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The per-CPU {$ifdef} chains in builtinheap.pas fail open

The shape

Every per-target syscall wrapper in this file is written as a run of {$ifdef CPU_x} blocks with no terminal {$else}. Adding a target means adding an arm; forgetting to means the routine silently does nothing, or worse. There is no place for the compiler to say "this target has no arm."

Four instances in this one file:

routine arms pre-chain default what an armless target gets
PXXSysWrite (1537) 5 + wasm32 Result := 0 "wrote nothing, no error" — fixed, see bug-a-pxxsyswrite-has-no-wasm32-arm
HeapMmap (681) 5 Result := 0 a heap that bumps from address 0 — filed, bug-a-heapmmap-has-no-wasm32-arm-so-the-heap-starts-at-address-zero
PXXSysOpenRO (1861) 4 none Result never assigned
PXXSysLseek (1877) 4 none Result never assigned

The first two at least return a defined wrong value. The last two do not.

The two that are worse, and who they hit

function PXXSysOpenRO(path: Pointer): Int64;
begin
{$ifdef CPUX86_64}  Result := __pxxrawsyscall(2,  Int64(path), 0, 0);    {$endif}
{$ifdef CPU_I386}   Result := __pxxrawsyscall(5,  Int64(path), 0, 0);    {$endif}
{$ifdef CPU_ARM32}  Result := __pxxrawsyscall(5,  Int64(path), 0, 0);    {$endif}
{$ifdef CPUAARCH64} Result := __pxxrawsyscall(56, -100, Int64(path), 0, 0); {$endif}
end;

PXXSysLseek is the same four arms. Both sit inside {$ifndef PXX_ESP} — ESP is excluded deliberately and says so in the header comment; nothing excludes riscv32 or wasm32. Meanwhile PXXSysRead and PXXSysWrite, twenty lines earlier and in the same group, do carry CPU_RISCV32 arms (63 / 64, "hosted linux (qemu-user)"). So the drift is visible in the file: riscv32 was added to read and write and not to open and lseek.

The caller makes the consequence concrete (PXXStrLoadFile, 1912):

  fd := PXXSysOpenRO(path);
  if fd < 0 then Exit;                     { garbage may well be >= 0 }
  size := PXXSysLseek(fd, 0, 2);           { garbage }
  ...
  base := Int64(PXXAlloc(size + PXX_HDR_SIZE + 1, 8));   { garbage-sized alloc }

So on hosted riscv32, LoadFile on a target with no open does not fail — it proceeds on an uninitialised fd and allocates an arbitrary amount of memory.

The fix that is NOT four more arms

Adding CPU_RISCV32 to both closes today's instance and leaves the generator running: the next target repeats it, exactly as wasm32 just did twice.

Give every chain a terminal {$else} that fails loud:

{$ifdef CPUAARCH64} ... {$endif}
{$if not (defined(CPUX86_64) or defined(CPU_I386) or defined(CPU_ARM32)
          or defined(CPUAARCH64) or defined(CPU_RISCV32))}
  {$error PXXSysOpenRO has no arm for this target}
{$endif}

A compile-time refusal is the right failure here for the same reason the wasm32 backend records an unlowered body rather than skipping it: a target that cannot do something must say so, not return a plausible value. If a target genuinely should be a no-op (ESP has no filesystem), that is an explicit arm saying Result := -1, not an absence.

Whether {$error} / {$if defined(...)} are available in this dialect is the first thing to check; if not, an explicit {$else} Result := -1; {$endif} per chain is the fallback, and at least the value is defined and negative.

PROVENANCE NOTE — frank-coordinator, 2026-08-29. Everything from here down was written on 2026-08-28 (f7bf2dfa3, 04b745294) and never reached this ticket. Both appends were addressed to backlog/<slug>.md while the ticket lived in backlog_new/; >> created an empty-headed file at the wrong path instead of failing, so the ranker offered TWO entries for one slug — this one, complete but missing all of the analysis, and an orphan fragment with no frontmatter carrying all of it. Reunited here. Nothing below is edited; the wrong table and its correction are both kept, in that order, because the wrong one is what a reader may already have seen.

The proposed fix has an in-repo POSITIVE CONTROL — measured 2026-08-28

Do not argue this fix from first principles. The same codebase already contains the same missing-arm shape with the opposite failure mode, and the difference is one else.

EmitZeroFrameSlot (compiler/ir_codegen.inc) dispatches per target and ends:

  else
    Error('compiler error: EmitZeroFrameSlot: unhandled target');

It has no wasm32 arm either. The observed result is compiler error: EmitZeroFrameSlot: unhandled target on test_dynarray_insert_delete.pas — a clean refusal naming the routine and the target, filed as bug-a-emitzeroframeslot-has-no-wasm32-arm at p55.

