at-container-registry/pkg/appview/storage/proxy_blob_store.go

package storage

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"sync"
	"time"

	"atcr.io/pkg/atproto"
	"atcr.io/pkg/auth"
	"github.com/distribution/distribution/v3"
	"github.com/opencontainers/go-digest"
)

const (
	// maxChunkSize is the maximum buffer size before flushing to hold service
	// Matches S3's minimum multipart upload size
	maxChunkSize = 10 * 1024 * 1024 // 10MB
)

// Global upload tracking (shared across all ProxyBlobStore instances)
// This is necessary because distribution creates new repository/blob store instances per request
var (
	globalUploads   = make(map[string]*ProxyBlobWriter)
	globalUploadsMu sync.RWMutex
)

// ProxyBlobStore proxies blob requests to an external storage service
type ProxyBlobStore struct {
	storageEndpoint string
	httpClient      *http.Client
	did             string
	database        DatabaseMetrics
	repository      string
	authorizer      auth.HoldAuthorizer
	holdDID         string
}

// NewProxyBlobStore creates a new proxy blob store
func NewProxyBlobStore(storageEndpoint, did string, database DatabaseMetrics, repository string, authorizer auth.HoldAuthorizer) *ProxyBlobStore {
	// Convert storage endpoint URL to did:web DID for authorization
	holdDID := atproto.ResolveHoldDIDFromURL(storageEndpoint)
	fmt.Printf("DEBUG [proxy_blob_store]: NewProxyBlobStore created with endpoint=%s, holdDID=%s, userDID=%s, repo=%s\n",
		storageEndpoint, holdDID, did, repository)

	return &ProxyBlobStore{
		storageEndpoint: storageEndpoint,
		httpClient: &http.Client{
			Timeout: 5 * time.Minute, // Timeout for presigned URL requests and uploads
			Transport: &http.Transport{
				DisableKeepAlives:   false, // Re-enable keep-alive
				MaxIdleConns:        100,
				MaxIdleConnsPerHost: 100,
				MaxConnsPerHost:     0, // unlimited
				IdleConnTimeout:     90 * time.Second,
			},
		},
		did:        did,
		database:   database,
		repository: repository,
		authorizer: authorizer,
		holdDID:    holdDID,
	}
}

// checkReadAccess verifies the user has read access to the hold
func (p *ProxyBlobStore) checkReadAccess(ctx context.Context) error {
	if p.authorizer == nil {
		// No authorizer configured - allow access (backward compatibility)
		return nil
	}

	hasAccess, err := p.authorizer.CheckReadAccess(ctx, p.holdDID, p.did)
	if err != nil {
		return fmt.Errorf("authorization check failed: %w", err)
	}

	if !hasAccess {
		return distribution.ErrBlobUnknown // Return same error as missing blob for security
	}

	return nil
}

// checkWriteAccess verifies the user has write access to the hold
func (p *ProxyBlobStore) checkWriteAccess(ctx context.Context) error {
	if p.authorizer == nil {
		// No authorizer configured - allow access (backward compatibility)
		return nil
	}

	hasAccess, err := p.authorizer.CheckWriteAccess(ctx, p.holdDID, p.did)
	if err != nil {
		return fmt.Errorf("authorization check failed: %w", err)
	}

	if !hasAccess {
		return fmt.Errorf("write access denied to hold %s", p.holdDID)
	}

	return nil
}

// Stat returns the descriptor for a blob
func (p *ProxyBlobStore) Stat(ctx context.Context, dgst digest.Digest) (distribution.Descriptor, error) {
	// Check read access
	if err := p.checkReadAccess(ctx); err != nil {
		return distribution.Descriptor{}, err
	}

	// Get presigned HEAD URL
	url, err := p.getHeadURL(ctx, dgst)
	if err != nil {
		return distribution.Descriptor{}, distribution.ErrBlobUnknown
	}

