deadlinks/client.go

package deadlinks

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"net/http"

	"git.sr.ht/~adnano/go-gemini"
)

// Client is a thread-safe type which fetches a resource at the given URL,
// returning its MIME type and body. If the MIME type is not known then empty
// string should be returned.
type Client interface {
	Get(context.Context, URL) (string, io.ReadCloser, error)
}

// ClientOpts are optional fields which can be provided to NewClient. A nil
// ClientOpts is equivalent to an empty one.
type ClientOpts struct {
	// GeminiClient will be used for retrieving resources via the gemini
	// protocol.
	//
	// Defaults to `new(gemini.Client)`.
	GeminiClient interface {
		Do(context.Context, *gemini.Request) (*gemini.Response, error)
	}

	// HTTPClient will be used for retrieving resources via the http protocol.
	//
	// Defaults to `new(http.Client)`.
	HTTPClient interface {
		Do(*http.Request) (*http.Response, error)
	}

	// HTTPUserAgent overwrites the user agent used by the HTTPClient.
	//
	// Defaults to whatever http.Client uses by default.
	HTTPUserAgent string

	// MaxRedirects indicates the maximum number of redirects which will be
	// allowed when resolving a resource. A negative value indicates no
	// redirects are allowed.
	//
	// Default: 10.
	MaxRedirects int
}

func (o *ClientOpts) withDefaults() *ClientOpts {
	if o == nil {
		o = new(ClientOpts)
	}

	if o.GeminiClient == nil {
		o.GeminiClient = new(gemini.Client)
	}

	if o.HTTPClient == nil {
		o.HTTPClient = &http.Client{
			Transport: http.DefaultTransport.(*http.Transport).Clone(),
		}
	}

	if o.MaxRedirects == 0 {
		o.MaxRedirects = 10
	}

	return o
}

type client struct {
	opts ClientOpts
}

// NewClient initializes and returns a Client which supports commonly used
// transport protocols. The returned Client will error when it encounters an
// unfamiliar protocol.
//
// Supported URL schemas:
// - gemini
// - http/https
//
// Some schemas automatically return success:
// - mailto
// - data
func NewClient(opts *ClientOpts) Client {
	return &client{*opts.withDefaults()}
}

func emptyReadCloser() io.ReadCloser {
	return io.NopCloser(new(bytes.Buffer))
}

func (c *client) getGemini(
	ctx context.Context, url URL, redirectDepth int,
) (
	string, io.ReadCloser, error,
) {
	req, err := gemini.NewRequest(string(url))
	if err != nil {
		return "", nil, fmt.Errorf("building request: %w", err)
	}

	// TODO allow specifying client cert

	res, err := c.opts.GeminiClient.Do(ctx, req)
	if err != nil {
		return "", nil, fmt.Errorf("performing request: %w", err)
	}

	// all status numbers are grouped by their first digit, and actions taken
	// can be entirely based on that.
	switch res.Status / 10 {

	case 1: // input required
		// Assume that input required is fine, even though we don't know the
		// MIME type.
		defer res.Body.Close()
		return "", emptyReadCloser(), nil

	case 2: // success
		return res.Meta, res.Body, nil

	case 3: // redirect
		defer res.Body.Close()
		if redirectDepth >= c.opts.MaxRedirects {
			return "", nil, errors.New("too many redirects")
		}

		metaURL, err := ParseURL(res.Meta)
		if err != nil {
			return "", nil, fmt.Errorf("parsing redirect URL %q: %w", res.Meta, err)
		}

		newURL := url.ResolveReference(metaURL)
		return c.get(ctx, newURL, redirectDepth+1)

	default:
		defer res.Body.Close()
		return "", nil, fmt.Errorf(
			"response code %d (%v): %q", res.Status, res.Status, res.Meta,
		)
	}
}

type concatHTTPBody struct {
	orig  io.ReadCloser
	multi io.Reader
}

func (b concatHTTPBody) Read(bb []byte) (int, error) {
	return b.multi.Read(bb)
}

func (b concatHTTPBody) Close() error {
	return b.orig.Close()
}

func httpResponseMIMEType(res *http.Response) (string, error) {
	if t := res.Header.Get("Content-Type"); t != "" {
		return t, nil
	}

