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/unzip/extract.go

@@ -0,0 +1,36 @@
+package unzip
+
+import (
+	"archive/zip"
+	"bytes"
+	"io/ioutil"
+	"path/filepath"
+	"strings"
+)
+
+func zipExtractAll(zipBytes []byte) (contents map[string][]byte, err error) {
+	r, err := zip.NewReader(bytes.NewReader(zipBytes), int64(len(zipBytes)))
+	if err != nil {
+		return nil, err
+	}
+	contents = map[string][]byte{}
+	for _, zf := range r.File {
+		fileName := filepath.Base(zf.Name)
+		if !strings.HasSuffix(fileName, ".ovpn") {
+			continue
+		}
+		f, err := zf.Open()
+		if err != nil {
+			return nil, err
+		}
+		defer f.Close()
+		contents[fileName], err = ioutil.ReadAll(f)
+		if err != nil {
+			return nil, err
+		}
+		if err := f.Close(); err != nil {
+			return nil, err
+		}
+	}
+	return contents, nil
+}

internal/updater/unzip/fetch.go

@@ -0,0 +1,52 @@
+package unzip
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+)
+
+var (
+	ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
+)
+
+func (u *unzipper) FetchAndExtract(ctx context.Context, url string) (
+	contents map[string][]byte, err error) {
+	contents = make(map[string][]byte)
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	response, err := u.client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%w: %s for %s", ErrHTTPStatusCodeNotOK, response.Status, url)
+	}
+
+	b, err := ioutil.ReadAll(response.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return nil, err
+	}
+
+	newContents, err := zipExtractAll(b)
+	if err != nil {
+		return nil, err
+	}
+	for fileName, content := range newContents {
+		contents[fileName] = content
+	}
+
+	return contents, nil
+}

internal/updater/unzip/unzip.go

@@ -0,0 +1,22 @@
+// Package unzip defines the Unzipper which fetches and extract a zip file
+// containing multiple files.
+package unzip
+
+import (
+	"context"
+	"net/http"
+)
+
+type Unzipper interface {
+	FetchAndExtract(ctx context.Context, url string) (contents map[string][]byte, err error)
+}
+
+type unzipper struct {
+	client *http.Client
+}
+
+func New(client *http.Client) Unzipper {
+	return &unzipper{
+		client: client,
+	}
+}