	// Make HEAD request to presigned URL
	req, err := http.NewRequestWithContext(ctx, "HEAD", url, nil)
	if err != nil {
		return distribution.Descriptor{}, distribution.ErrBlobUnknown
	}

	resp, err := p.httpClient.Do(req)
	if err != nil {
		return distribution.Descriptor{}, distribution.ErrBlobUnknown
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return distribution.Descriptor{}, distribution.ErrBlobUnknown
	}

	// Return a minimal descriptor with size from Content-Length if available
	size := int64(0)
	if contentLength := resp.Header.Get("Content-Length"); contentLength != "" {
		fmt.Sscanf(contentLength, "%d", &size)
	}

	return distribution.Descriptor{
		Digest:    dgst,
		Size:      size,
		MediaType: "application/octet-stream",
	}, nil
}

// Get retrieves a blob
func (p *ProxyBlobStore) Get(ctx context.Context, dgst digest.Digest) ([]byte, error) {
	// Check read access
	if err := p.checkReadAccess(ctx); err != nil {
		return nil, err
	}

	url, err := p.getDownloadURL(ctx, dgst)
	if err != nil {
		return nil, err
	}

	// Download the blob
	resp, err := http.Get(url)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, distribution.ErrBlobUnknown
	}

	return io.ReadAll(resp.Body)
}

// Open returns a reader for a blob
func (p *ProxyBlobStore) Open(ctx context.Context, dgst digest.Digest) (io.ReadSeekCloser, error) {
	// Check read access
	if err := p.checkReadAccess(ctx); err != nil {
		return nil, err
	}

	url, err := p.getDownloadURL(ctx, dgst)
	if err != nil {
		return nil, err
	}

	// Download the blob
	resp, err := http.Get(url)
	if err != nil {
		return nil, err
	}

	if resp.StatusCode != http.StatusOK {
		resp.Body.Close()
		return nil, distribution.ErrBlobUnknown
	}

	// Wrap in a ReadSeekCloser
	return &readSeekCloser{
		ReadCloser: resp.Body,
	}, nil
}

// Put stores a blob using the multipart upload flow
// This ensures all uploads go through the same XRPC path
func (p *ProxyBlobStore) Put(ctx context.Context, mediaType string, content []byte) (distribution.Descriptor, error) {
	// Check write access (fast-fail before starting multipart upload)
	if err := p.checkWriteAccess(ctx); err != nil {
		return distribution.Descriptor{}, err
	}

	// Calculate digest
	dgst := digest.FromBytes(content)

	// Use Create() flow for all uploads (goes through multipart XRPC endpoints)
	writer, err := p.Create(ctx)
	if err != nil {
		fmt.Printf("[proxy_blob_store/Put] Failed to create writer: %v\n", err)
		return distribution.Descriptor{}, err
	}

	// Write the content
	if _, err := writer.Write(content); err != nil {
		writer.Cancel(ctx)
		fmt.Printf("[proxy_blob_store/Put] Failed to write content: %v\n", err)
		return distribution.Descriptor{}, err
	}

	// Commit with the calculated digest
	desc, err := writer.Commit(ctx, distribution.Descriptor{
		Digest:    dgst,
		Size:      int64(len(content)),
		MediaType: mediaType,
	})
	if err != nil {
		fmt.Printf("[proxy_blob_store/Put] Failed to commit: %v\n", err)
		return distribution.Descriptor{}, err
	}

	fmt.Printf("[proxy_blob_store/Put] Upload successful: digest=%s, size=%d\n", dgst, len(content))
	return desc, nil
}

// Delete removes a blob
func (p *ProxyBlobStore) Delete(ctx context.Context, dgst digest.Digest) error {
	// Not implemented - storage service would need a delete endpoint
	return fmt.Errorf("delete not supported for proxy blob store")
}

// ServeBlob serves a blob via HTTP redirect
func (p *ProxyBlobStore) ServeBlob(ctx context.Context, w http.ResponseWriter, r *http.Request, dgst digest.Digest) error {
	// Check read access
	if err := p.checkReadAccess(ctx); err != nil {
		return err
	}

