Maintenance: refactor servers updater code

- Require at least 80% of number of servers now to pass
- Each provider is in its own package with a common structure
- Unzip package with unzipper interface
- Openvpn package with extraction and download functions

internal/updater/openvpn/multifetch.go

@@ -0,0 +1,59 @@
+package openvpn
+
+import (
+	"context"
+	"net/http"
+)
+
+// FetchMultiFiles fetches multiple Openvpn files in parallel and
+// parses them to extract each of their host. A mapping from host to
+// URL is returned.
+func FetchMultiFiles(ctx context.Context, client *http.Client, urls []string) (
+	hostToURL map[string]string, err error) {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	hostToURL = make(map[string]string, len(urls))
+
+	type Result struct {
+		url  string
+		host string
+	}
+
+	results := make(chan Result)
+	defer close(results)
+	errors := make(chan error)
+	defer close(errors)
+
+	for _, url := range urls {
+		go func(url string) {
+			host, err := FetchFile(ctx, client, url)
+			if err != nil {
+				errors <- err
+				return
+			}
+			results <- Result{
+				url:  url,
+				host: host,
+			}
+		}(url)
+	}
+
+	for range urls {
+		select {
+		case newErr := <-errors:
+			if err == nil { // only assign to the first error
+				err = newErr
+				cancel() // stop other operations, this will trigger other errors we ignore
+			}
+		case result := <-results:
+			hostToURL[result.host] = result.url
+		}
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return hostToURL, nil
+}