/// <summary>
/// Fetches an artwork for the specified combination of artist and album.
/// </summary>
/// <returns>
/// The cache key of the fetched artwork, or <c>null</c> , if no artwork could be found.
/// </returns>
/// <exception cref="ArtworkCacheException">
/// An error occured while fetching or storing the artwork.
/// </exception>
public async Task <string> FetchOnline(string artist, string album)
{
if (artist == null)
{
throw new ArgumentNullException(nameof(artist));
}
if (album == null)
{
throw new ArgumentNullException(nameof(album));
}
string lookupKey = BlobCacheKeys.GetKeyForOnlineArtwork(artist, album);
// Requests with the same lookup key have to wait on the first and then get the cached
// artwork key. That won't happen often, but when it does, we are save.
await this.keyedMemoizingSemaphore.Wait(lookupKey);
string artworkCacheKey = null;
// Each lookup key gets an artwork key assigned, let's see if it's already in the cache
if (await this.cache.GetObjectCreatedAt <string>(lookupKey) != null)
{
artworkCacheKey = await this.queue.EnqueueObservableOperation(1, () => this.cache.GetObject <string>(lookupKey));
}
// Previously failed lookups are marked as failed, it doesn't make sense to let it fail again
if (artworkCacheKey == OnlineFailMark)
{
this.Log().Debug($"Key {lookupKey} is marked as failed, returning.");
this.keyedMemoizingSemaphore.Release(lookupKey);
return(null);
}
if (artworkCacheKey != null)
{
// We already have the artwork cached? Great!
this.keyedMemoizingSemaphore.Release(lookupKey);
return(artworkCacheKey);
}
this.Log().Info($"Fetching online link for artwork {artist} - {album}");
Uri artworkLink;
try
{
artworkLink = await this.artworkFetcher.RetrieveAsync(artist, album);
}
catch (ArtworkFetchException ex)
{
this.keyedMemoizingSemaphore.Release(lookupKey);
throw new ArtworkCacheException("Could not retrieve the artwork information", ex);
}
if (artworkLink == null)
{
await this.MarkOnlineLookupKeyAsFailed(lookupKey);
this.keyedMemoizingSemaphore.Release(lookupKey);
return(null);
}
byte[] imageData;
using (var client = new HttpClient())
{
this.Log().Info($"Downloading artwork data for {artist} - {album} from {artworkLink}");
try
{
imageData = await client.GetByteArrayAsync(artworkLink);
}
catch (HttpRequestException ex)
{
this.keyedMemoizingSemaphore.Release(lookupKey);
throw new ArtworkCacheException($"Unable to download artwork from {artworkLink}", ex);
}
}
artworkCacheKey = BlobCacheKeys.GetKeyForArtwork(imageData);
await this.Store(artworkCacheKey, imageData);
await this.queue.EnqueueObservableOperation(1, () => this.cache.InsertObject(lookupKey, artworkCacheKey));
this.keyedMemoizingSemaphore.Release(lookupKey);
return(artworkCacheKey);
}
