fix: properly close HTTP responses after sending each email

This change extracts per-email sending logic into a helper function
for which proper resource closing can be performed. Previously it
invoke defer res.Body.Close in a for-loop but Go's specifications
and operations dictate that the defer will only be invoked when
the surrounding function is returning or panicking: this means then
that if very many emails are being sent, that risks potential
resource exhaustion because so many connections won't be closed yet
nor can they be garbage collected.

Fixes cds-snc#13

notify_client.go

@@ -92,28 +92,35 @@ func sendEmail(client *NotifyClient, email *NotifyEmail) error {
 		req.Header.Set("Content-Type", contentType)
 		req.Header.Set("Authorization", fmt.Sprintf("ApiKey-v1 %s", client.ApiKey))
 
-		resp, err := client.Client.Do(req)
-
-		if err != nil {
-			log.Error().Msgf("Error sending email: %s", err)
+		if err := doSendEmail(client.Client, req); err != nil {
 			return err
 		}
+	}
+
+	return nil
+}
 
-		defer resp.Body.Close()
+func doSendEmail(client *http.Client, req *http.Request) error {
+	resp, err := client.Do(req)
+	if err != nil {
+		log.Error().Msgf("Error sending email: %s", err)
+		return err
+	}
+	defer resp.Body.Close()
 
-		if resp.StatusCode != 201 {
-			log.Error().Msgf("Unexpected status code: %d", resp.StatusCode)
-			respbody, err := io.ReadAll(resp.Body)
+	if resp.StatusCode == 201 {
+		return nil
+	}
 
-			if err != nil {
-				log.Error().Msgf("Error reading response body: %s", err)
-				return err
-			}
-			log.Error().Msgf("Response: %s", respbody)
+	// A non-201 status code so craft the error.
+	log.Error().Msgf("Unexpected status code: %d", resp.StatusCode)
+	respbody, err := io.ReadAll(resp.Body)
 
-			return fmt.Errorf("unexpected status code: %d", resp.StatusCode)
-		}
+	if err != nil {
+		log.Error().Msgf("Error reading response body: %s", err)
+		return err
 	}
+	log.Error().Msgf("Response: %s", respbody)
 
-	return nil
+	return fmt.Errorf("unexpected status code: %d", resp.StatusCode)
 }