discovery/internal/proxy: implement GET zip for proxy client

GetZip is implemented, which makes a request to
$GOPROXY/<module>/@v/<version>.zip and transforms that data into a
*zip.Reader

This will be used by the discovery fetch service to download modules
from the proxy.

Change-Id: I5aacbbc12fd01f77a22a415206e64698d8849d40
Reviewed-on: https://team-review.git.corp.google.com/c/417749
Reviewed-by: Andrew Bonventre <andybons@google.com>

internal/proxy/proxy_client.go

@@ -5,8 +5,11 @@
 package proxy
 
 import (
+	"archive/zip"
+	"bytes"
 	"encoding/json"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 	"strings"
 	"time"
@@ -36,12 +39,13 @@ func New(rawurl string) *Client {
 	return &Client{url: cleanURL(rawurl)}
 }
 
-// infoURL constructs a url for a GET request to $GOPROXY/<module>/@v/list.
+// infoURL constructs a url for a request to
+// $GOPROXY/<module>/@v/list.
 func (c *Client) infoURL(name, version string) string {
 	return fmt.Sprintf("%s/%s/@v/%s.info", c.url, name, version)
 }
 
-// GetInfo makes a GET request to $GOPROXY/<module>/@v/<version>.info and
+// GetInfo makes a request to $GOPROXY/<module>/@v/<version>.info and
 // transforms that data into a *VersionInfo.
 func (c *Client) GetInfo(name, version string) (*VersionInfo, error) {
 	r, err := http.Get(c.infoURL(name, version))
@@ -56,3 +60,38 @@ func (c *Client) GetInfo(name, version string) (*VersionInfo, error) {
 	}
 	return &v, nil
 }
+
+// zipURL constructs a url for a request to $GOPROXY/<module>/@v/<version>.zip.
+func (c *Client) zipURL(name, version string) string {
+	return fmt.Sprintf("%s/%s/@v/%s.zip", c.url, name, version)
+}
+
+// GetZip makes a request to $GOPROXY/<module>/@v/<version>.zip and transforms
+// that data into a *zip.Reader.
+func (c *Client) GetZip(name, version string) (*zip.Reader, error) {
+	u := c.zipURL(name, version)
+	r, err := http.Get(u)
+	if err != nil {
+		return nil, fmt.Errorf("http.Get(%q): %v",
+			c.zipURL(name, version), err)
+	}
+	defer r.Body.Close()
+
+	if r.StatusCode < 200 || r.StatusCode >= 300 {
+		return nil, fmt.Errorf("http.Get(%q) returned response: %d (%q)",
+			c.zipURL(name, version), r.StatusCode, r.Status)
+	}
+
+	body, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		return nil, fmt.Errorf("http.Get(%q): %v",
+			c.zipURL(name, version), err)
+	}
+
+	zipReader, err := zip.NewReader(bytes.NewReader(body), int64(len(body)))
+	if err != nil {
+		return nil, fmt.Errorf("http.Get(%q): %v",
+			c.zipURL(name, version), err)
+	}
+	return zipReader, nil
+}

internal/proxy/proxy_client_test.go

@@ -5,6 +5,7 @@
 package proxy
 
 import (
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -73,3 +74,49 @@ func TestGetInfoVersionDoesNotExist(t *testing.T) {
 		t.Errorf("GetInfo(%q, %q) = %v, want %v", name, version, info, nil)
 	}
 }
+
+func TestGetZip(t *testing.T) {
+	teardownTestCase, testCase := setupTestCase(t)
+	defer teardownTestCase(t)
+
+	name := "my/module"
+	version := "v1.0.0"
+	zipReader, err := testCase.client.GetZip(name, version)
+	if err != nil {
+		t.Errorf("GetZip(%q, %q) error: %v", name, version, err)
+	}
+
+	expectedFiles := map[string]bool{
+		"my/":                        true,
+		"my/module@v1.0.0/":          true,
+		"my/module@v1.0.0/LICENSE":   true,
+		"my/module@v1.0.0/README.md": true,
+		"my/module@v1.0.0/go.mod":    true,
+	}
+	if len(zipReader.File) != len(expectedFiles) {
+		t.Errorf("GetZip(%q, %q) returned number of files: got %d, want %d",
+			name, version, len(zipReader.File), len(expectedFiles))
+	}
+
+	for _, zipFile := range zipReader.File {
+		if !expectedFiles[zipFile.Name] {
+			t.Errorf("GetZip(%q, %q) returned unexpected file: %q", name,
+				version, zipFile.Name)
+		}
+		delete(expectedFiles, zipFile.Name)
+	}
+}
+
+func TestGetZipNonExist(t *testing.T) {
+	teardownTestCase, testCase := setupTestCase(t)
+	defer teardownTestCase(t)
+
+	name := "my/nonexistmodule"
+	version := "v1.0.0"
+	expectedErr := fmt.Sprintf("http.Get(%q) returned response: %d (%q)",
+		testCase.client.zipURL(name, version), 404, "404 Not Found")
+
+	if _, err := testCase.client.GetZip(name, version); err.Error() != expectedErr {
+		t.Errorf("GetZip(%q, %q) returned error %v, want %v", name, version, err, expectedErr)
+	}
+}