	// content type header not provided, do mime type sniffing.
	// http.DetectContentType only requires up to the first 512 bytes of the
	// body, according to its documentation, so we pull that off.
	head := new(bytes.Buffer)
	_, err := io.CopyN(head, res.Body, 512)
	if err != nil && !errors.Is(err, io.EOF) {
		return "", fmt.Errorf("reading head of response body: %w", err)
	}

	mimeType := http.DetectContentType(head.Bytes())

	// since some of the body has been read off the original reader, we have to
	// re-concattenate that portion to the beginning of the stream.
	res.Body = concatHTTPBody{
		orig:  res.Body,
		multi: io.MultiReader(head, res.Body),
	}

	return mimeType, nil
}

func (c *client) getHTTP(
	ctx context.Context, url URL, redirectDepth int,
) (
	string, io.ReadCloser, error,
) {
	req, err := http.NewRequestWithContext(ctx, "GET", string(url), nil)
	if err != nil {
		return "", nil, fmt.Errorf("building request: %w", err)
	}

	if c.opts.HTTPUserAgent != "" {
		req.Header.Set("User-Agent", c.opts.HTTPUserAgent)
	}

	res, err := c.opts.HTTPClient.Do(req)
	if err != nil {
		return "", nil, fmt.Errorf("performing request: %w", err)
	}

	mimeType, err := httpResponseMIMEType(res)
	if err != nil {
		res.Body.Close()
		return "", nil, fmt.Errorf("determining response MIME type: %w", err)
	}

	statusCodeCategory := res.StatusCode / 100

	switch {
	case statusCodeCategory == 1: // informational
		defer res.Body.Close()
		return "", emptyReadCloser(), nil

	case statusCodeCategory == 2: // success
		return mimeType, res.Body, nil

	// redirects
	case res.StatusCode == 301,
		res.StatusCode == 302,
		res.StatusCode == 307,
		res.StatusCode == 308:
		defer res.Body.Close()

		loc, err := res.Location()
		if err != nil {
			return "", nil, fmt.Errorf(
				"getting Location header of response with code %v", res.StatusCode,
			)
		}

		locURL, err := ParseURL(loc.String())
		if err != nil {
			return "", nil, fmt.Errorf("parsing redirect URL %v: %w", loc, err)
		}

		newURL := url.ResolveReference(locURL)
		return c.get(ctx, newURL, redirectDepth+1)

	case statusCodeCategory == 3: // unsupported redirections
		defer res.Body.Close()
		return "", emptyReadCloser(), nil

		// all other response codes, 4xx and 5xx, are considered errors
	default:
		defer res.Body.Close()
		return "", nil, fmt.Errorf(
			"response code %d (%v)", res.StatusCode, res.Status,
		)
	}
}

func (c *client) noOpGet() (
	string, io.ReadCloser, error,
) {
	return "", emptyReadCloser(), nil
}

func (c *client) get(
	ctx context.Context, url URL, redirectDepth int,
) (
	string, io.ReadCloser, error,
) {

	scheme := url.toStd().Scheme
	switch scheme {
	case "gemini":
		return c.getGemini(ctx, url, redirectDepth)
	case "http", "https":
		return c.getHTTP(ctx, url, redirectDepth)
	case "mailto", "data":
		return c.noOpGet()
	default:
		return "", nil, fmt.Errorf("unsupported scheme %q", scheme)
	}
}

func (c *client) Get(
	ctx context.Context, url URL,
) (
	string, io.ReadCloser, error,
) {
	return c.get(ctx, url, 0)
}