Warp/apps/desktop/internal/shell/discover.go

package shell

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
)

// Discover turns what someone typed on the first-run page into a homeserver
// base URL, the way Matrix clients do (spec: client-server API, server
// discovery). The input is a server name ("example.org") or a URL; a
// /.well-known/matrix/client on that host may delegate to another base URL;
// and whatever comes out has to answer /_matrix/client/versions.
//
// The errors are written for the person who typed the input.
func Discover(ctx context.Context, c *http.Client, input string) (string, error) {
	input = strings.TrimSpace(input)
	if input == "" {
		return "", errors.New("Enter a server name, like example.org, or its address.")
	}
	raw := input
	if !strings.Contains(raw, "://") {
		raw = "https://" + raw
	}
	u, err := parseBase(raw)
	if err != nil {
		return "", fmt.Errorf("“%s” isn’t a server name or address.", input)
	}

	delegated, err := wellKnown(ctx, c, u)
	if err != nil {
		return "", err
	}
	if delegated == nil || *delegated == *u {
		if err := checkVersions(ctx, c, u); err != nil {
			return "", err
		}
		return u.String(), nil
	}
	err = checkVersions(ctx, c, delegated)
	if err == nil {
		return delegated.String(), nil
	}
	// A server whose well-known names an address that doesn't work (a
	// development server advertising its public name, say) is still
	// usable at the address that was typed.
	if checkVersions(ctx, c, u) == nil {
		return u.String(), nil
	}
	return "", fmt.Errorf("%s points to %s. %w", u.Host, delegated, err)
}

// parseBase accepts an absolute http(s) URL with no query or fragment and
// returns it without a trailing slash.
func parseBase(raw string) (*url.URL, error) {
	u, err := url.Parse(raw)
	if err != nil {
		return nil, err
	}
	if (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" || u.User != nil ||
		u.RawQuery != "" || u.Fragment != "" {
		return nil, errors.New("not an http(s) base URL")
	}
	u.Path = strings.TrimRight(u.Path, "/")
	u.RawPath = ""
	return u, nil
}

// wellKnown returns the base URL the host delegates to, or nil when it
// doesn't say (no document, or one without m.homeserver). A host that can't
// be reached at all is not an error here: the versions check reports it.
func wellKnown(ctx context.Context, c *http.Client, u *url.URL) (*url.URL, error) {
	wk := url.URL{Scheme: u.Scheme, Host: u.Host, Path: "/.well-known/matrix/client"}
	body, status, err := get(ctx, c, wk.String())
	if err != nil || status != http.StatusOK {
		return nil, nil
	}
	var doc struct {
		Homeserver *struct {
			BaseURL string `json:"base_url"`
		} `json:"m.homeserver"`
	}
	if json.Unmarshal(body, &doc) != nil || doc.Homeserver == nil {
		return nil, nil
	}
	base, err := parseBase(doc.Homeserver.BaseURL)
	if err != nil {
		return nil, fmt.Errorf("%s points to “%s”, which isn’t a valid homeserver address.", u.Host, doc.Homeserver.BaseURL)
	}
	return base, nil
}

func checkVersions(ctx context.Context, c *http.Client, base *url.URL) error {
	body, status, err := get(ctx, c, base.String()+"/_matrix/client/versions")
	if err != nil {
		if ctx.Err() != nil {
			return fmt.Errorf("%s took too long to answer.", base.Host)
		}
		return fmt.Errorf("Can’t reach %s.", base.Host)
	}
	var v struct {
		Versions []string `json:"versions"`
	}
	if status != http.StatusOK || json.Unmarshal(body, &v) != nil || len(v.Versions) == 0 {
		return fmt.Errorf("%s doesn’t look like a Matrix homeserver.", base.Host)
	}
	return nil
}

func get(ctx context.Context, c *http.Client, u string) ([]byte, int, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
	if err != nil {
		return nil, 0, err
	}
	req.Header.Set("Accept", "application/json")
	res, err := c.Do(req)
	if err != nil {
		return nil, 0, err
	}
	defer res.Body.Close()
	body, err := io.ReadAll(io.LimitReader(res.Body, 64<<10))
	return body, res.StatusCode, err
}