fix(lastfm): report a failure when the artist page has no image (#6198)

GetArtistImages scrapes the og:image tag off the Last.fm artist page, because
the API only ever returns the placeholder image. Last.fm now answers non-browser
clients with a Fastly bot challenge, served as a 200 with valid HTML, so the
query found no og:image and the agent returned an empty list with no error. The
artwork worker read that as a definitive "this artist has no image" and settled
the state as absent, silently and with nothing in the log.

A real artist page always carries an og:image, so its absence now returns an
error instead. The worker keeps the previous state, other agents still get their
turn, and its per-agent circuit breaker bounds the retries. The error is
deliberately not a RetryLaterError: that would park the whole Last.fm agent,
including the API-backed biography, similar-artists and top-songs calls, which
the page block does not affect.

The new fixture is the real 3038-byte challenge page.

Fixes #6192

adapters/lastfm/agent.go

@@ -241,6 +241,10 @@ func (l *lastfmAgent) GetSimilarSongsByTrack(ctx context.Context, id, name, arti
 var (
 	artistOpenGraphQuery = cascadia.MustCompile(`html > head > meta[property="og:image"]`)
 	artistIgnoredImage   = "2a96cbd8b46e442fc41c2b86b821562f" // Last.fm artist placeholder image name
+
+	// Not a RetryLaterError on purpose: parking the agent would also stall its API-backed
+	// methods, which the page block does not affect.
+	errNoArtistPage = errors.New("no artist image in Last.fm page")
 )
 
 func (l *lastfmAgent) GetArtistImages(ctx context.Context, _, name, mbid string) ([]agents.ExternalImage, error) {
@@ -267,7 +271,9 @@ func (l *lastfmAgent) GetArtistImages(ctx context.Context, _, name, mbid string)
 	var res []agents.ExternalImage
 	n := cascadia.Query(node, artistOpenGraphQuery)
 	if n == nil {
-		return res, nil
+		// A real artist page always has og:image; its absence means a bot challenge or a redesign.
+		log.Warn(ctx, "Last.fm did not return a usable artist page", "name", name, "url", a.URL)
+		return nil, errNoArtistPage
 	}
 	for _, attr := range n.Attr {
 		if attr.Key != "content" {

adapters/lastfm/agent_test.go

@@ -648,18 +648,41 @@ var _ = Describe("lastfmAgent", func() {
 			Expect(images).To(BeEmpty())
 		})
 
-		It("returns empty list if page has no meta tags", func() {
+		It("errors when the page has no meta tags", func() {
 			fApi, _ := os.Open("tests/fixtures/lastfm.artist.getinfo.json")
 			apiClient.Res = http.Response{Body: fApi, StatusCode: 200}
 
 			fScraper, _ := os.Open("tests/fixtures/lastfm.artist.page.no_meta.html")
 			httpClient.Res = http.Response{Body: fScraper, StatusCode: 200}
 
+			_, err := agent.GetArtistImages(ctx, "123", "U2", "")
+			Expect(err).To(MatchError(errNoArtistPage))
+		})
+
+		It("errors when Last.fm serves a bot challenge page", func() {
+			fApi, _ := os.Open("tests/fixtures/lastfm.artist.getinfo.json")
+			apiClient.Res = http.Response{Body: fApi, StatusCode: 200}
+
+			fScraper, _ := os.Open("tests/fixtures/lastfm.artist.page.challenge.html")
+			httpClient.Res = http.Response{Body: fScraper, StatusCode: 200}
+
 			images, err := agent.GetArtistImages(ctx, "123", "U2", "")
-			Expect(err).ToNot(HaveOccurred())
+			Expect(err).To(MatchError(errNoArtistPage))
 			Expect(images).To(BeEmpty())
 		})
 
+		It("does not park the agent: the failure is not a retry-later", func() {
+			// A RetryLaterError would cool down the agent's API-backed methods too.
+			fApi, _ := os.Open("tests/fixtures/lastfm.artist.getinfo.json")
+			apiClient.Res = http.Response{Body: fApi, StatusCode: 200}
+
+			fScraper, _ := os.Open("tests/fixtures/lastfm.artist.page.challenge.html")
+			httpClient.Res = http.Response{Body: fScraper, StatusCode: 200}
+
+			_, err := agent.GetArtistImages(ctx, "123", "U2", "")
+			Expect(errors.Is(err, agents.ErrRetryLater)).To(BeFalse())
+		})
+
 		It("returns error if API call fails", func() {
 			apiClient.Err = errors.New("api error")
 			_, err := agent.GetArtistImages(ctx, "123", "U2", "")

tests/fixtures/lastfm.artist.page.challenge.html

@@ -0,0 +1,88 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta
+      http-equiv="Content-Security-Policy"
+      content="default-src 'self'; img-src 'self' data:; media-src 'self' data:; object-src 'none'; style-src 'self' 'sha256-o4vzfmmUENEg4chMjjRP9EuW9ucGnGIGVdbl8d0SHQQ='; script-src 'self' 'sha256-a9bHdQGvRzDwDVzx8m+Rzw+0FHZad8L0zjtBwkxOIz4=';"
+    />
+    <link
+      href="/_fs-ch-1T1wmsGaOgGaSxcX/assets/inter-var.woff2"
+      rel="preload"
+      as="font"
+      type="font/woff2"
+      crossorigin
+    />
+    <link href="/_fs-ch-1T1wmsGaOgGaSxcX/assets/styles.css" rel="stylesheet" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <title>Client Challenge</title>
+    <style>
+      #loading-error {
+        font-size: 16px;
+        font-family: 'Inter', sans-serif;
+        margin-top: 10px;
+        margin-left: 10px;
+        display: none;
+      }
+    </style>
+  </head>
+  <body>
+    <noscript>
+      <div class="noscript-container">
+        <div class="noscript-content">
+          <img
+            src="/_fs-ch-1T1wmsGaOgGaSxcX/assets/errorIcon.svg"
+            alt=""
+            role="presentation"
+            class="error-icon"
+          />
+          <span class="noscript-span"
+            >JavaScript is disabled in your browser.</span
+          >
+          <p>Please enable JavaScript to proceed.</p>
+        </div>
+      </div>
+    </noscript>
+    <div id="loading-error" role="alert" aria-live="polite">
+      A required part of this site couldn’t load. This may be due to a browser
+      extension, network issues, or browser settings. Please check your
+      connection, disable any ad blockers, or try using a different browser.
+    </div>
+    <script>
+      function loadScript(src) {
+        return new Promise((resolve, reject) => {
+          const script = document.createElement('script');
+          script.onload = resolve;
+          script.onerror = (event) => {
+            console.error('Script load error event:', event);
+            document.getElementById('loading-error').style.display = 'block';
+            reject(
+              new Error(
+                `Failed to load script: ${src}, Please contact the service administrator.`
+              )
+            );
+          };
+          script.src = src;
+          document.body.appendChild(script);
+        });
+      }
+
+      loadScript('/_fs-ch-1T1wmsGaOgGaSxcX/errors.js')
+        .then(() => {
+          const script = document.createElement('script');
+          script.src = '/_fs-ch-1T1wmsGaOgGaSxcX/script.js?reload=true';
+          script.onerror = (event) => {
+            console.error('Script load error event:', event);
+            const errorMsg = new Error(
+              `Failed to load script: ${script.src}. Please contact the service administrator.`
+            );
+            console.error(errorMsg);
+            handleScriptError();
+          };
+          document.body.appendChild(script);
+        })
+        .catch((error) => {
+          console.error(error);
+        });
+    </script>
+  </body>
+</html>