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nix_alloc_primop: args: fix constness and use all arguments #16519

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nix_alloc_primop: args: fix constness and use all arguments #16519

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7243e84
nix_alloc_primop: args: fix constness and use all arguments
yorickvP Sep 25, 2026
74bf5ac
nix_alloc_primop: Check that argNames matches the specified arity
yorickvP Oct 2, 2026
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17 changes: 12 additions & 5 deletions src/libexpr-c/nix_api_value.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -105,27 +105,34 @@ PrimOp * nix_alloc_primop(
PrimOpFun fun,
int arity,
const char * name,
const char ** args,
const char * const * args,
const char * doc,
void * user_data)
{
if (context)
context->last_err_code = NIX_OK;
try {
std::vector<std::string> argNames;
if (args)
for (size_t i = 0; args[i]; i++)
argNames.emplace_back(args[i]);
// `RegisterPrimOp` raises the arity to `args.size()`, which would
// desync it from the arity bound into `nix_c_primop_wrapper`.
if (!argNames.empty() && argNames.size() != (size_t) arity)
throw nix::Error(
"primop '%s' has arity %d, but %d argument names were given", name, arity, argNames.size());

using namespace std::placeholders;
auto p = new
#if NIX_USE_BOEHMGC
(GC)
#endif
nix::PrimOp{
.name = name,
.args = {},
.args = std::move(argNames),
.arity = (size_t) arity,
.doc = doc ? std::optional<std::string>{doc} : std::nullopt,
.impl = std::bind(nix_c_primop_wrapper, fun, user_data, arity, _1, _2, _3, _4)};
if (args)
for (size_t i = 0; args[i]; i++)
p->args.emplace_back(*args);
nix_gc_incref(nullptr, p);
return (PrimOp *) p;
}
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5 changes: 3 additions & 2 deletions src/libexpr-c/nix_api_value.h
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Original file line number Diff line number Diff line change
Expand Up @@ -196,7 +196,8 @@ typedef void (*PrimOpFun)(
* @param[in] fun callback
* @param[in] arity expected number of function arguments
* @param[in] name function name
* @param[in] args array of argument names, NULL-terminated
* @param[in] args optional, NULL-terminated array of argument names, used for documentation.
* If non-empty, it must contain exactly `arity` names.
* @param[in] doc optional, documentation for this primop
* @param[in] user_data optional, arbitrary data, passed to the callback when it's called
* @return primop, or null in case of errors
Expand All @@ -207,7 +208,7 @@ PrimOp * nix_alloc_primop(
PrimOpFun fun,
int arity,
const char * name,
const char ** args,
const char * const * args,
const char * doc,
void * user_data);

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40 changes: 40 additions & 0 deletions src/libexpr-tests/nix_api_expr.cc
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Expand Up @@ -3,6 +3,7 @@
#include "nix_api_expr.h"
#include "nix_api_value.h"

#include "nix/expr/eval.hh"
#include "nix/expr/tests/nix_api_expr.hh"
#include "nix/util/tests/string_callback.hh"
#include "nix/util/file-system.hh"
Expand Down Expand Up @@ -350,6 +351,45 @@ TEST_F(nix_api_expr_test, nix_alloc_primop_without_doc)
assert_ctx_ok();
}

TEST_F(nix_api_expr_test, nix_alloc_primop_arg_names)
{
const char * const args[] = {"s", "n", nullptr};
PrimOp * primop = nix_alloc_primop(ctx, primop_square, 2, "named", args, nullptr, nullptr);
assert_ctx_ok();
ASSERT_NE(nullptr, primop);
EXPECT_THAT(((nix::PrimOp *) primop)->args, testing::ElementsAre("s", "n"));

nix_gc_decref(ctx, primop);
assert_ctx_ok();
}

TEST_F(nix_api_expr_test, nix_alloc_primop_empty_arg_names)
{
const char * const args[] = {nullptr};
PrimOp * primop = nix_alloc_primop(ctx, primop_square, 1, "unnamed", args, nullptr, nullptr);
assert_ctx_ok();
ASSERT_NE(nullptr, primop);
EXPECT_THAT(((nix::PrimOp *) primop)->args, testing::IsEmpty());

nix_gc_decref(ctx, primop);
assert_ctx_ok();
}

TEST_F(nix_api_expr_test, nix_alloc_primop_arg_names_arity_mismatch)
{
const char * const tooMany[] = {"a", "b", nullptr};
ASSERT_EQ(nullptr, nix_alloc_primop(ctx, primop_square, 1, "tooMany", tooMany, nullptr, nullptr));
ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("argument names were given"));
ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("tooMany"));

const char * const tooFew[] = {"a", nullptr};
ASSERT_EQ(nullptr, nix_alloc_primop(ctx, primop_square, 2, "tooFew", tooFew, nullptr, nullptr));
ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);

@xokdvium xokdvium Oct 2, 2026

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I never quite understood what use the error codes have an how they are converted (or rather how they design is supposed to work...). At least my understanding was that NIX_ERROR was supposed to represent evaluation errors (and this is more akin EINVAL which we don't have).

@xokdvium xokdvium Oct 2, 2026

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Not a blocker, because that part of the C API seems very undercooked in general. But I'm just wondering if I'm misunderstanding or if we really don't have any way to represent "invalid argument" errors.

ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("argument names were given"));
ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("tooFew"));
}

TEST_F(nix_api_expr_test, nix_expr_primop)
{
PrimOp * primop = nix_alloc_primop(
Expand Down
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