app/eth2wrap: fix cancelled context race (#1378)

Fixes a race in `eth2wrap` forkjoin code when context is cancelled before any results are returned. This resulted in `<zero>,nil` being returned which resulted in unexpected behaviour, including "divide by zero".

category: bug
ticket: #1326

app/eth2wrap/eth2wrap.go

@@ -163,8 +163,9 @@ func provide[O any](ctx context.Context, clients []Client,
 	defer cancel()
 
 	var (
-		nokResp forkjoin.Result[Client, O]
-		zero    O
+		nokResp    forkjoin.Result[Client, O]
+		hasNokResp bool
+		zero       O
 	)
 	for res := range join() {
 		if ctx.Err() != nil {
@@ -173,9 +174,16 @@ func provide[O any](ctx context.Context, clients []Client,
 			return res.Output, nil
 		} else {
 			nokResp = res
+			hasNokResp = true
 		}
 	}
 
+	if ctx.Err() != nil {
+		return zero, ctx.Err()
+	} else if !hasNokResp {
+		return zero, errors.New("bug: no forkjoin results")
+	}
+
 	return nokResp.Output, nokResp.Err
 }
 

app/eth2wrap/eth2wrap_test.go

@@ -226,3 +226,19 @@ func TestErrors(t *testing.T) {
 		require.ErrorContains(t, err, "beacon api genesis_time: network operation error: :")
 	})
 }
+
+func TestCtxCancel(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		ctx, cancel := context.WithCancel(context.Background())
+
+		bmock, err := beaconmock.New()
+		require.NoError(t, err)
+		eth2Cl, err := eth2wrap.NewMultiHTTP(ctx, time.Second, bmock.Address())
+		require.NoError(t, err)
+
+		cancel() // Cancel context before calling method.
+
+		_, err = eth2Cl.SlotDuration(ctx)
+		require.ErrorIs(t, err, context.Canceled)
+	}
+}