selfupdate: fix --version X.Y depending on ./ in the download site links

To find the latest patch release of a minor version, for example
`rclone selfupdate --version 1.75`, selfupdate searched the listing of
downloads.rclone.org for href="./vX.Y.Z/". The leading ./ is a detail
of how Caddy's file server writes its links. A listing which linked to
the same directories as "vX.Y.Z/", which is an equally valid relative
URL, made selfupdate fail with "could not find the minor release".

This makes the ./ optional in the pattern, so selfupdate no longer
depends on which program wrote the listing, and takes the version from
a capture group rather than from fixed offsets into the match.

The listings written by rclone index now include the ./ for the
benefit of rclone versions without this fix, so this is to remove the
dependency for the future.

cmd/selfupdate/selfupdate.go

@@ -133,18 +133,28 @@ func GetVersion(ctx context.Context, beta bool, version string) (newVersion, sit
 		if err != nil {
 			return "", siteURL, fmt.Errorf("failed to get list of releases: %w", err)
 		}
-		reSubver := fmt.Sprintf(`href="\./%s\.\d+/"`, regexp.QuoteMeta(newVersion))
-		allSubvers := regexp.MustCompile(reSubver).FindAllString(string(html), -1)
-		if allSubvers == nil {
-			return "", siteURL, errors.New("could not find the minor release")
+		newVersion, err = latestMicroRelease(html, newVersion)
+		if err != nil {
+			return "", siteURL, err
 		}
-		// Use the fact that releases in the index are sorted by date
-		lastSubver := allSubvers[len(allSubvers)-1]
-		newVersion = lastSubver[8 : len(lastSubver)-2]
 	}
 	return
 }
 
+// latestMicroRelease returns the latest micro release of the
+// major.minor release version linked to from the HTML index of
+// releases.
+func latestMicroRelease(html []byte, version string) (string, error) {
+	// The links may or may not start with ./ depending on what made the index
+	reSubver := fmt.Sprintf(`href="(?:\./)?(%s\.\d+)/"`, regexp.QuoteMeta(version))
+	allSubvers := regexp.MustCompile(reSubver).FindAllSubmatch(html, -1)
+	if allSubvers == nil {
+		return "", errors.New("could not find the minor release")
+	}
+	// Use the fact that releases in the index are sorted by date
+	return string(allSubvers[len(allSubvers)-1][1]), nil
+}
+
 // InstallUpdate performs rclone self-update
 func InstallUpdate(ctx context.Context, opt *Options) error {
 	// Find the latest release number

cmd/selfupdate/selfupdate_test.go

@@ -45,6 +45,50 @@ func TestGetVersion(t *testing.T) {
 	assert.Equal(t, "v1.52.3", resultVer)
 }
 
+func TestLatestMicroRelease(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		html string
+		want string
+	}{
+		{
+			name: "caddy",
+			html: `<a href="./v1.51.0/">v1.51.0/</a>
+<a href="./v1.52.0/">v1.52.0/</a>
+<a href="./v1.52.3/">v1.52.3/</a>
+<a href="./v1.53.0/">v1.53.0/</a>
+<a href="./v1.52-DEV/">v1.52-DEV/</a>
+<a href="./v1.52.3/rclone-v1.52.3-linux-amd64.zip">rclone</a>`,
+			want: "v1.52.3",
+		},
+		{
+			name: "no prefix",
+			html: `<a href="v1.51.0/">v1.51.0/</a>
+<a href="v1.52.0/">v1.52.0/</a>
+<a href="v1.52.3/">v1.52.3/</a>
+<a href="v1.53.0/">v1.53.0/</a>`,
+			want: "v1.52.3",
+		},
+		{
+			name: "other prefix",
+			html: `<a href="../v1.52.0/">v1.52.0/</a>
+<a href="https://example.com/v1.52.1/">v1.52.1/</a>
+<a href="v1.52.2/index.html">v1.52.2/</a>
+<a href="v1x52.3/">v1x52.3/</a>`,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := latestMicroRelease([]byte(test.html), "v1.52")
+			if test.want == "" {
+				assert.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+			assert.Equal(t, test.want, got)
+		})
+	}
+}
+
 func TestInstallOnLinux(t *testing.T) {
 	testy.SkipUnreliable(t)
 	if runtime.GOOS != "linux" {