feat: background thumbnail worker with smaller thumbs

Replace on-demand thumbnail generation (blocking HTTP requests) with a
background goroutine that scans gallery directories every 10 seconds.
Uploads trigger an immediate scan via a channel for fast availability.

1 files changed, 132 insertions, 71 deletions

diff --git a/gallery_test.go b/gallery_test.go
index 125a19a..29539fa 100644
--- a/gallery_test.go
+++ b/gallery_test.go
@@ -15,8 +15,6 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
-	"sync"
-	"sync/atomic"
 	"testing"
 	"time"
 
@@ -238,6 +236,92 @@ func TestThumbServing(t *testing.T) {
 	assert.NoError(t, err, "thumbnail should be a decodable JPEG")
 }
 
+func TestThumbServesSourceWhenThumbNotReady(t *testing.T) {
+	// When the background worker hasn't generated a thumb yet,
+	// handleThumb should serve the source image as fallback.
+	ts, srv := setupTestServer(t)
+	cookies := login(t, ts, "test-gallery", "secret123")
+
+	// Write a photo directly — don't start the thumb worker, so no thumb exists.
+	dir := srv.galleryDir("test-gallery")
+	original := makeTestJPEG()
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "nothumb.jpg"), original, 0644))
+
+	resp := authedGet(t, ts.URL+"/g/test-gallery/thumb/nothumb.jpg", cookies)
+	body, err := io.ReadAll(resp.Body)
+	resp.Body.Close()
+	require.NoError(t, err)
+
+	assert.Equal(t, 200, resp.StatusCode)
+	assert.Equal(t, original, body,
+		"when thumbnail is not ready, should serve the source image as fallback")
+}
+
+func TestThumbWorkerGeneratesThumbnails(t *testing.T) {
+	dir := t.TempDir()
+	srv := NewServer(dir, 20<<20)
+	require.NoError(t, srv.createGallery("test-gallery", "secret123"))
+
+	// Write a photo directly.
+	galleryDir := srv.galleryDir("test-gallery")
+	require.NoError(t, os.WriteFile(filepath.Join(galleryDir, "worker-test.jpg"), makeTestJPEG(), 0644))
+
+	// Start the worker and trigger a scan.
+	srv.startThumbWorker()
+	srv.triggerThumbScan("test-gallery")
+
+	// Wait for the thumbnail to appear (up to 5s).
+	thumbPath := filepath.Join(galleryDir, ".thumbs-v2", "worker-test.jpg")
+	require.Eventually(t, func() bool {
+		_, err := os.Stat(thumbPath)
+		return err == nil
+	}, 5*time.Second, 50*time.Millisecond, "thumbnail should be generated by the background worker")
+
+	// Verify it's a valid JPEG.
+	data, err := os.ReadFile(thumbPath)
+	require.NoError(t, err)
+	_, err = jpeg.Decode(bytes.NewReader(data))
+	assert.NoError(t, err, "generated thumbnail should be a valid JPEG")
+}
+
+func TestUploadTriggersThumbWorker(t *testing.T) {
+	dir := t.TempDir()
+	srv := NewServer(dir, 20<<20)
+	require.NoError(t, srv.createGallery("test-gallery", "secret123"))
+	srv.startThumbWorker()
+
+	ts := httptest.NewServer(srv.Handler())
+	t.Cleanup(ts.Close)
+
+	cookies := login(t, ts, "test-gallery", "secret123")
+	resp := uploadPhoto(t, ts, "test-gallery", cookies, "upload-trigger.jpg", makeTestJPEG())
+	resp.Body.Close()
+
+	// The upload should have triggered an immediate scan. Wait for the thumb.
+	thumbPath := filepath.Join(srv.galleryDir("test-gallery"), ".thumbs-v2", "upload-trigger.jpg")
+	require.Eventually(t, func() bool {
+		_, err := os.Stat(thumbPath)
+		return err == nil
+	}, 5*time.Second, 50*time.Millisecond,
+		"upload should trigger background thumbnail generation")
+}
+
+func TestOldThumbsDirCleanedUp(t *testing.T) {
+	dir := t.TempDir()
+	srv := NewServer(dir, 20<<20)
+	require.NoError(t, srv.createGallery("test-gallery", "secret123"))
+
+	// Create the old .thumbs directory with some content.
+	oldThumbsDir := filepath.Join(srv.galleryDir("test-gallery"), ".thumbs")
+	require.NoError(t, os.MkdirAll(oldThumbsDir, 0755))
+	require.NoError(t, os.WriteFile(filepath.Join(oldThumbsDir, "old.jpg"), []byte("old"), 0644))
+
+	srv.cleanupOldThumbs()
+
+	_, err := os.Stat(oldThumbsDir)
+	assert.True(t, os.IsNotExist(err), "old .thumbs directory should be removed")
+}
+
 func TestPhotoServing(t *testing.T) {
 	ts, _, cookies := setupWithPhoto(t, "original.jpg")
 
