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Give each DEF call its own environment, so recursion returns

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The function's environment was owned by the funcdef and re-initialised on every
call, which made a function not re-entrant and cost two silent defects:

    DEF DBL(N#) = N# * 2
    PRINT DBL(10) + DBL(1)      was 4, should be 22

The result was a pointer into the funcdef's own environment, so the second call
overwrote the first before the operator saw it -- both operands became the last
call's answer. Two *different* functions in one expression were fine, which is
most of why it was invisible.

    DEF FACT(N#)
    IF N# <= 1 THEN RETURN 1
    RETURN N# * FACT(N# - 1)
    PRINT FACT(5)               never returned

The recursive call re-initialised the environment the outer call was still
using, so the loop waiting for control to come back could not see it. No error,
no bound, no diagnostic -- the one place in this interpreter that looped forever
rather than raising.

A call takes an environment from the pool now, exactly as GOSUB does. The result
is copied into a caller-scope scratch before that environment goes back, because
handing back a pointer into the callee is what made two calls collide and would
now be a pointer into a released slot as well. RETURN parks its result on the
*parent* rather than on the environment it is about to release, so nothing reads
a freed slot to find it.

Recursion depth answers to AKBASIC_MAX_ENVIRONMENTS like every other nesting, so
too deep is "Environment pool exhausted" -- a diagnosis where there was none.

akbasic_FunctionDef.environment goes with it, as dead state.

One thing this exposed but did not cause, measured against a stashed build and
recorded rather than fixed: a statement containing a failed multi-line DEF call
still completes and prints a junk value. It is visible more often now only
because runaway recursion reaches it where it used to hang.
tests/language/functions/recursion.bas deliberately does not pin that answer.

