fix(hbrt): variadic PValue support — snapshot pushed args per frame

Variadic Harbour functions (`FUNCTION foo(...)`) declare 0 params,
so Frame() copied no args into the locals slots PValue/CallerLocal
read from. Result: PValue(n) returned whatever happened to live in
the caller's LOCAL slot n (i.e. the first LOCAL declared after `(...)`,
which loops typically use as a counter). fivenode's AP_RPUTS, AP_ECHO
and any other variadic dispatcher that walks PValue() saw garbage —
fivenode_go shipped a workaround (AP_RPUTS collapsed to a single arg)
that this commit now lets us revert.

Mechanism

  CallFrame now carries `actualArgs []Value`. Frame() snapshots every
  value the caller pushed (the full pendingParams, not the clipped
  declared count) into this slice before moving sp. The locals[]
  declared-param region is unchanged so positional LOCAL access keeps
  working. CallerLocal reads from actualArgs first.

  Stack handling tightens slightly: t.sp now ends at `sp - pushedArgs`
  instead of `sp - localsCopy`, dropping the extra-args slots that
  variadic callers used to leave on the stack. They're no longer needed
  — actualArgs is the canonical home — and leaving them on the stack
  was the root of the original "PValue returns the caller's LOCAL"
  bug because the next push would overwrite them.

  Slice reuse: when capacity permits, we slice down rather than
  reallocating, so the hot path (0/1/few args) keeps its no-alloc
  characteristics.

Verified

  • Variadic 2/1/0-arg case and fixed-arg comparison all print the
    expected values (test_variadic_only.prg).
  • Full suite: go test ./compiler/... ./hbrt/... ./hbrtl/...,
    Compat 56/56, std.ch 17/17, FRB 7/7, FiveSql2 43/43 all green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hbrt/thread.go

@@ -28,6 +28,13 @@ type CallFrame struct {
 	module     string  // current PRG source file (updated by DebugLine)
 	line       int     // current PRG source line
 	localNames []string // PRG-source names of params+locals (nil = none registered)
+	// actualArgs snapshots every value the caller pushed, regardless of
+	// the callee's declared param count. PValue / CallerLocal read this
+	// so variadic functions (declared params = 0) can see their args —
+	// the locals[] slots are reserved for declared LOCALs in that case,
+	// so the args have nowhere else to live. Reused across calls when
+	// capacity allows.
+	actualArgs []Value
 }
 
 // CurFrame returns the current call frame (for closure capture).
@@ -292,17 +299,20 @@ func (t *Thread) Frame(params, locals int) {
 	}
 
 	// Save frame
-	// Handle case where fewer args were pushed than declared params
-	actual := t.pendingParams
-	if actual > params {
-		actual = params
+	// pushedArgs is the actual count the caller put on the stack.
+	// localsCopy is how many of those map onto the callee's declared
+	// LOCAL slot indexes (clipped at the declared param count).
+	pushedArgs := t.pendingParams
+	if pushedArgs > t.sp {
+		pushedArgs = t.sp
 	}
-	if actual > t.sp {
-		actual = t.sp
+	localsCopy := pushedArgs
+	if localsCopy > params {
+		localsCopy = params
 	}
 
 	frame := &t.calls[t.callSP]
-	frame.base = t.sp - actual // only actual args on stack
+	frame.base = t.sp - localsCopy // unchanged: only declared args bind to LOCAL slots
 	frame.localBase = localBase
 	frame.localCount = params + locals
 	frame.paramCount = t.pendingParams // actual args passed by caller (not declared count)
@@ -310,18 +320,36 @@ func (t *Thread) Frame(params, locals int) {
 	frame.symbol = t.pendingCallSym
 	t.pendingCallSym = nil
 
-	// Copy actual parameters from stack to locals
-	for i := 0; i < actual; i++ {
+	// Snapshot every pushed arg BEFORE we move sp — extras would be
+	// overwritten as soon as this frame's body started pushing for its
+	// own expression evaluation. Reuse the slice when capacity allows
+	// to keep the no-arg / few-arg path allocation-free.
+	if pushedArgs > 0 {
+		snapStart := t.sp - pushedArgs
+		if cap(frame.actualArgs) >= pushedArgs {
+			frame.actualArgs = frame.actualArgs[:pushedArgs]
+		} else {
+			frame.actualArgs = make([]Value, pushedArgs)
+		}
+		copy(frame.actualArgs, t.stack[snapStart:snapStart+pushedArgs])
+	} else if frame.actualArgs != nil {
+		frame.actualArgs = frame.actualArgs[:0]
+	}
+
+	// Copy actual parameters from stack to locals (declared-param portion).
+	for i := 0; i < localsCopy; i++ {
 		t.locals[localBase+i] = t.stack[frame.base+i]
 	}
 
 	// Initialize missing params and locals to NIL
-	for i := actual; i < params+locals; i++ {
+	for i := localsCopy; i < params+locals; i++ {
 		t.locals[localBase+i] = MakeNil()
 	}
 
-	// Pop args from stack (they're now in locals)
-	t.sp = frame.base
+	// Pop args from stack (they're now in locals + actualArgs).
+	// Extras beyond declared params (variadic) live only in actualArgs;
+	// drop the stack slots so the caller can't accidentally re-read.
+	t.sp = t.sp - pushedArgs
 
 	t.curFrame = frame
 	t.callSP++
@@ -673,11 +701,16 @@ func (t *Thread) CallerParamCount() int {
 // CallerLocal returns the n-th parameter of the calling PRG function
 // (1-based). Returns NIL if no caller frame exists or n is out of range.
 // Pairs with CallerParamCount for implementing the PValue() RTL.
+//
+// Reads from the caller frame's actualArgs snapshot — works for both
+// plain and variadic callers. The locals[] slots would only carry the
+// first declared-param-count values, which fails for FUNCTION foo(...)
+// where declared params == 0.
 func (t *Thread) CallerLocal(n int) Value {
 	if t.callSP >= 2 {
 		caller := &t.calls[t.callSP-2]
-		if n >= 1 && n <= caller.paramCount {
-			return caller.GetLocal(n, t.locals)
+		if n >= 1 && n <= len(caller.actualArgs) {
+			return caller.actualArgs[n-1]
 		}
 	}
 	return MakeNil()