Fix the error-handler stack index in xpcall

lua.go

@@ -397,7 +397,10 @@ func (l *State) ProtectedCallWithContinuation(argCount, resultCount, errorFuncti
 	l.checkResults(argCount, resultCount)
 	if errorFunction != 0 {
 		apiCheckStackIndex(errorFunction, l.indexToValue(errorFunction))
-		errorFunction = l.AbsIndex(errorFunction)
+		// The error handler is addressed relative to this frame, but it is
+		// called later from whichever frame raises the error, so remember its
+		// absolute stack position.
+		errorFunction = l.callInfo.function + l.AbsIndex(errorFunction)
 	}
 
 	f := l.top - (argCount + 1)

xpcall_test.go

@@ -0,0 +1,42 @@
+package lua
+
+import "testing"
+
+// TestXPCallInvokesErrorHandler checks that xpcall calls the error handler
+// passed as its second argument. The handler lives in the caller's frame, so
+// the VM has to remember its absolute stack position while the protected
+// function runs.
+func TestXPCallInvokesErrorHandler(t *testing.T) {
+	l := NewState()
+	OpenLibraries(l)
+	// Put a non-callable value in the first stack slot. A stale frame-relative
+	// error-handler index resolves to this slot, so the bug surfaces as
+	// "error in error handling" instead of an endless recursion into the
+	// outermost chunk.
+	l.PushNumber(0)
+	err := DoString(l, `
+		local ok, msg = xpcall(function() error("boom") end,
+			function(m) return "handled: " .. m end)
+		assert(ok == false, "xpcall reported success for a failing function")
+		assert(msg:sub(1, 9) == "handled: ", "handler result: " .. tostring(msg))
+		assert(msg:sub(-4) == "boom", "handler result: " .. tostring(msg))
+	`)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+// TestXPCallReturnsResults checks that a successful xpcall still returns the
+// protected function's results and true.
+func TestXPCallReturnsResults(t *testing.T) {
+	l := NewState()
+	OpenLibraries(l)
+	err := DoString(l, `
+		local ok, a, b = xpcall(function() return 1, 2 end, function() return "unused" end)
+		assert(ok == true, "xpcall reported failure for a successful function")
+		assert(a == 1 and b == 2, "results: " .. tostring(a) .. ", " .. tostring(b))
+	`)
+	if err != nil {
+		t.Fatal(err)
+	}
+}