[onechat] / botapi.go

package main

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

const (
        // pollSeconds is how long one getUpdates call waits for an update.
        pollSeconds = 50

        // pollSlack is how much longer than pollSeconds a getUpdates call can
        // take before the bot gives it up.
        pollSlack = 20 * time.Second

        // sendTimeout is how long a sendMessage or sendChatAction call can
        // take. A connection that stops without a close, such as after a
        // network change, then fails the call, and the sender tries again,
        // instead of a wait until TCP gives up. The client has no timeout of
        // its own, because the long poll of getUpdates needs a longer limit.
        sendTimeout = 30 * time.Second

        // botAnswerBytes is the most bytes of one answer of the Bot API.
        botAnswerBytes = 16 << 20
)

// errBotNetwork tells that a Bot API call got no valid answer, such as after a
// network error. A later call can work.
var errBotNetwork = errors.New("no valid answer from the Bot API")

// botAPI calls the Telegram Bot API. The bot token is a part of each URL, so no
// error of botAPI holds a URL, a net/http error, or the token. SendTimeout
// limits each sendMessage and sendChatAction call.
type botAPI struct {
        base        string
        token       string
        http        *http.Client
        sendTimeout time.Duration
}

// botError is an answer of the Bot API that tells of a failure. RetryAfter is
// the wait that the Bot API asks for after HTTP 429.
type botError struct {
        Method      string
        Status      int
        Description string
        RetryAfter  time.Duration
}

// botAnswer is the envelope of each answer of the Bot API.
type botAnswer struct {
        OK          bool            `json:"ok"`
        Result      json.RawMessage `json:"result"`
        ErrorCode   int             `json:"error_code"`
        Description string          `json:"description"`
        Parameters  struct {
                RetryAfter int `json:"retry_after"`
        } `json:"parameters"`
}

// botUpdate is one update of getUpdates. Message is nil for the kinds of
// updates that are not a new message.
type botUpdate struct {
        UpdateID int64       `json:"update_id"`
        Message  *botMessage `json:"message"`
}

// botMessage is a message to the bot. Text is empty for a message that is not
// text, such as a photo.
type botMessage struct {
        Chat botChat  `json:"chat"`
        From *botUser `json:"from"`
        Text string   `json:"text"`
}

// botChat is the chat of a message. Type is private for a chat with one user.
type botChat struct {
        ID   int64  `json:"id"`
        Type string `json:"type"`
}

// botUser is the sender of a message.
type botUser struct {
        ID int64 `json:"id"`
}

// newBotAPI returns the client of the Bot API at base with token.
func newBotAPI(base, token string) *botAPI {
        return &botAPI{
                base:        base,
                token:       token,
                http:        &http.Client{},
                sendTimeout: sendTimeout,
        }
}

// GetUpdates waits for the updates from offset on, up to pollSeconds, and
// returns them. Telegram forgets each update before offset. Only new messages
// are asked for.
func (b *botAPI) GetUpdates(ctx context.Context, offset int64) ([]botUpdate, error) {
        ctx, cancel := context.WithTimeout(ctx, pollSeconds*time.Second+pollSlack)
        defer cancel()

        var updates []botUpdate
        err := b.call(ctx, "getUpdates", map[string]any{
                "offset":          offset,
                "timeout":         pollSeconds,
                "allowed_updates": []string{"message"},
        }, &updates)

        return updates, err
}

// SendMessage sends text to chat as plain text, with no parse mode. A call that
// takes longer than sendTimeout fails with errBotNetwork.
func (b *botAPI) SendMessage(ctx context.Context, chat int64, text string) error {
        ctx, cancel := context.WithTimeout(ctx, b.sendTimeout)
        defer cancel()

        return b.call(ctx, "sendMessage", map[string]any{
                "chat_id": chat,
                "text":    text,
        }, nil)
}

// SendTyping shows the typing action in chat for about five seconds. A call
// that takes longer than sendTimeout fails with errBotNetwork.
func (b *botAPI) SendTyping(ctx context.Context, chat int64) error {
        ctx, cancel := context.WithTimeout(ctx, b.sendTimeout)
        defer cancel()

        return b.call(ctx, "sendChatAction", map[string]any{
                "chat_id": chat,
                "action":  "typing",
        }, nil)
}

// call sends one call of method with the JSON params and decodes its result
// into result, when result is not nil.
func (b *botAPI) call(ctx context.Context, method string, params, result any) error {
        body, err := json.Marshal(params)
        if err != nil {
                return fmt.Errorf("%s: encode request: %w", method, err)
        }

        request, err := http.NewRequestWithContext(ctx, http.MethodPost, b.base+"/bot"+b.token+"/"+method, bytes.NewReader(body))
        if err != nil {
                return fmt.Errorf("%s: make request: %s", method, b.redact(err))
        }

        request.Header.Set("Content-Type", "application/json")
        response, err := b.http.Do(request)
        if err != nil {
                return fmt.Errorf("%s: %w: %s", method, errBotNetwork, b.redact(err))
        }

        data, err := io.ReadAll(io.LimitReader(response.Body, botAnswerBytes))
        err = errors.Join(err, response.Body.Close())
        if err != nil {
                return fmt.Errorf("%s: %w: read answer: %s", method, errBotNetwork, b.redact(err))
        }

        var answer botAnswer
        err = json.Unmarshal(data, &answer)
        if err != nil && response.StatusCode != http.StatusOK {
                return &botError{
                        Method:      method,
                        Status:      response.StatusCode,
                        Description: http.StatusText(response.StatusCode),
                }
        }

        if err != nil {
                return fmt.Errorf("%s: %w: decode answer: %s", method, errBotNetwork, b.redact(err))
        }

        if !answer.OK {
                return &botError{
                        Method:      method,
                        Status:      response.StatusCode,
                        Description: b.redact(errors.New(answer.Description)),
                        RetryAfter:  time.Duration(answer.Parameters.RetryAfter) * time.Second,
                }
        }

        if result == nil {
                return nil
        }

        err = json.Unmarshal(answer.Result, result)
        if err != nil {
                return fmt.Errorf("%s: %w: decode result: %s", method, errBotNetwork, b.redact(err))
        }

        return nil
}

// redact returns the text of err without a URL and without the token. A
// *url.Error holds the URL of the call, which holds the token, so only its
// cause is kept.
func (b *botAPI) redact(err error) string {
        var urlError *url.Error
        if errors.As(err, &urlError) {
                err = urlError.Err
        }

        return strings.ReplaceAll(err.Error(), b.token, "TOKEN")
}

func (e *botError) Error() string {
        return fmt.Sprintf("%s: Bot API answered %d: %s", e.Method, e.Status, e.Description)
}

// retryable tells if a later try of a failed Bot API call can work: after a
// network error, HTTP 429, or a server error. It returns the wait that the Bot
// API asks for, or 0.
func retryable(err error) (bool, time.Duration) {
        if errors.Is(err, errBotNetwork) {
                return true, 0
        }

        var answer *botError
        if errors.As(err, &answer) {
                return answer.Status == http.StatusTooManyRequests || answer.Status >= 500, answer.RetryAfter
        }

        return false, 0
}