@@ -382,33 +466,38 @@ func makeUnsupportedJPEG() []byte {
 }
 
 func TestThumbFallbackToImageMagick(t *testing.T) {
-	ts, srv := setupTestServer(t)
-	cookies := login(t, ts, "test-gallery", "secret123")
+	dir := t.TempDir()
+	srv := NewServer(dir, 20<<20)
+	require.NoError(t, srv.createGallery("test-gallery", "secret123"))
 
 	// Create a JPEG that Go's stdlib can't decode but ImageMagick can.
 	unsupportedJPEG := makeUnsupportedJPEG()
 
-	// Write directly to the gallery dir (bypass upload, test thumbnail generation).
-	dir := srv.galleryDir("test-gallery")
-	require.NoError(t, os.WriteFile(filepath.Join(dir, "tricky.jpg"), unsupportedJPEG, 0644))
+	// Write directly to the gallery dir.
+	galleryDir := srv.galleryDir("test-gallery")
+	require.NoError(t, os.WriteFile(filepath.Join(galleryDir, "tricky.jpg"), unsupportedJPEG, 0644))
 
-	// Request the thumbnail — should use ImageMagick fallback and return a valid JPEG.
-	resp := authedGet(t, ts.URL+"/g/test-gallery/thumb/tricky.jpg", cookies)
-	defer resp.Body.Close()
+	// Start the worker and trigger a scan.
+	srv.startThumbWorker()
+	srv.triggerThumbScan("test-gallery")
 
-	assert.Equal(t, 200, resp.StatusCode)
-	assert.Contains(t, resp.Header.Get("Content-Type"), "image/jpeg")
+	// Wait for the thumbnail to appear.
+	thumbPath := filepath.Join(galleryDir, ".thumbs-v2", "tricky.jpg")
+	require.Eventually(t, func() bool {
+		_, err := os.Stat(thumbPath)
+		return err == nil
+	}, 5*time.Second, 50*time.Millisecond, "thumbnail should be generated by the background worker")
 
 	// The thumbnail should be a valid, fully decodable JPEG.
-	thumbData, err := io.ReadAll(resp.Body)
+	thumbData, err := os.ReadFile(thumbPath)
 	require.NoError(t, err)
 
 	_, err = jpeg.Decode(bytes.NewReader(thumbData))
 	assert.NoError(t, err, "thumbnail should be a fully decodable JPEG even when Go's stdlib can't decode the original")
 
-	// The thumbnail must not be the raw original served as fallback.
+	// The thumbnail must not be the raw original.
 	assert.NotEqual(t, unsupportedJPEG, thumbData,
-		"thumbnail must not be the broken original served as fallback")
+		"thumbnail must not be the broken original")
 }
 
 // makeLargeTestJPEG creates a larger JPEG that takes more time to thumbnail.
@@ -424,70 +513,42 @@ func makeLargeTestJPEG() []byte {
 	return buf.Bytes()
 }
 
