tapir/internal/adapters/transcribe/gateway.go

// Package transcribe turns a WAV file into a timed Swedish transcript by calling
// the LiteLLM gateway's OpenAI-compatible /audio/transcriptions endpoint
// (ADR-033). A Gateway is one model alias; a Chain tries a local one and falls
// back to a second.
package transcribe

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"mime/multipart"
	"net/http"
	"os"
	"strings"

	"git.d-ma.be/mathias/tapir/internal/domain"
)

// ErrEmptyTranscript is returned when the endpoint answered 200 but with no text.
// An empty transcript is not a success: chat over it could only answer "nothing".
var ErrEmptyTranscript = errors.New("transcribe: endpoint returned an empty transcript")

// StatusError is a non-2xx answer from the gateway. Nothing may branch on the
// body — {"error":"failed to process audio"} has had at least five distinct
// causes — so only the status code is exposed for logic; Snippet is for logs.
type StatusError struct {
	Code    int
	Snippet string
}

func (e *StatusError) Error() string {
	return fmt.Sprintf("transcribe: gateway returned HTTP %d: %s", e.Code, e.Snippet)
}

// Gateway transcribes through one model alias on the gateway. baseURL is the
// gateway's /v1 root — the in-cluster NodePort, not the public hostname, whose
// ingress rejects bodies over ~8-10 MB with 413 (docs/research/audio-length-ceiling).
type Gateway struct {
	baseURL string
	key     string
	model   string
	client  *http.Client
}

// NewGateway builds a Gateway. The client should carry no overall Timeout: a 90
// minute recording takes minutes, so the deadline belongs to the caller's context.
func NewGateway(baseURL, key, model string, client *http.Client) *Gateway {
	return &Gateway{baseURL: strings.TrimRight(baseURL, "/"), key: key, model: model, client: client}
}

// Name is the model alias; it is what gets recorded as the serving endpoint.
func (g *Gateway) Name() string { return g.model }

// Transcribe uploads the WAV at path and returns the normalised transcription.
// The body is streamed from disk with a COMPUTED Content-Length: a 90 minute WAV
// is ~170 MB, so it is neither buffered in memory nor sent chunked.
func (g *Gateway) Transcribe(ctx context.Context, path string) (domain.Transcription, error) {
	f, err := os.Open(path)
	if err != nil {
		return domain.Transcription{}, fmt.Errorf("transcribe: open audio: %w", err)
	}
	defer f.Close()
	info, err := f.Stat()
	if err != nil {
		return domain.Transcription{}, fmt.Errorf("transcribe: stat audio: %w", err)
	}

	var prefix bytes.Buffer
	mw := multipart.NewWriter(&prefix)
	for _, kv := range [][2]string{{"model", g.model}, {"response_format", "verbose_json"}, {"language", "sv"}} {
		if err := mw.WriteField(kv[0], kv[1]); err != nil {
			return domain.Transcription{}, fmt.Errorf("transcribe: build request: %w", err)
		}
	}
	// The multipart field is `file`; `audio_file` is rejected as "Invalid request".
	if _, err := mw.CreateFormFile("file", "audio.wav"); err != nil {
		return domain.Transcription{}, fmt.Errorf("transcribe: build request: %w", err)
	}
	suffix := []byte("\r\n--" + mw.Boundary() + "--\r\n")

	body := io.MultiReader(bytes.NewReader(prefix.Bytes()), f, bytes.NewReader(suffix))
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, g.baseURL+"/audio/transcriptions", body)
	if err != nil {
		return domain.Transcription{}, fmt.Errorf("transcribe: build request: %w", err)
	}
	req.ContentLength = int64(prefix.Len()) + info.Size() + int64(len(suffix))
	req.Header.Set("Content-Type", mw.FormDataContentType())
	req.Header.Set("Authorization", "Bearer "+g.key)
	// No "Expect: 100-continue": it trips the proxy for any body over 1 KB.

	resp, err := g.client.Do(req)
	if err != nil {
		return domain.Transcription{}, fmt.Errorf("transcribe: %s: %w", g.model, err)
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		snip, _ := io.ReadAll(io.LimitReader(resp.Body, 200))
		return domain.Transcription{}, &StatusError{Code: resp.StatusCode, Snippet: strings.TrimSpace(string(snip))}
	}

	t, err := parseVerbose(resp.Body)
	if err != nil {
		return domain.Transcription{}, fmt.Errorf("transcribe: %s: %w", g.model, err)
	}
	t.Endpoint = g.model
	return t, nil
}

// verboseJSON is the subset of the verbose_json response used. Both backends
// return it; they differ in segment granularity (iguana: whole seconds, berget:
// sub-second) and in the language token, neither of which matters here.
type verboseJSON struct {
	Text     string  `json:"text"`
	Duration float64 `json:"duration"`
	Segments []struct {
		Start float64 `json:"start"`
		End   float64 `json:"end"`
		Text  string  `json:"text"`
	} `json:"segments"`
}

// parseVerbose normalises a verbose_json body. Whisper's final window can end past
// the audio (9.0 s for 4.25 s of audio was observed), so segment ends are clamped
// to the reported duration.
func parseVerbose(r io.Reader) (domain.Transcription, error) {
	var v verboseJSON
	if err := json.NewDecoder(r).Decode(&v); err != nil {
		return domain.Transcription{}, fmt.Errorf("decode response: %w", err)
	}
	text := strings.TrimSpace(v.Text)
	if text == "" {
		return domain.Transcription{}, ErrEmptyTranscript
	}
	t := domain.Transcription{Text: text, AudioSeconds: v.Duration}
	for _, s := range v.Segments {
		seg := domain.Segment{Start: s.Start, End: s.End, Text: strings.TrimSpace(s.Text)}
		if v.Duration > 0 {
			seg.End = min(seg.End, v.Duration)
			seg.Start = min(seg.Start, seg.End)
		}
		t.Segments = append(t.Segments, seg)
	}
	return t, nil
}