From d5f34f242f91e030b77879f29400c0b584c23c68 Mon Sep 17 00:00:00 2001
From: Caleb Allen <caleballen1276@gmail.com>
Date: Wed, 25 Mar 2026 14:14:30 -0400
Subject: [PATCH] fix: don't cache transient cover art failures as permanent
 misses

The CAA proxy was caching all fetch failures (including 503s and
timeouts) as empty files, treating them as permanent 'no art' misses.
During Internet Archive outages, this meant every album got cached
as having no cover art, and the cache persisted after IA recovered.

Now only 404 responses (no cover art exists) are cached as permanent
misses. 503, timeouts, and other transient errors are not cached,
so the next request retries the fetch.

Also cleared 33 incorrectly cached 0-byte miss files from a
concurrent IA outage.
---
 backend/explore/coverartproxy.go | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/backend/explore/coverartproxy.go b/backend/explore/coverartproxy.go
index 67cf651..9a46459 100644
--- a/backend/explore/coverartproxy.go
+++ b/backend/explore/coverartproxy.go
@@ -77,11 +77,13 @@ func (p *CoverArtProxy) GetThumbnail(releaseGroupMBID string) string {
 
 	// Fetch from CAA.
 	url := CoverArtGroupURL(releaseGroupMBID)
-	data, err := p.fetch(url)
+	data, cacheable, err := p.fetch(url)
 
 	if err != nil || len(data) == 0 {
-		// Cache the miss as an empty file so we don't retry.
-		p.writeCache(releaseGroupMBID, nil)
+		// Only cache permanent misses (404), not transient errors.
+		if cacheable {
+			p.writeCache(releaseGroupMBID, nil)
+		}
 
 		return ""
 	}
@@ -91,37 +93,43 @@ func (p *CoverArtProxy) GetThumbnail(releaseGroupMBID string) string {
 	return "data:image/jpeg;base64," + base64.StdEncoding.EncodeToString(data)
 }
 
-func (p *CoverArtProxy) fetch(url string) ([]byte, error) {
+func (p *CoverArtProxy) fetch(url string) ([]byte, bool, error) {
 	// Rate-limit CAA requests.
 	ctx := context.Background()
 	if err := p.limiter.Wait(ctx); err != nil {
-		return nil, err
+		return nil, false, err
 	}
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
-		return nil, err
+		return nil, false, err
 	}
 
 	req.Header.Set("User-Agent", lbUserAgent)
 
 	resp, err := p.client.Do(req)
 	if err != nil {
-		return nil, err
+		return nil, false, err
 	}
 
 	defer func() { _ = resp.Body.Close() }()
 
+	// 404 = no cover art exists — permanent, safe to cache as miss.
+	if resp.StatusCode == http.StatusNotFound {
+		return nil, true, nil
+	}
+
+	// Other non-200 = transient error — don't cache.
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("%w: %d", ErrCoverArt, resp.StatusCode)
+		return nil, false, fmt.Errorf("%w: %d", ErrCoverArt, resp.StatusCode)
 	}
 
 	data, err := io.ReadAll(io.LimitReader(resp.Body, thumbnailMaxSize))
 	if err != nil {
-		return nil, err
+		return nil, false, err
 	}
 
-	return data, nil
+	return data, true, nil
 }
 
 func (p *CoverArtProxy) cachePath(mbid string) string {