heimdall: better error logging for clerk/event-record/list nil response (#9103)

Users reported this error
```
[bor.heimdall] an error while trying fetching path=clerk/event-record/list attempt=5 error="unexpected end of JSON input"
```

Which may happen if:

1. Heimdall is behind and not sync-ed - for more info check
https://github.com/maticnetwork/heimdall/pull/993
2. Or the header time erigon is sending is far into the future

The logs in this PR will help us see which of the 2 is the culprit but
most likely it is 1. We will investigate further 2. if it ever happens.

Changes:
1. Improves logging upon heimdall client retries - prints out the full
url that failed.
2. Fixes a bug where the body was incorrectly checked if it is empty -
`len(body) == 0` vs `body == nil`
3. Unit test for the bug regression
4. Adds a log to indicate to users to check their heimdall process if
they run into this scenario since that may be the culprit


Example output with new logs
<img width="1465" alt="Screenshot 2023-12-29 at 20 16 57"
src="https://github.com/ledgerwatch/erigon/assets/94537774/1ebfde68-aa93-41d6-889a-27bef5414f25">

consensus/bor/heimdall/client.go

@@ -121,6 +121,14 @@ func (c *Client) StateSyncEvents(ctx context.Context, fromID uint64, to int64) (
 
 		response, err := FetchWithRetry[StateSyncEventsResponse](ctx, c, url)
 		if err != nil {
+			if errors.Is(err, ErrNoResponse) {
+				// for more info check https://github.com/maticnetwork/heimdall/pull/993
+				c.logger.Warn(
+					"[bor.heimdall] check heimdall logs to see if it is in sync - no response when querying state sync events",
+					"path", url.Path,
+					"queryParams", url.RawQuery,
+				)
+			}
 			return nil, err
 		}
 
@@ -327,7 +335,7 @@ func FetchWithRetryEx[T any](ctx context.Context, client *Client, url *url.URL,
 		// yet in heimdall. E.g. when the hard fork hasn't hit yet but heimdall
 		// is upgraded.
 		if errors.Is(err, ErrServiceUnavailable) {
-			client.logger.Debug("[bor.heimdall] service unavailable at the moment", "path", url.Path, "attempt", attempt, "err", err)
+			client.logger.Debug("[bor.heimdall] service unavailable at the moment", "path", url.Path, "queryParams", url.RawQuery, "attempt", attempt, "err", err)
 			return nil, err
 		}
 
@@ -335,14 +343,14 @@ func FetchWithRetryEx[T any](ctx context.Context, client *Client, url *url.URL,
 			return nil, err
 		}
 
-		client.logger.Warn("[bor.heimdall] an error while fetching", "path", url.Path, "attempt", attempt, "err", err)
+		client.logger.Warn("[bor.heimdall] an error while fetching", "path", url.Path, "queryParams", url.RawQuery, "attempt", attempt, "err", err)
 
 		select {
 		case <-ctx.Done():
-			client.logger.Debug("[bor.heimdall] request canceled", "reason", ctx.Err(), "path", url.Path, "attempt", attempt)
+			client.logger.Debug("[bor.heimdall] request canceled", "reason", ctx.Err(), "path", url.Path, "queryParams", url.RawQuery, "attempt", attempt)
 			return nil, ctx.Err()
 		case <-client.closeCh:
-			client.logger.Debug("[bor.heimdall] shutdown detected, terminating request", "path", url.Path)
+			client.logger.Debug("[bor.heimdall] shutdown detected, terminating request", "path", url.Path, "queryParams", url.RawQuery)
 			return nil, ErrShutdownDetected
 		case <-ticker.C:
 			// retry
@@ -369,7 +377,7 @@ func Fetch[T any](ctx context.Context, request *Request) (*T, error) {
 		return nil, err
 	}
 
-	if body == nil {
+	if len(body) == 0 {
 		return nil, ErrNoResponse
 	}
 

consensus/bor/heimdall/client_test.go

@@ -31,7 +31,10 @@ func TestHeimdallClientFetchesTerminateUponTooManyErrors(t *testing.T) {
 	httpClient := mock.NewMockHttpClient(ctrl)
 	httpClient.EXPECT().
 		Do(gomock.Any()).
-		Return(&http.Response{StatusCode: 404, Body: emptyBodyReadCloser{}}, nil).
+		Return(&http.Response{
+			StatusCode: 404,
+			Body:       emptyBodyReadCloser{},
+		}, nil).
 		Times(5)
 	logger := testlog.Logger(t, log.LvlDebug)
 	heimdallClient := newHeimdallClient("https://dummyheimdal.com", httpClient, 100*time.Millisecond, 5, logger)
@@ -40,3 +43,22 @@ func TestHeimdallClientFetchesTerminateUponTooManyErrors(t *testing.T) {
 	require.Nil(t, spanRes)
 	require.Error(t, err)
 }
+
+func TestHeimdallClientStateSyncEventsReturnsErrNoResponseWhenHttp200WithEmptyBody(t *testing.T) {
+	ctx := context.Background()
+	ctrl := gomock.NewController(t)
+	httpClient := mock.NewMockHttpClient(ctrl)
+	httpClient.EXPECT().
+		Do(gomock.Any()).
+		Return(&http.Response{
+			StatusCode: 200,
+			Body:       emptyBodyReadCloser{},
+		}, nil).
+		Times(2)
+	logger := testlog.Logger(t, log.LvlDebug)
+	heimdallClient := newHeimdallClient("https://dummyheimdal.com", httpClient, time.Millisecond, 2, logger)
+
+	spanRes, err := heimdallClient.StateSyncEvents(ctx, 100, time.Now().Unix())
+	require.Nil(t, spanRes)
+	require.ErrorIs(t, err, ErrNoResponse)
+}