func install()

in internal/version/version.go [80:134]
53 lines of code
17 McCabe index (conditional complexity)

func install(targetDir, version string) error {
	if _, err := os.Stat(filepath.Join(targetDir, unpackedOkay)); err == nil {
		log.Printf("%s: already downloaded in %v", version, targetDir)
		return nil
	}

	if err := os.MkdirAll(targetDir, 0755); err != nil {
		return err
	}
	goURL := versionArchiveURL(version)
	res, err := http.Head(goURL)
	if err != nil {
		return err
	}
	if res.StatusCode == http.StatusNotFound {
		return fmt.Errorf("no binary release of %v for %v/%v at %v", version, getOS(), runtime.GOARCH, goURL)
	}
	if res.StatusCode != http.StatusOK {
		return fmt.Errorf("server returned %v checking size of %v", http.StatusText(res.StatusCode), goURL)
	}
	base := path.Base(goURL)
	archiveFile := filepath.Join(targetDir, base)
	if fi, err := os.Stat(archiveFile); err != nil || fi.Size() != res.ContentLength {
		if err != nil && !os.IsNotExist(err) {
			// Something weird. Don't try to download.
			return err
		}
		if err := copyFromURL(archiveFile, goURL); err != nil {
			return fmt.Errorf("error downloading %v: %v", goURL, err)
		}
		fi, err = os.Stat(archiveFile)
		if err != nil {
			return err
		}
		if fi.Size() != res.ContentLength {
			return fmt.Errorf("downloaded file %s size %v doesn't match server size %v", archiveFile, fi.Size(), res.ContentLength)
		}
	}
	wantSHA, err := slurpURLToString(goURL + ".sha256")
	if err != nil {
		return err
	}
	if err := verifySHA256(archiveFile, strings.TrimSpace(wantSHA)); err != nil {
		return fmt.Errorf("error verifying SHA256 of %v: %v", archiveFile, err)
	}
	log.Printf("Unpacking %v ...", archiveFile)
	if err := unpackArchive(targetDir, archiveFile); err != nil {
		return fmt.Errorf("extracting archive %v: %v", archiveFile, err)
	}
	if err := ioutil.WriteFile(filepath.Join(targetDir, unpackedOkay), nil, 0644); err != nil {
		return err
	}
	log.Printf("Success. You may now run '%v'", version)
	return nil
}