internal/fetch: implement parse url for fetch service

The fetch service accepts HTTP requests at
http(s)://<fetchURL>/<module>@<version>.

ParseNameAndVersion is implemented, which validates the module name and
version for requests to this endpoint.

Change-Id: Iacf7f4ae8dfef54a024172807d8bc9ff01749c84
Reviewed-on: https://team-review.git.corp.google.com/c/422997
Reviewed-by: Channing Kimble-Brown <ckimblebrown@google.com>
Reviewed-by: Andrew Bonventre <andybons@google.com>

internal/fetch/fetch.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -20,6 +21,25 @@ import (
 
 var errReadmeNotFound = errors.New("fetch: zip file does not contain a README")
 
+// ParseNameAndVersion returns the module and version specified by u. u is
+// assumed to be a valid url following the structure http(s)://<fetchURL>/<module>@<version>.
+func ParseNameAndVersion(u *url.URL) (string, string, error) {
+	parts := strings.Split(strings.TrimPrefix(u.Path, "/"), "/@v/")
+	if len(parts) != 2 {
+		return "", "", fmt.Errorf("invalid path: %q", u)
+	}
+
+	// TODO(julieqiu): Check module name is valid using
+	// https://github.com/golang/go/blob/c97e576/src/cmd/go/internal/module/module.go#L123
+	// Check version is valid using
+	// https://github.com/golang/go/blob/c97e576/src/cmd/go/internal/modload/query.go#L183
+	if parts[0] == "" || parts[1] == "" {
+		return "", "", fmt.Errorf("invalid path: %q", u)
+	}
+
+	return parts[0], parts[1], nil
+}
+
 // isReadme checks if file is the README. It is case insensitive.
 func isReadme(file string) bool {
 	base := filepath.Base(file)

internal/fetch/fetch_test.go

@@ -9,10 +9,72 @@ import (
 	"bytes"
 	"errors"
 	"io/ioutil"
+	"net/url"
 	"strings"
 	"testing"
 )
 
+func TestParseNameAndVersion(t *testing.T) {
+	testCases := []struct {
+		name    string
+		url     string
+		module  string
+		version string
+		err     error
+	}{
+		{
+			name:    "ValidFetchURL",
+			url:     "https://proxy.com/module/@v/v1.0.0",
+			module:  "module",
+			version: "v1.0.0",
+			err:     nil,
+		},
+		{
+			name: "InvalidFetchURL",
+			url:  "https://proxy.com/",
+			err:  errors.New(`invalid path: "https://proxy.com/"`),
+		},
+		{
+			name: "InvalidFetchURLNoModule",
+			url:  "https://proxy.com/@v/version",
+			err:  errors.New(`invalid path: "https://proxy.com/@v/version"`),
+		},
+		{
+			name: "InvalidFetchURLNoVersion",
+			url:  "https://proxy.com/module/@v/",
+			err:  errors.New(`invalid path: "https://proxy.com/module/@v/"`),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			u, err := url.Parse(tc.url)
+			if err != nil {
+				t.Errorf("url.Parse(%q): %v", tc.url, err)
+			}
+
+			m, v, err := ParseNameAndVersion(u)
+			if tc.err != nil {
+				if err == nil {
+					t.Fatalf("ParseNameAndVersion(%v) error = (%v); want = (%v)", u, err, tc.err)
+				}
+				if tc.err.Error() != err.Error() {
+					t.Fatalf("ParseNameAndVersion(%v) error = (%v); want = (%v)", u, err, tc.err)
+				} else {
+					return
+				}
+			} else if err != nil {
+				t.Fatalf("ParseNameAndVersion(%v) error = (%v); want = (%v)", u, err, tc.err)
+			}
+
+			if tc.module != m || tc.version != v {
+				t.Fatalf("ParseNameAndVersion(%v): %q, %q, %v; want = %q, %q, %v",
+					u, m, v, err, tc.module, tc.version, tc.err)
+			}
+		})
+	}
+}
+
 func TestIsReadme(t *testing.T) {
 	for input, want := range map[string]bool{
 		"rEaDme":         true,