Chapter 16 loses its "walk a list with a loop, not a recursive DEF" caveat and
gains the one that is still true: a function cannot take a structure parameter
yet, so it reaches a record by name.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Andrew Kesterson
2026-08-01 12:29:35 -04:00
parent 6bac929901
commit 00daa17a47
10 changed files with 332 additions and 31 deletions
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176
tests/user_functions.c Normal file
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/**
* @file user_functions.c
* @brief Tests that a `DEF` call is re-entrant.
*
* Both of the defects here came from one cause: the function's environment used
* to be owned by the funcdef and reset on every call, so a second call trampled
* the first. A call takes one from the pool now, exactly as `GOSUB` does.
*
* Neither failure was visible in an ordinary program. The aliasing one gave a
* wrong number rather than an error, and *only* when the same function appeared
* twice in one expression -- two different functions were fine, which is what
* made it hard to see at all. The recursion one produced no output whatsoever.
* So both are pinned here against the language rather than left to a listing
* somebody might not write.
*/
#include <string.h>
#include <akbasic/error.h>
#include <akbasic/runtime.h>
#include "harness.h"
#include "testutil.h"
/** @brief Run a program to completion, bounded so a hang fails rather than waits. */
static akerr_ErrorContext AKERR_NOIGNORE *run_program(const char *source)
{
PREPARE_ERROR(errctx);
PASS(errctx, harness_start(NULL));
PASS(errctx, akbasic_runtime_load(&HARNESS_RUNTIME, source));
PASS(errctx, akbasic_runtime_start(&HARNESS_RUNTIME, AKBASIC_MODE_RUN));
PASS(errctx, akbasic_runtime_run(&HARNESS_RUNTIME, 20000));
SUCCEED_RETURN(errctx);
}
/**
* @brief Two calls to one function in one expression do not share a slot.
*
* The result used to be a pointer into the funcdef's own environment, so the
* second call overwrote the first before the operator saw it: `DBL(10) + DBL(1)`
* came out as 4 rather than 22, both operands having become the last call's
* answer. A silent wrong number from a two-line program.
*/
static void test_two_calls_in_one_expression(void)
{
TEST_REQUIRE_OK(run_program("10 DEF DBL(N#) = N# * 2\n"
"20 DEF TPL(N#) = N# * 3\n"
"30 PRINT DBL(10) + DBL(1)\n"
"40 PRINT DBL(1) + DBL(10) + DBL(100)\n"));
TEST_REQUIRE_STR(HARNESS_OUTPUT, "22\n222\n");
harness_stop();
/*
* Two *different* functions were always correct, because each funcdef owned
* its own environment. Asserted so a future fix that reintroduces sharing
* cannot pass by getting this case right.
*/
TEST_REQUIRE_OK(run_program("10 DEF DBL(N#) = N# * 2\n"
"20 DEF TPL(N#) = N# * 3\n"
"30 PRINT DBL(1) + TPL(10)\n"));
TEST_REQUIRE_STR(HARNESS_OUTPUT, "32\n");
harness_stop();
}
/**
* @brief A recursive multi-line `DEF` returns.
*
* It used to hang: the recursive call re-initialised the environment the outer
* call was still using, so the loop waiting for control to come back never saw
* it. No error, no bound, no diagnostic -- the one place in this interpreter
* that looped forever rather than raising.
*/
static void test_recursion_returns(void)
{
TEST_REQUIRE_OK(run_program("10 DEF FACT(N#)\n"
"20 IF N# <= 1 THEN RETURN 1\n"
"30 RETURN N# * FACT(N# - 1)\n"
"40 PRINT FACT(5)\n"));
TEST_REQUIRE_STR(HARNESS_OUTPUT, "120\n");
harness_stop();
}
/**
* @brief Each call gets its own arguments, which is what recursion needs.
*
* Distinct from the test above: a function could return the right answer for
* `FACT` by luck if the argument happened to be re-read before being clobbered.
* This one unwinds and uses the argument *after* the inner call has returned, so
* a shared parameter slot gives the wrong answer rather than no answer.
*/
static void test_arguments_are_per_call(void)
{
TEST_REQUIRE_OK(run_program("10 DEF SUMTO(N#)\n"
"20 IF N# <= 0 THEN RETURN 0\n"
"30 RETURN N# + SUMTO(N# - 1)\n"
"40 PRINT SUMTO(4)\n"));
/* 4+3+2+1 = 10, and only if each frame kept its own N#. */
TEST_REQUIRE_STR(HARNESS_OUTPUT, "10\n");
harness_stop();
}
/**
* @brief Runaway recursion is reported, not hung.
*
* Depth now answers to the environment pool like every other nesting, so too
* deep is the same diagnosis `GOSUB` already gave. That is the whole change in
* one sentence: a bound where there was none.
*/
static void test_runaway_recursion_is_diagnosed(void)
{
TEST_REQUIRE_OK(run_program("10 DEF INF(N#)\n"
"20 RETURN INF(N# + 1)\n"
"30 PRINT INF(1)\n"));
TEST_REQUIRE(strstr(HARNESS_OUTPUT, "Environment pool exhausted") != NULL,
"unbounded recursion should report the pool bound, got \"%s\"", HARNESS_OUTPUT);
harness_stop();
}
/** @brief A single-expression function still works, and still nests. */
static void test_single_expression_form(void)
{
TEST_REQUIRE_OK(run_program("10 DEF SQ(N#) = N# * N#\n"
"20 PRINT SQ(4)\n"
"30 PRINT SQ(SQ(2))\n"));
TEST_REQUIRE_STR(HARNESS_OUTPUT, "16\n16\n");
harness_stop();
}
/**
* @brief A function reaches a structure through the caller's scope.
*
* **Not through a parameter**: `DEF F(B@ AS CRATE)` is not implemented, and a
* bare `DEF F(B@)` is refused because `@` alone does not say *which* type. So
* the way a function works with a record today is by name, which the dynamic
* scoping already allows -- a call's environment has the caller's as its parent.
*
* Pinned rather than left implicit, because it is the only route there is and a
* reader would otherwise reasonably assume the parameter form works.
*/
static void test_structures_reached_through_scope(void)
{
TEST_REQUIRE_OK(run_program("10 TYPE CRATE\n"
"20 W#\n"
"30 END TYPE\n"
"40 DIM A@ AS CRATE\n"
"50 A@.W# = 5\n"
"60 DEF SCALED(N#) = A@.W# * N#\n"
"70 PRINT SCALED(3)\n"));
TEST_REQUIRE_STR(HARNESS_OUTPUT, "15\n");
harness_stop();
/* And a structure parameter is refused rather than silently misbehaving. */
TEST_REQUIRE_OK(run_program("10 TYPE CRATE\n"
"20 W#\n"
"30 END TYPE\n"
"40 DEF WID(B@) = B@.W#\n"
"50 PRINT 1\n"));
TEST_REQUIRE(strstr(HARNESS_OUTPUT, "ERROR") != NULL,
"a structure parameter should be refused for now, got \"%s\"", HARNESS_OUTPUT);
harness_stop();
}
int main(void)
{
TEST_REQUIRE_OK(akbasic_error_register());
test_two_calls_in_one_expression();
test_recursion_returns();
test_arguments_are_per_call();
test_runaway_recursion_is_diagnosed();
test_single_expression_form();
test_structures_reached_through_scope();
return akbasic_test_failures;
}