/// <summary>
/// Attempts to schedule a write of the specified length into the file.
/// </summary>
/// <returns>
/// A valid block address representing the location where the written
/// block will be found, or a 'None' address if the file does not
/// currently have enough room for this.
/// </returns>
/// <remarks>
/// The provided writer will be invoked at some point in the future.
///
/// Thread-safe.
/// </remarks>
public BlockAddress TryScheduleWrite(
uint realm,
Hash hash,
int length,
WithSpan.ReadWrite writer)
{
if (length < 0)
{
throw new ArgumentException($"Negative length: {length}", nameof(length));
}
int rank;
long offset;
lock (_syncRoot)
{
offset = _offset;
_offset = BlockAddress.Ceiling(offset + BlockHeader.Size + length);
if (_offset > _file.Length)
{
// Since this file is about to be flushed and closed,
// because it's "full enough", we prevent additional writes.
_offset = _file.Length;
return(BlockAddress.None);
}
rank = _flags.Count;
_flags.Append(ReadFlag.Triggered(() => PerformWrite(offset, writer)));
}
// We touched '_offset' and '_flags' in the critical section, the remaining
// work on the actual byte span can be done without mutex.
var span = _file.AsMemory(offset, BlockHeader.Size).Span;
ref var header = ref MemoryMarshal.Cast <byte, BlockHeader>(span)[0];
/// <summary>
/// Schedule the writing of a block, returns the address where the block will be
/// available once written.
/// </summary>
/// <remarks>
/// It is of course possible to immediately query the data at that address, in
/// which case the data will be returned.
///
/// Can be called from multiple threads.
/// </remarks>
public BlockAddress ScheduleWrite(
uint realm,
Hash hash,
int length,
WithSpan.ReadWrite writer)
{
for (var retries = 0; retries < 3; ++retries)
{
var currentWriter = _writer.GetCurrent();
var addr = currentWriter.TryScheduleWrite(realm, hash, length, writer);
if (!addr.IsNone())
{
return(addr);
}
// Scheduling a write failed because the current writer is full,
// so close it and loop to get a new writer.
_writer.RequestRecycle(currentWriter);
}
throw new Exception("Could not schedule write after three retries");
}
/// <summary>
/// Write a block to the scratch space. It will be possible to read it
/// back from the scratch space using the (realm, hash) that was
/// provided to create it.
/// </summary>
/// <remarks>
/// <paramref name="hash"/> should be the hash of the data passed in,
/// according to <see cref="BlockHasher"/>.
///
/// <paramref name="writer"/> will be called at some point in the future,
/// but not necessarily while <see cref="Write"/> is running.
/// </remarks>
public void Write(uint realm, Hash hash, int length, WithSpan.ReadWrite writer)
{
var addr = _wheel.ScheduleWrite(realm, hash, length, writer);
_index.Add(realm, hash, addr);
}