	// For HEAD requests, redirect to presigned HEAD URL
	if r.Method == http.MethodHead {
		url, err := p.getHeadURL(ctx, dgst)
		if err != nil {
			return err
		}

		// Redirect to presigned HEAD URL
		http.Redirect(w, r, url, http.StatusTemporaryRedirect)
		return nil
	}

	// For GET requests, redirect to presigned URL
	url, err := p.getDownloadURL(ctx, dgst)
	if err != nil {
		return err
	}

	// Redirect to presigned URL
	http.Redirect(w, r, url, http.StatusTemporaryRedirect)
	return nil
}

// Create returns a blob writer for uploading using multipart upload
func (p *ProxyBlobStore) Create(ctx context.Context, options ...distribution.BlobCreateOption) (distribution.BlobWriter, error) {
	// Check write access
	if err := p.checkWriteAccess(ctx); err != nil {
		return nil, err
	}

	// Parse options
	var opts distribution.CreateOptions
	for _, option := range options {
		if err := option.Apply(&opts); err != nil {
			return nil, err
		}
	}

	// Generate unique writer ID
	writerID := fmt.Sprintf("upload-%d", time.Now().UnixNano())

	// Use temp digest for upload location (will be moved to final digest on commit)
	tempDigest := fmt.Sprintf("uploads/temp-%s", writerID)

	// Start multipart upload via hold service
	uploadID, err := p.startMultipartUpload(ctx, tempDigest)
	if err != nil {
		return nil, fmt.Errorf("failed to start multipart upload: %w", err)
	}

	fmt.Printf("   Started multipart upload: uploadID=%s\n", uploadID)

	writer := &ProxyBlobWriter{
		store:      p,
		options:    opts,
		uploadID:   uploadID,
		parts:      make([]CompletedPart, 0),
		partNumber: 1,
		buffer:     bytes.NewBuffer(make([]byte, 0, maxChunkSize)),
		id:         writerID,
		startedAt:  time.Now(),
	}

	// Store in global uploads map for resume support
	globalUploadsMu.Lock()
	globalUploads[writer.id] = writer
	globalUploadsMu.Unlock()

	return writer, nil
}

// Resume returns a blob writer for resuming an upload
func (p *ProxyBlobStore) Resume(ctx context.Context, id string) (distribution.BlobWriter, error) {
	// Retrieve upload from global map
	globalUploadsMu.RLock()
	writer, ok := globalUploads[id]
	globalUploadsMu.RUnlock()

	if !ok {
		return nil, distribution.ErrBlobUploadUnknown
	}

	// Just return the writer - parts are buffered and flushed on demand
	return writer, nil
}

// getDownloadURL returns the XRPC getBlob URL for downloading a blob
// The hold service will redirect to a presigned S3 URL
func (p *ProxyBlobStore) getDownloadURL(ctx context.Context, dgst digest.Digest) (string, error) {
	// Use XRPC endpoint: GET /xrpc/com.atproto.sync.getBlob?did={holdDID}&cid={digest}
	// Per migration doc: hold accepts OCI digest directly as cid parameter (checks for sha256: prefix)
	url := fmt.Sprintf("%s/xrpc/com.atproto.sync.getBlob?did=%s&cid=%s",
		p.storageEndpoint, p.holdDID, dgst.String())
	return url, nil
}

// getHeadURL returns the XRPC getBlob URL for HEAD requests
// The hold service will redirect to a presigned S3 URL
func (p *ProxyBlobStore) getHeadURL(ctx context.Context, dgst digest.Digest) (string, error) {
	// Same as GET - hold service handles HEAD method on getBlob endpoint
	url := fmt.Sprintf("%s/xrpc/com.atproto.sync.getBlob?did=%s&cid=%s",
		p.storageEndpoint, p.holdDID, dgst.String())
	return url, nil
}

// getUploadURL is deprecated - single blob uploads should use Create() instead
// XRPC migration: No direct presigned upload URL endpoint, use multipart flow for all uploads
func (p *ProxyBlobStore) getUploadURL(ctx context.Context, dgst digest.Digest, size int64) (string, error) {
	return "", fmt.Errorf("single blob upload via Put() not supported with XRPC endpoints - use Create() instead")
}