-func TestConcurrentThumbnailGeneration(t *testing.T) {
-	ts, srv := setupTestServer(t)
-	cookies := login(t, ts, "test-gallery", "secret123")
+func TestWorkerGeneratesAllThumbnails(t *testing.T) {
+	dir := t.TempDir()
+	srv := NewServer(dir, 20<<20)
+	require.NoError(t, srv.createGallery("test-gallery", "secret123"))
 
-	// Upload 40 photos directly to the gallery dir (bypass upload handler for speed).
-	dir := srv.galleryDir("test-gallery")
-	largeJPEG := makeLargeTestJPEG()
-	numPhotos := 40
+	// Write 20 photos directly to the gallery dir.
+	galleryDir := srv.galleryDir("test-gallery")
+	numPhotos := 20
 	for i := 0; i < numPhotos; i++ {
 		filename := fmt.Sprintf("photo_%03d.jpg", i)
-		require.NoError(t, os.WriteFile(filepath.Join(dir, filename), largeJPEG, 0644))
+		require.NoError(t, os.WriteFile(filepath.Join(galleryDir, filename), makeTestJPEG(), 0644))
 	}
 
-	// Fire all thumbnail requests concurrently, like a browser would.
-	var wg sync.WaitGroup
-	var failures atomic.Int32
-	deadline := 30 * time.Second
-
-	for i := 0; i < numPhotos; i++ {
-		wg.Add(1)
-		go func(idx int) {
-			defer wg.Done()
-			filename := fmt.Sprintf("photo_%03d.jpg", idx)
-
-			client := &http.Client{Timeout: deadline}
-			req, err := http.NewRequest(http.MethodGet, ts.URL+"/g/test-gallery/thumb/"+filename, nil)
-			if err != nil {
-				failures.Add(1)
-				return
-			}
-			for _, c := range cookies {
-				req.AddCookie(c)
-			}
-
-			resp, err := client.Do(req)
-			if err != nil {
-				failures.Add(1)
-				return
-			}
-			defer resp.Body.Close()
+	// Start the worker and trigger a scan.
+	srv.startThumbWorker()
+	srv.triggerThumbScan("test-gallery")
 
-			if resp.StatusCode != 200 {
-				failures.Add(1)
-				return
-			}
+	// Wait for all thumbnails to appear.
+	thumbsDir := filepath.Join(galleryDir, ".thumbs-v2")
+	require.Eventually(t, func() bool {
+		entries, err := os.ReadDir(thumbsDir)
+		if err != nil {
+			return false
+		}
+		return len(entries) >= numPhotos
+	}, 10*time.Second, 100*time.Millisecond,
+		"all %d thumbnails should be generated by the background worker", numPhotos)
 
-			// Must be a valid JPEG
-			_, err = jpeg.Decode(resp.Body)
-			if err != nil {
-				failures.Add(1)
-			}
-		}(i)
+	// Verify each is a valid JPEG.
+	for i := 0; i < numPhotos; i++ {
+		filename := fmt.Sprintf("photo_%03d.jpg", i)
+		data, err := os.ReadFile(filepath.Join(thumbsDir, filename))
+		require.NoError(t, err)
+		_, err = jpeg.Decode(bytes.NewReader(data))
+		assert.NoError(t, err, "thumbnail %s should be a valid JPEG", filename)
 	}
-
-	wg.Wait()
-	assert.Equal(t, int32(0), failures.Load(),
-		"all %d concurrent thumbnail requests should succeed, but %d failed", numPhotos, failures.Load())
-
-	// Verify that thumbnail generation concurrency was bounded.
-	// With thumbSemaphore in place, peak concurrency must not exceed the limit.
-	peak := srv.peakThumbConcurrency.Load()
-	t.Logf("peak thumbnail generation concurrency: %d", peak)
-	assert.LessOrEqual(t, peak, int32(maxThumbConcurrency),
-		"peak concurrency (%d) should not exceed maxThumbConcurrency (%d)", peak, maxThumbConcurrency)
 }
 
 func setupTestServerNoThumbs(t *testing.T) (*httptest.Server, *Server) {