test: extract getSlot and unit test it

pkg/workflows/wasm/host/wasip1.go

@@ -2,6 +2,7 @@ package host
 
 import (
 	"encoding/binary"
+	"fmt"
 	"io"
 	"math/rand"
 	"time"
@@ -122,8 +123,11 @@ func pollOneoff(caller *wasmtime.Caller, subscriptionptr int32, eventsptr int32,
 		eventType := subs[inOffset+8]
 		argBuf := subs[inOffset+8+8:]
 
-		outOffset := i * eventsLen
-		slot := events[outOffset : outOffset+eventsLen]
+		slot, err := getSlot(events, i)
+		if err != nil {
+			return ErrnoFault
+		}
+
 		switch eventType {
 		case eventTypeClock:
 			// We want to stub out clock events,
@@ -227,3 +231,14 @@ func createRandomGet(cfg *ModuleConfig) func(caller *wasmtime.Caller, buf, bufLe
 		return ErrnoSuccess
 	}
 }
+
+func getSlot(events []byte, i int32) ([]byte, error) {
+	outOffset := i * eventsLen
+
+	if outOffset+eventsLen > int32(len(events)) {
+		return nil, fmt.Errorf("slot %d out of bounds", i)
+	}
+
+	slot := events[outOffset : outOffset+eventsLen]
+	return slot, nil
+}

pkg/workflows/wasm/host/wasip1_test.go

@@ -0,0 +1,25 @@
+package host
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetSlot(t *testing.T) {
+	events := []byte{
+		0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
+		250, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
+	}
+
+	expectedSlots := [][]byte{
+		{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31},
+		{250, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63},
+	}
+
+	for i := int32(0); i < 2; i++ {
+		slot, err := getSlot(events, i)
+		assert.NoError(t, err)
+		assert.Equal(t, expectedSlots[i], slot)
+	}
+}