// startMultipartUpload initiates a multipart upload via XRPC uploadBlob endpoint
func (p *ProxyBlobStore) startMultipartUpload(ctx context.Context, digest string) (string, error) {
	reqBody := map[string]any{
		"action": "start",
		"digest": digest,
	}

	body, err := json.Marshal(reqBody)
	if err != nil {
		return "", err
	}

	url := fmt.Sprintf("%s/xrpc/com.atproto.repo.uploadBlob", p.storageEndpoint)
	req, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewReader(body))
	if err != nil {
		return "", err
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := p.httpClient.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		bodyBytes, _ := io.ReadAll(resp.Body)
		return "", fmt.Errorf("start multipart failed: status %d, body: %s", resp.StatusCode, string(bodyBytes))
	}

	var result struct {
		UploadID string `json:"uploadId"`
		Mode     string `json:"mode"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
		return "", err
	}

	return result.UploadID, nil
}

// PartUploadInfo contains structured information for uploading a part
type PartUploadInfo struct {
	URL     string            `json:"url"`
	Method  string            `json:"method,omitempty"`
	Headers map[string]string `json:"headers,omitempty"`
}

// getPartUploadInfo gets structured upload info for uploading a specific part via XRPC
func (p *ProxyBlobStore) getPartUploadInfo(ctx context.Context, digest, uploadID string, partNumber int) (*PartUploadInfo, error) {
	reqBody := map[string]any{
		"action":     "part",
		"uploadId":   uploadID,
		"partNumber": partNumber,
		"digest":     digest,
	}

	body, err := json.Marshal(reqBody)
	if err != nil {
		return nil, err
	}

	url := fmt.Sprintf("%s/xrpc/com.atproto.repo.uploadBlob", p.storageEndpoint)
	req, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := p.httpClient.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		bodyBytes, _ := io.ReadAll(resp.Body)
		return nil, fmt.Errorf("get part URL failed: status %d, body: %s", resp.StatusCode, string(bodyBytes))
	}

	var uploadInfo PartUploadInfo
	if err := json.NewDecoder(resp.Body).Decode(&uploadInfo); err != nil {
		return nil, err
	}

	return &uploadInfo, nil
}

// completeMultipartUpload completes a multipart upload via XRPC uploadBlob endpoint
// The XRPC complete action handles the move from temp to final location internally
func (p *ProxyBlobStore) completeMultipartUpload(ctx context.Context, digest, uploadID string, parts []CompletedPart) error {
	// Convert parts to XRPC format (partNumber instead of part_number)
	xrpcParts := make([]map[string]any, len(parts))
	for i, part := range parts {
		xrpcParts[i] = map[string]any{
			"partNumber": part.PartNumber,
			"etag":       part.ETag,
		}
	}

	reqBody := map[string]any{
		"action":   "complete",
		"uploadId": uploadID,
		"digest":   digest,
		"parts":    xrpcParts,
	}

	body, err := json.Marshal(reqBody)
	if err != nil {
		return err
	}

	url := fmt.Sprintf("%s/xrpc/com.atproto.repo.uploadBlob", p.storageEndpoint)
	req, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewReader(body))
	if err != nil {
		return err
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := p.httpClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		bodyBytes, _ := io.ReadAll(resp.Body)
		return fmt.Errorf("complete multipart failed: status %d, body: %s", resp.StatusCode, string(bodyBytes))
	}

	return nil
}

// abortMultipartUpload aborts a multipart upload via XRPC uploadBlob endpoint
func (p *ProxyBlobStore) abortMultipartUpload(ctx context.Context, digest, uploadID string) error {
	reqBody := map[string]any{
		"action":   "abort",
		"uploadId": uploadID,
		"digest":   digest,
	}

	body, err := json.Marshal(reqBody)
	if err != nil {
		return err
	}

	url := fmt.Sprintf("%s/xrpc/com.atproto.repo.uploadBlob", p.storageEndpoint)
	req, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewReader(body))
	if err != nil {
		return err
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := p.httpClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		bodyBytes, _ := io.ReadAll(resp.Body)
		return fmt.Errorf("abort multipart failed: status %d, body: %s", resp.StatusCode, string(bodyBytes))
	}

	return nil
}

// CompletedPart represents an uploaded part with its ETag
type CompletedPart struct {
	PartNumber int    `json:"part_number"`
	ETag       string `json:"etag"`
}

