pixa/internal/imgcache/service.go

package imgcache

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net/url"
	"time"
)

// Service implements the ImageCache interface, orchestrating cache, fetcher, and processor.
type Service struct {
	cache     *Cache
	fetcher   *HTTPFetcher
	processor Processor
	signer    *Signer
	whitelist *HostWhitelist
	log       *slog.Logger
}

// ServiceConfig holds configuration for the image service.
type ServiceConfig struct {
	// Cache is the cache instance
	Cache *Cache
	// FetcherConfig configures the upstream fetcher
	FetcherConfig *FetcherConfig
	// SigningKey is the HMAC signing key (empty disables signing)
	SigningKey string
	// Whitelist is the list of hosts that don't require signatures
	Whitelist []string
	// Logger for logging
	Logger *slog.Logger
}

// NewService creates a new image service.
func NewService(cfg *ServiceConfig) (*Service, error) {
	if cfg.Cache == nil {
		return nil, errors.New("cache is required")
	}

	fetcherCfg := cfg.FetcherConfig
	if fetcherCfg == nil {
		fetcherCfg = DefaultFetcherConfig()
	}

	var signer *Signer
	if cfg.SigningKey != "" {
		signer = NewSigner(cfg.SigningKey)
	}

	log := cfg.Logger
	if log == nil {
		log = slog.Default()
	}

	return &Service{
		cache:     cfg.Cache,
		fetcher:   NewHTTPFetcher(fetcherCfg),
		processor: NewImageProcessor(),
		signer:    signer,
		whitelist: NewHostWhitelist(cfg.Whitelist),
		log:       log,
	}, nil
}

// Get retrieves a processed image, fetching and processing if necessary.
func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, error) {
	// Check cache first
	result, err := s.cache.Lookup(ctx, req)
	if err != nil {
		if errors.Is(err, ErrNegativeCache) {
			return nil, fmt.Errorf("upstream returned error (cached)")
		}

		s.log.Warn("cache lookup failed", "error", err)
	}

	// Cache hit - serve from cache
	if result != nil && result.Hit {
		s.cache.IncrementStats(ctx, true, 0)

		reader, err := s.cache.GetOutput(result.OutputHash)
		if err != nil {
			s.log.Error("failed to get cached output", "hash", result.OutputHash, "error", err)
			// Fall through to re-fetch
		} else {
			return &ImageResponse{
				Content:       reader,
				ContentLength: -1, // Unknown until read
				ContentType:   result.ContentType,
				CacheStatus:   CacheHit,
			}, nil
		}
	}

	// Cache miss - need to fetch, process, and cache
	s.cache.IncrementStats(ctx, false, 0)

	response, err := s.fetchAndProcess(ctx, req)
	if err != nil {
		return nil, err
	}

	response.CacheStatus = CacheMiss

	return response, nil
}

// fetchAndProcess fetches from upstream, processes, and caches the result.
func (s *Service) fetchAndProcess(ctx context.Context, req *ImageRequest) (*ImageResponse, error) {
	// Fetch from upstream
	sourceURL := req.SourceURL()

	s.log.Debug("fetching from upstream", "url", sourceURL)

	fetchResult, err := s.fetcher.Fetch(ctx, sourceURL)
	if err != nil {
		// Store negative cache for certain errors
		if isNegativeCacheable(err) {
			statusCode := extractStatusCode(err)
			_ = s.cache.StoreNegative(ctx, req, statusCode, err.Error())
		}

		return nil, fmt.Errorf("upstream fetch failed: %w", err)
	}
	defer func() { _ = fetchResult.Content.Close() }()

	// Read and validate the source content
	sourceData, err := io.ReadAll(fetchResult.Content)
	if err != nil {
		return nil, fmt.Errorf("failed to read upstream response: %w", err)
	}

	// Validate magic bytes match content type
	if err := ValidateMagicBytes(sourceData, fetchResult.ContentType); err != nil {
		return nil, fmt.Errorf("content validation failed: %w", err)
	}

	// Store source content
	_, err = s.cache.StoreSource(ctx, req, bytes.NewReader(sourceData), fetchResult)
	if err != nil {
		s.log.Warn("failed to store source content", "error", err)
		// Continue even if caching fails
	}

	// Process the image
	processResult, err := s.processor.Process(ctx, bytes.NewReader(sourceData), req)
	if err != nil {
		return nil, fmt.Errorf("image processing failed: %w", err)
	}

	// Read processed data to cache it
	processedData, err := io.ReadAll(processResult.Content)
	if err != nil {
		return nil, fmt.Errorf("failed to read processed image: %w", err)
	}
	_ = processResult.Content.Close()

	// Store output content
	metaID, err := s.cache.GetSourceMetadataID(ctx, req)
	if err == nil {
		err = s.cache.StoreOutput(ctx, req, metaID, bytes.NewReader(processedData), processResult.ContentType)
		if err != nil {
			s.log.Warn("failed to store output content", "error", err)
		}
	}

	return &ImageResponse{
		Content:       io.NopCloser(bytes.NewReader(processedData)),
		ContentLength: int64(len(processedData)),
		ContentType:   processResult.ContentType,
		FetchedBytes:  int64(len(sourceData)),
	}, nil
}

// Warm pre-fetches and caches an image without returning it.
func (s *Service) Warm(ctx context.Context, req *ImageRequest) error {
	_, err := s.Get(ctx, req)

	return err
}

// Purge removes a cached image.
func (s *Service) Purge(_ context.Context, _ *ImageRequest) error {
	// TODO: Implement purge
	return errors.New("purge not implemented")
}

// Stats returns cache statistics.
func (s *Service) Stats(ctx context.Context) (*CacheStats, error) {
	return s.cache.Stats(ctx)
}

// ValidateRequest validates the request signature if required.
func (s *Service) ValidateRequest(req *ImageRequest) error {
	// Check if host is whitelisted (no signature required)
	sourceURL := req.SourceURL()

	parsedURL, err := url.Parse(sourceURL)
	if err != nil {
		return fmt.Errorf("invalid source URL: %w", err)
	}

	if s.whitelist.IsWhitelisted(parsedURL) {
		return nil
	}

	// Signature required
	if s.signer == nil {
		return errors.New("signing key not configured but host not whitelisted")
	}

	return s.signer.Verify(req)
}

// GenerateSignedURL generates a signed URL for the given request.
func (s *Service) GenerateSignedURL(
	baseURL string,
	req *ImageRequest,
	ttl time.Duration,
) (string, error) {
	if s.signer == nil {
		return "", errors.New("signing key not configured")
	}

	path, sig, exp := s.signer.GenerateSignedURL(req, ttl)

	return fmt.Sprintf("%s%s?sig=%s&exp=%d", baseURL, path, sig, exp), nil
}

// HTTP status codes for error responses.
const (
	httpStatusBadGateway    = 502
	httpStatusInternalError = 500
)

// isNegativeCacheable returns true if the error should be cached.
func isNegativeCacheable(err error) bool {
	return errors.Is(err, ErrUpstreamError)
}

// extractStatusCode extracts HTTP status code from error message.
func extractStatusCode(err error) int {
	// Default to 502 Bad Gateway for upstream errors
	if errors.Is(err, ErrUpstreamError) {
		return httpStatusBadGateway
	}

	return httpStatusInternalError
}