MM-27918 In-Product notices support (#15316)

utils/utils.go

@@ -4,6 +4,8 @@
 package utils
 
 import (
+	"github.com/pkg/errors"
+	"io/ioutil"
 	"net"
 	"net/http"
 	"net/url"
@@ -121,3 +123,52 @@ func GetHostnameFromSiteURL(siteURL string) string {
 
 	return u.Hostname()
 }
+
+type RequestCache struct {
+	Data []byte
+	Date string
+	Key  string
+}
+
+// Fetch JSON data from the notices server
+// if skip is passed, does a fetch without touching the cache
+func GetUrlWithCache(url string, cache *RequestCache, skip bool) ([]byte, error) {
+	// Build a GET Request, including optional If-None-Match header.
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		cache.Data = nil
+		return nil, err
+	}
+	if !skip && cache.Data != nil {
+		req.Header.Add("If-None-Match", cache.Key)
+		req.Header.Add("If-Modified-Since", cache.Date)
+	}
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	if err != nil {
+		cache.Data = nil
+		return nil, err
+	}
+	defer resp.Body.Close()
+	// No change from latest known Etag?
+	if resp.StatusCode == http.StatusNotModified {
+		return cache.Data, nil
+	}
+
+	if resp.StatusCode != 200 {
+		cache.Data = nil
+		return nil, errors.Errorf("Fetching notices failed with status code %d", resp.StatusCode)
+	}
+
+	cache.Data, err = ioutil.ReadAll(resp.Body)
+	if err != nil {
+		cache.Data = nil
+		return nil, err
+	}
+
+	// If etags headers are missing, ignore.
+	cache.Key = resp.Header.Get("ETag")
+	cache.Date = resp.Header.Get("Date")
+	return cache.Data, err
+}