// ProxyBlobWriter implements distribution.BlobWriter for proxy uploads using multipart upload
type ProxyBlobWriter struct {
	store       *ProxyBlobStore
	options     distribution.CreateOptions
	uploadID    string          // S3 multipart upload ID
	parts       []CompletedPart // Track uploaded parts with ETags
	partNumber  int             // Current part number (starts at 1)
	buffer      *bytes.Buffer   // Buffer for current part
	size        int64           // Total bytes written
	closed      bool
	id          string // Distribution's upload ID (for state)
	startedAt   time.Time
	finalDigest string // Set on Commit
}

// ID returns the upload ID
func (w *ProxyBlobWriter) ID() string {
	return w.id
}

// StartedAt returns when the upload started
func (w *ProxyBlobWriter) StartedAt() time.Time {
	return w.startedAt
}

// Write writes data to the upload
// Buffers data and flushes when buffer reaches 5MB
func (w *ProxyBlobWriter) Write(p []byte) (int, error) {
	if w.closed {
		return 0, fmt.Errorf("writer closed")
	}

	n, err := w.buffer.Write(p)
	w.size += int64(n)

	// Flush if buffer reaches limit (S3 part size)
	if w.buffer.Len() >= maxChunkSize {
		if err := w.flushPart(); err != nil {
			return n, err
		}
	}

	return n, err
}

// flushPart uploads the current buffer as a part
func (w *ProxyBlobWriter) flushPart() error {
	if w.buffer.Len() == 0 {
		return nil
	}

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
	defer cancel()

	// Get structured upload info for this part
	tempDigest := fmt.Sprintf("uploads/temp-%s", w.id)
	uploadInfo, err := w.store.getPartUploadInfo(ctx, tempDigest, w.uploadID, w.partNumber)
	if err != nil {
		return fmt.Errorf("failed to get part upload info: %w", err)
	}

	// Determine HTTP method (default to PUT)
	method := uploadInfo.Method
	if method == "" {
		method = "PUT"
	}

	// Upload part (either to S3 presigned URL or back to XRPC with headers)
	req, err := http.NewRequestWithContext(ctx, method, uploadInfo.URL, bytes.NewReader(w.buffer.Bytes()))
	if err != nil {
		return err
	}
	req.Header.Set("Content-Type", "application/octet-stream")

	// Apply any additional headers from the response (for buffered mode)
	for key, value := range uploadInfo.Headers {
		req.Header.Set(key, value)
	}

	resp, err := w.store.httpClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusCreated {
		bodyBytes, _ := io.ReadAll(resp.Body)
		return fmt.Errorf("part upload failed: status %d, body: %s", resp.StatusCode, string(bodyBytes))
	}

	// Store ETag for completion
	// For buffered mode, ETag might be in JSON response body
	etag := resp.Header.Get("ETag")
	if etag == "" {
		// Try to parse JSON response for buffered mode
		var result struct {
			ETag string `json:"etag"`
		}
		if err := json.NewDecoder(resp.Body).Decode(&result); err == nil && result.ETag != "" {
			etag = result.ETag
		} else {
			return fmt.Errorf("no ETag in response")
		}
	}

	w.parts = append(w.parts, CompletedPart{
		PartNumber: w.partNumber,
		ETag:       etag,
	})

	fmt.Printf("[flushPart] Part %d uploaded successfully: ETag=%s\n", w.partNumber, etag)

