deadlinks/client.go

package deadlinks

import (
	"context"
	"errors"
	"fmt"
	"io"

	"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)
	}

	// 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.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 (TODO)
func NewClient(opts *ClientOpts) Client {
	return &client{*opts.withDefaults()}
}

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.
		return "", res.Body, 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,
		)
	}
}

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)
	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)
}