/// <summary>
/// Reads lines from the current position to the end of the file.
/// </summary>
private static async Task <CacheRetrievalResult> RetrieveInternalAsync(FileStream fs)
{
List <Scrobble> result = new List <Scrobble>();
List <string> failedLines = new List <string>();
using (var reader = GetReader(fs))
{
while (!reader.EndOfStream)
{
// Try to parse each line to a scrobble, ignoring failures.
var line = await reader.ReadLineAsync();
if (!string.IsNullOrWhiteSpace(line))
{
try
{
var scrobble = ScrobbleSerializer.Deserialize(line);
result.Add(scrobble);
}
catch
{
failedLines.Add(line);
}
}
}
}
return(new CacheRetrievalResult(result, failedLines));
}
/// <summary>
/// Asynchronously stores a scrobble in the underlying cache file.
/// Will throw an exception if the cache file cannot be accessed.
/// This method is thread safe.
/// </summary>
public async Task StoreAsync(Scrobble scrobble)
{
EnsureFileLockIsAcquired();
using (await _fileOperationAsyncLock.LockAsync())
{
// Note about the file content:
// The file is line separated.
// Instead of writing the data followed by a new line, we do the opposite:
// We always start by writing a new line, followed by the data.
// This way, we don't risk continuing a corrupted line without a proper line ending.
// Unreadable lines can then be discarded, and we lose only one record instead of two.
await EnsureCorrectHeaderAndGetFileVersionAsync(_fileStream);
// Skip to the end of the file to append a new scrobble.
_fileStream.Seek(0, SeekOrigin.End);
var serializedScrobble = ScrobbleSerializer.Serialize(scrobble);
byte[] buffer = Encoding.UTF8.GetBytes(serializedScrobble);
byte[] newLineBuffer = Encoding.UTF8.GetBytes("\n");
await _fileStream.WriteAsync(newLineBuffer, 0, newLineBuffer.Length);
await _fileStream.WriteAsync(buffer, 0, buffer.Length);
}
}
/// <summary>
/// Erases the file, write a new header, then write the scrobbles.
/// </summary>
private static async Task OverwriteFileAsync(FileStream fs, IEnumerable <Scrobble> scrobbles)
{
// Truncate the file.
fs.SetLength(0);
// Write the header.
await EnsureCorrectHeaderAndGetFileVersionAsync(fs);
// Skip to the end of the file to append the scrobbles.
fs.Seek(0, SeekOrigin.End);
// To minimize file writes, we use a StringBuilder that will be written in the file
// in as few buffer lengths as possible.
StringBuilder sb = new StringBuilder();
foreach (var scrobble in scrobbles)
{
var serialized = ScrobbleSerializer.Serialize(scrobble);
sb.AppendLine();
sb.Append(serialized);
}
byte[] buffer = Encoding.UTF8.GetBytes(sb.ToString());
await fs.WriteAsync(buffer, 0, buffer.Length);
}