to the thunk, but that's
// ok because we don't see a need for this yet (e.g. inspecting thunks,
// or maybe something to make blackholes work better; we don't know).
nix::Value vTmp;
f(userdata, &ctx, (EvalState *) &state, (Value **) args, (Value *) &vTmp);
if (ctx.last_err_code != NIX_OK) {
/* TODO: Throw different errors depending on the error code */
state.error<nix::EvalError>("Error from builtin function: %s", *ctx.last_err).atPos(pos).debugThrow();
}
v = vTmp;
}
PrimOp * nix_alloc_primop(

35
tests/unit/libexpr/nix_api_expr.cc
View file

@ -191,4 +191,39 @@ TEST_F(nix_api_expr_test, nix_expr_realise_context)
nix_realised_string_free(r);
}
const char * SAMPLE_USER_DATA = "whatever";
static void primop_square(void * user_data, nix_c_context * context, EvalState * state, Value ** args, Value * ret)
{
assert(context);
assert(state);
assert(user_data == SAMPLE_USER_DATA);
auto i = nix_get_int(context, args[0]);
nix_init_int(context, ret, i * i);
}
TEST_F(nix_api_expr_test, nix_expr_primop)
{
PrimOp * primop =
nix_alloc_primop(ctx, primop_square, 1, "square", nullptr, "square an integer", (void *) SAMPLE_USER_DATA);
assert_ctx_ok();
Value * primopValue = nix_alloc_value(ctx, state);
assert_ctx_ok();
nix_init_primop(ctx, primopValue, primop);
assert_ctx_ok();
Value * three = nix_alloc_value(ctx, state);
assert_ctx_ok();
nix_init_int(ctx, three, 3);
assert_ctx_ok();
Value * result = nix_alloc_value(ctx, state);
assert_ctx_ok();
nix_value_call(ctx, state, primopValue, three, result);
assert_ctx_ok();
auto r = nix_get_int(ctx, result);
ASSERT_EQ(9, r);
}
} // namespace nixC