Avro adapter - Read and write Avro container files

### Describe the enhancement requested

Part 4 in the Avro series, following on from #731. This will allow reading and writing whole files in the Avro container format as a series of batches. Each batch will correspond to one Avro file block and fill a single VSR. The VSR can be recycled between batches. Input and output can be to Avro encoder / decoder (set up externally) or to Java's native byte channels (which are set up with default binary encoder / decoder). To cater for async scenarios, the reader API should know how many bytes are required for a block before attempting to read it.

I'd like to propose the following API - hopefully this is going in the right direction. I've taken some inspiration from ArrowFilleReader / Writer and Json Reader / Writer, but it's not identical (and they're not identical to each other). If there is a desire to line up on specific naming / conventions then certainly happy to do that, in which case I'll need a steer on exactly how it should be. Otherwise if anyone has radically different ideas of what it should look like, please do share!

class AvroFileWriter {

// Writer owns a channel / encoder and will close them
// VSR and optional dictionaries are not owned and will not be closed
// VSR can be recycled or supplied as a stream

// Avro encoder configured externally
public AvroFileWriter(
Encoder encoder,
VectorSchemaRoot firstBatch,
DictionaryProvider dictionaries)

// Sets up a defaulr binary encoder for the channel
public AvroFileWriter(
WritableByteChannel channel,
VectorSchemaRoot firstBatch,
DictionaryProvider dictionaries)

// Write the Avro header (throws if already written)
void writeHeader()

// Write the contents of the VSR as an Avro data block
// Writes header if not yet written
// Expects new data to be in the batch (i.e. VSR can be recycled)
void writeBatch()

// Reset vectors in all the producders
// Supports a stream of VSRs if source VSR is not recycled
void resetBatch(VectorSchemaRoot batch)

// Closes encoder and / or channel
// Does not close VSR or dictionary vectors
void close()

}

Now writing data looks like this:

void writeAvro(MyApp app) {

var root = app.prepareVsr();
var dictionaries = app.prepareDictionaries()

try (var writer = new AvroFileWriter(app.openChannel(), root, dictionaries)) {

writer.writeHeader();

// Assume recycling, loadBatch() puts fresh data into root
while (app.loadBatch()) {
writer.writeBatch()
}
}
}

And then for the reader:

class AvroFileReader implements DictionaryProvider {

// Writer owns a channel / decoder and will close them
// Schema / VSR / dictionaries are created when header is read
// VSR / dictionaries are cleaned up on close
// Dictionaries accessible through DictionaryProvider iface

// Avro decoder configured externally
public AvroFileWriter(
Decoder decoder,
BufferAllocator allocator)

// Sets up a defaulr binary deocder for the channel
// Avro read sequentially so seekable channel not needed
public AvroFileWriter(
ReadableByteChannel channel,
BufferAllocator allocator)

// Read the Avro header and set up schema / VSR / dictionaries
void readHeader()

// Schema and VSR available after readHeader()
Schema getSchema()
VectorSchemaRoot getVectorSchemaRoot()

// Read the next Avro block and load it into the VSR
// Return true if successful, false if EOS
// Also false in non-blocking mode if need more data
boolean readBatch()

// Check for position and size of the next Avro data block
// Provides a mechanism for non-blocking / reactive styles
boolean hasNextBatch();
long nextBatchPosition();
long nextBatchSize();

// Closes encoder and / or channel
// Also closes VSR and dictionary vectors
void close()

}

So reading looks like this:

// Blocking style
void readAvro(MyApp app) {

try (var reader = new AvroFileReader(app.openChannel(), app.allocator()) {

reader.readHeader();

app.setSchema(reader.getSchema());
app.setVsr(reader.getVectorSchemaRoot());
app.setDictionaries(reader);

while (reader.readBatch())) {
app.saveBatch();
}
}
}

// Non-blocking stage to process one batch
CompletionStage readAvroAsync(AvroFileReader reader) {

if (reader.hasNextBatch()) {

var start = reader.nextBatchStart();
var end = reader.nextBatchEnd();

return app.ensureBytesAvailable(start, end)
.thenApply(x -> {

if (reader.readBatch()) {
app.saveBatch();
}

return reader;
})
.thenCompose(this::readAsync);
}
else {
return CompletableFuture.completedFuture(true);
}
}

The non-blocking read is quite important for me as I have a web service that receives bytes in a stream. There is a slight gotcha because we need the first 8 bytes of the next batch before we know its size, but we can implement hasNextBatch() without them and the probably expose the batch padding size as a constant.

Compression is probably worth thinking about now - each block is compressed individually so the implementation needs to treat the contents of each block as a separate chunk, that can be fed through a codec. My guess is this is fairly straightforward for codecs that are already available so we might as well include it rather than reworking later.

If this looks broadly right I'll make a start on top of #779

Comece revisando as APIs propostas AvroFileWriter e AvroFileReader juntamente com o trabalho anterior de Avro em #731 e #779, e compare as convenções existentes de ArrowFileReader/Writer e dos readers/writers JSON. Done deve abranger a leitura e a escrita de arquivos contêiner baseadas em batches, codecs configurados externamente ou respaldados por canais, a inspeção do tamanho dos blocos para leituras não bloqueantes, a propriedade dos recursos e a compressão por bloco.

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