The chains in this ticket have no terminal else, so the same omission produces PXXStrLoadFile allocating on an uninitialised size — p70, and it took a deliberate audit to find.

missing arm, terminal else missing arm, no terminal else
example EmitZeroFrameSlot PXXSysOpenRO, PXXSysLseek
result compiler error naming routine + target uninitialised return, arbitrary allocation
found by the first program that hit it a deliberate audit, weeks later
prio it earned 55 70

The 15-point gap between them IS the value of the terminal else, priced by this repo's own triage rather than asserted. That is the whole argument for this ticket, and it is empirical.

CORRECTION 2026-08-28 — the positive control above is WRONG in two ways

The table above was written by frank-coordinator and is corrected here rather than edited away, because the wrong version is the one someone may already have read.

Error 1 — wrong file. EmitZeroFrameSlot is defined at compiler/symtab.inc:10074, not in ir_codegen.inc. The original grep ranged over compiler/*.inc and the file was never checked.

Error 2 — and it inverts the point. EmitZeroFrameSlot has TWO per-target chains, one per size class, and the table described only one:

chain targets named terminal arm behaviour
wide (nBytes > TARGET_PTR_SIZE) i386, arm32, aarch64, riscv32, xtensa Error(...) fails LOUD — what the table described
narrow (nBytes <= TARGET_PTR_SIZE) i386, arm32, aarch64, xtensa, riscv32 bare else that IS the x86-64 implementation falls OPEN

The narrow chain is the one every managed scalar goes through. So the routine offered as this family's counter-example is itself a member of it.

The corrected argument is STRONGER, which is why it is worth restating rather than deleting

The 55-vs-70 contrast still holds, but not for the reason given. Both behaviours live in one routine, forty lines apart — so this is no longer a comparison across two files with other differences, it is a controlled one:

A reader sees the loud arm. Every program runs the silent one.

That is the whole cost of a missing terminal else, demonstrated inside a single routine with everything else held constant.

The general rule this yields

A dispatch chain whose last arm is a REAL TARGET rather than an error is a fall-open chain wearing the shape of an exhaustive one.

Six named arms and an unnamed seventh reads as "the default" when it is in fact x86-64 — the bytes are mov qword [rbp+off], 0. That is why reading missed it and a probe found it, and it is the third instance of refactor-a-target-dispatch-chains-fail-open's general case.

Severity, measured rather than assumed (frankwasm, ed1d37a8b): a probe build emitting nothing there produces byte-identical .wasm for three slices — Code[] is unread on that target, so nothing wrong has ever come out of it. Latent, not active. Prio stays 55, for the opposite reason to the one recorded.

And the guarantee whose own header says it has ONE owner has three mechanisms on wasm32 — the backend's prologue pass, the x86-64 fall-through, and the loud Error. root-cause-over-microfix.md calls three mechanisms for one concept a design flaw, so the fix here is plausibly deleting an arm rather than adding one.

Log

Resolved 2026-08-29 — fixed as the shape, with three corrections to this ticket

All five chains in the file (PXXSysOpenRO, PXXSysLseek, PXXSysClose, PXXSysRead, PXXSysWrite) converted from runs of separate {$ifdef} blocks into single {$if defined}/{$elseif}/{$else} chains with a terminal arm. Closes this and bug-a-three-pxxsys-primitives-return-a-plausible-fd-on-wasm32 together — one defect that was counted twice.

Correction 1 — {$error} does not exist in this dialect, so the fix this ticket proposes cannot be written. The directive dispatcher in lexer.inc handles ifdef/ifndef/if/ifopt/elseif/else/endif/define/undef and the strict-* flags; there is no error/fatal/message. The ticket flagged this as "the first thing to check" and it was, and the answer was no. {$if defined(X)}, {$elseif}, {$else} and not/and/or all work, which is what made an exhaustive chain possible at all. The absent directive is the real missing mechanism here — with no way to refuse at compile time, every chain in the tree can only fail to a defined-but-wrong value. Worth its own Track A ticket; lexer.inc was outside this grant.

Correction 2 — the armless set is larger than the ticket's table. The file header records that xtensa under IDF compiles the {$ifndef PXX_ESP} bodies ("riscv32 under IDF compiled all of them, which is the proof the bodies are fine on an ESP target"). So open/lseek/close are reached with no arm on hosted riscv32, on wasm32, AND on xtensa/IDF — three live targets, one of them Track S's active campaign, not the two this ticket names.