	// Reset buffer and increment part number
	w.buffer.Reset()
	w.partNumber++

	return nil
}

// ReadFrom reads from a reader
func (w *ProxyBlobWriter) ReadFrom(r io.Reader) (int64, error) {
	if w.closed {
		return 0, fmt.Errorf("writer closed")
	}

	// Read in chunks and flush when needed
	buf := make([]byte, 32*1024) // 32KB read buffer
	var total int64

	for {
		nr, err := r.Read(buf)
		if nr > 0 {
			nw, werr := w.Write(buf[:nr])
			total += int64(nw)
			if werr != nil {
				return total, werr
			}
		}
		if err == io.EOF {
			break
		}
		if err != nil {
			return total, err
		}
	}

	return total, nil
}

// Size returns the current size
func (w *ProxyBlobWriter) Size() int64 {
	return w.size
}

// Commit finalizes the upload by completing multipart upload and moving to final location
func (w *ProxyBlobWriter) Commit(ctx context.Context, desc distribution.Descriptor) (distribution.Descriptor, error) {
	if w.closed {
		return distribution.Descriptor{}, fmt.Errorf("writer closed")
	}
	w.closed = true

	// Remove from global uploads map
	globalUploadsMu.Lock()
	delete(globalUploads, w.id)
	globalUploadsMu.Unlock()

	// Flush any remaining buffered data
	if w.buffer.Len() > 0 {
		fmt.Printf("[Commit] Flushing final buffer: %d bytes\n", w.buffer.Len())
		if err := w.flushPart(); err != nil {
			// Try to abort multipart on error
			tempDigest := fmt.Sprintf("uploads/temp-%s", w.id)
			w.store.abortMultipartUpload(ctx, tempDigest, w.uploadID)
			return distribution.Descriptor{}, fmt.Errorf("failed to flush final part: %w", err)
		}
	}

	// Complete multipart upload - XRPC complete action handles move internally
	tempDigest := fmt.Sprintf("uploads/temp-%s", w.id)
	fmt.Printf("🔒 [Commit] Completing multipart upload: uploadID=%s, parts=%d\n", w.uploadID, len(w.parts))
	if err := w.store.completeMultipartUpload(ctx, tempDigest, w.uploadID, w.parts); err != nil {
		return distribution.Descriptor{}, fmt.Errorf("failed to complete multipart upload: %w", err)
	}

	fmt.Printf("[Commit] Upload completed successfully: digest=%s, size=%d, parts=%d\n", desc.Digest, w.size, len(w.parts))

	return distribution.Descriptor{
		Digest:    desc.Digest,
		Size:      w.size,
		MediaType: desc.MediaType,
	}, nil
}

// Cancel cancels the upload by aborting the multipart upload
func (w *ProxyBlobWriter) Cancel(ctx context.Context) error {
	w.closed = true

	fmt.Printf("[Cancel] Cancelling upload: id=%s\n", w.id)

	// Remove from global uploads map
	globalUploadsMu.Lock()
	delete(globalUploads, w.id)
	globalUploadsMu.Unlock()

	// Abort multipart upload
	tempDigest := fmt.Sprintf("uploads/temp-%s", w.id)
	if err := w.store.abortMultipartUpload(ctx, tempDigest, w.uploadID); err != nil {
		fmt.Printf("⚠️  [Cancel] Failed to abort multipart upload: %v\n", err)
		// Continue anyway - we want to mark upload as cancelled
	}

	fmt.Printf("[Cancel] Upload cancelled: id=%s\n", w.id)
	return nil
}

// Close closes the writer
// Parts are flushed on demand, so this is a no-op
func (w *ProxyBlobWriter) Close() error {
	// Don't set w.closed = true - allow resuming for next PATCH
	return nil
}

// readSeekCloser wraps an io.ReadCloser to implement ReadSeekCloser
type readSeekCloser struct {
	io.ReadCloser
}

func (r *readSeekCloser) Seek(offset int64, whence int) (int64, error) {
	// Not implemented - would need buffering or re-downloading
	return 0, fmt.Errorf("seek not supported")
}