Correction 3 — PXXSysWrite is marked "fixed" in the table above and was not. Adding the wasm32 arm closed that instance and left the generator running: its pre-chain Result := 0 still meant an armless target reported writing nothing successfully. PXXSysRead is the same, where 0 reads as EOF. Both are converted here. This is the ticket's own prediction — "adding an arm closes today's instance and leaves the generator running" — having already come true once inside the ticket, unnoticed.

Measured, per the playbook, not reasoned

PXXSysOpenRO IR on riscv32 (PXXDBG=a.ir:PXXSysOpenRO):

IR meaning
before IR count=1 — one empty block body is EMPTY; Result never assigned
after IR count=4 — const 1; neg; store Result Result := -1

And frankwasm's reproducer from the sibling ticket, run end to end:

native x86-64 riscv32 (no arms)
PXXSysOpenRO(nil) -14 unchanged -1 (was: return-slot leftovers)
PXXSysClose(7) -9 unchanged -1
PXXSysLseek(7,0,0) -9 unchanged -1

x86-64 is provably untouched by the first three chains: that build produced a byte-identical compiler (da54007a8f92, equal to HEAD) because the else arms are dead on the host. The read/write conversion does change the binary — it removes the now-redundant pre-chain Result := 0 dead store on every target that has an arm. riscv32 keeps syscalls 63/64, confirmed by IR dump. 12 of 12 loadfile/readln/writeln assertions pass, i386 and aarch64 cross sites included.

Why -1 rather than something louder

For these five, -1 is not a consolation for the missing {$error} — it is the value they already signal failure with, and PXXStrLoadFile's if fd < 0 then Exit consumes it correctly. The routine now fails the way its own caller already expects, instead of returning a plausible success.

Explicitly NOT done

Log

CORRECTION 2026-08-29 — {$error} DOES exist. Correction 1 above is wrong.

Caught by frankA within minutes of the resolve, and corrected here rather than edited away, because the commit message 1ea3bdb85 carries the false claim and someone will read it.

lexer.inc:1697 recognises warning/message/error; lexer.inc:1976 calls Error(messageText). Measured both ways:

{$if not defined(NEVER_DEFINED_XYZ)}{$error the terminal arm fired}{$endif}
  -> pascal26:1: error: the terminal arm fired
{$if defined(NEVER_DEFINED_XYZ)}{$error should not fire}{$endif}
  -> compiles clean

How the wrong answer was produced, because the mechanism matters more than the fact. The first grep was case-sensitive over uppercase directive names ('(IFDEF|...|ERROR|...)') against source that spells them lowercase. It returned nothing at all — not even ifdef — and that empty output was read as "no such directive" rather than as "this pattern matched nothing". A second grep then ranged over line 1745-1900 and missed 1976 by 76 lines. Two instruments, both silent, both read as evidence of absence.

That is the exact shape this session spent the day cataloguing — an instrument whose aperture is invisible in its own output — committed in the step that was supposed to be checking. An empty grep result and a does-not-exist are the same output, and only one of them was true.

The fix is still right, for a reason NEITHER TICKET GAVE — this is frankA's

{$error} fires when the file is compiled, not when the routine is called. builtinheap.pas is compiled into every program on every target — the empty program p; begin end. used for the IR probe above compiled it on riscv32 — so an {$error} terminal in PXXSysOpenRO would refuse every wasm32 and xtensa build outright, including the overwhelming majority that never open a file. It would convert a latent wrong value into a hard build failure for unrelated programs.

So the terminal arm is chosen by reachability, and that is the rule to carry forward:

{$error} is right where a missing arm cannot be reached at run time; a defined failure value is right where the routine is compiled into everything and called by almost nothing.

Both tickets prescribed {$error} unconditionally, without that distinction — which is why "the dialect lacks it" landed as a plausible explanation instead of being re-tested. A wrong premise reached the right code, and only the premise needed correcting.

The rule both tickets flattened — coordinator, same day

pxx-a5's account above and frankA's independently reached the same fix; neither ticket stated the rule that decides the NEXT missing arm:

the missing arm is use
unreachable at runtime — a case that cannot occur in a valid build {$error}, fail at compile time
reachable at runtime — the program may legitimately hit it a defined failure value the caller checks

Prescribing {$error} flatly turns the second row into a build refusal for every target not yet ported. Prescribing a sentinel flatly turns the first row into a silent wrong answer — the original fail-open bug this ticket was filed about.

Confirmed independently at lexer.inc:1697 / 1976 by the coordinator before propagating, from a checkout that did not share pxx-a5's grep.