Implement the BatchingProcessor (#5093)

* [WIP] Implement the BatchingProcessor

* Add TestExportSync

* Add TestChunker

* Test export error default to ErrorHandler

* Fix lint

* Fix chunk smaller than size error

* Add batch tests

* Fix lint

* Update OnEmit test

Check the len of records in eventually assertion given that is what we
are going to measure.

* Revert unneeded change to BatchingProcessor doc

* Add batch type

* Refactor testing of batching config

The BatchingProcessor is not expected to ultimately contain
configuration fields for queue size or export parameters (see #5093).
This will break TestNewBatchingProcessorConfiguration which tests the
configuration by evaluating the BatchingProcessor directly.

Instead, test the batchingConfig and rename the test to
TestNewBatchingConfig to match what is being tested.

* Implement the BatchingProcessor without polling

* Add TestBatchingProcessor

* Add ConcurrentSafe test

* Expand Shutdown tests

* Test context canceled for ForceFlush

* Refactor batch to queue

* Use exportSync

* Update docs and naming

* Split buffered export to its own type

* Update comments and naming

* Fix lint

* Remove redundant triggered type

* Add interval polling

* Refactor test structure

* Add custom ring implimementation

* Add BenchmarkBatchingProcessor

* Fix merge

* Remove custom ring impl

* Remove BenchmarkBatchingProcessor

* Update dev docs

* Test nil exporter

* Update OnEmit test

Ensure the poll goroutine will completely flush the queue of batches.

* Test RetriggerFlushNonBlocking

* Update ascii diagram

* Fix flaky OnEmit

* Revert unnecessary change to test pkg name

* Use batching term in docs

* Document EnqueueExport

* Return from EnqueueExport if blocked

Do not wait for the enqueue to succeed.

* Do not drop failed flush log records

* Use cancelable ctx in concurrency test

* Fix comments

* Apply feedback

Do not spawn a goroutine for the flush operation.

* Return true from EnqueueExport when stopped

* Update sdk/log/batch.go

Co-authored-by: Robert Pająk <pellared@hotmail.com>

* Remove TODO

* Comment re-trigger in poll

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

sdk/log/batch.go

@@ -6,7 +6,10 @@ package log // import "go.opentelemetry.io/otel/sdk/log"
 import (
 	"container/ring"
 	"context"
+	"errors"
+	"slices"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -26,56 +29,220 @@ const (
 var _ Processor = (*BatchingProcessor)(nil)
 
 // BatchingProcessor is a processor that exports batches of log records.
+// A BatchingProcessor must be created with [NewBatchingProcessor].
 type BatchingProcessor struct {
-	exporter Exporter
-
-	maxQueueSize       int
-	exportInterval     time.Duration
-	exportTimeout      time.Duration
-	exportMaxBatchSize int
+	// The BatchingProcessor is designed to provide the highest throughput of
+	// log records possible while being compatible with OpenTelemetry. The
+	// entry point of log records is the OnEmit method. This method is designed
+	// to receive records as fast as possible while still honoring shutdown
+	// commands. All records received are enqueued to queue.
+	//
+	// In order to block OnEmit as little as possible, a separate "poll"
+	// goroutine is spawned at the creation of a BatchingProcessor. This
+	// goroutine is responsible for batching the queue at regular polled
+	// intervals, or when it is directly signaled to.
+	//
+	// To keep the polling goroutine from backing up, all batches it makes are
+	// exported with a bufferedExporter. This exporter allows the poll
+	// goroutine to enqueue an export payload that will be handled in a
+	// separate goroutine dedicated to the export. This asynchronous behavior
+	// allows the poll goroutine to maintain accurate interval polling.
+	//
+	//   __BatchingProcessor__     __Poll Goroutine__     __Export Goroutine__
+	// ||                     || ||                  || ||                    ||
+	// ||          ********** || ||                  || ||     **********     ||
+	// || Records=>* OnEmit * || ||   | - ticker     || ||     * export *     ||
+	// ||          ********** || ||   | - trigger    || ||     **********     ||
+	// ||             ||      || ||   |              || ||         ||         ||
+	// ||             ||      || ||   |              || ||         ||         ||
+	// ||   __________\/___   || ||   |***********   || ||   ______/\_______  ||
+	// ||  (____queue______)>=||=||===|*  batch  *===||=||=>[_export_buffer_] ||
+	// ||                     || ||   |***********   || ||                    ||
+	// ||_____________________|| ||__________________|| ||____________________||
+	//
+	//
+	// The "release valve" in this processing is the record queue. This queue
+	// is a ring buffer. It will overwrite the oldest records first when writes
+	// to OnEmit are made faster than the queue can be flushed. If batches
+	// cannot be flushed to the export buffer, the records will remain in the
+	// queue.
+
+	// exporter is the bufferedExporter all batches are exported with.
+	exporter *bufferExporter
+
+	// q is the active queue of records that have not yet been exported.
+	q *queue
+	// batchSize is the minimum number of records needed before an export is
+	// triggered (unless the interval expires).
+	batchSize int
+
+	// pollTrigger triggers the poll goroutine to flush a batch from the queue.
+	// This is sent to when it is known that the queue contains at least one
+	// complete batch.
+	//
+	// When a send is made to the channel, the poll loop will be reset after
+	// the flush. If there is still enough records in the queue for another
+	// batch the reset of the poll loop will automatically re-trigger itself.
+	// There is no need for the original sender to monitor and resend.
+	pollTrigger chan struct{}
+	// pollKill kills the poll goroutine. This is only expected to be closed
+	// once by the Shutdown method.
+	pollKill chan struct{}
+	// pollDone signals the poll goroutine has completed.
+	pollDone chan struct{}
+
+	// stopped holds the stopped state of the BatchingProcessor.
+	stopped atomic.Bool
 }
 
 // NewBatchingProcessor decorates the provided exporter
 // so that the log records are batched before exporting.
 //
 // All of the exporter's methods are called synchronously.
 func NewBatchingProcessor(exporter Exporter, opts ...BatchProcessorOption) *BatchingProcessor {
+	cfg := newBatchingConfig(opts)
 	if exporter == nil {
 		// Do not panic on nil export.
 		exporter = defaultNoopExporter
 	}
-	cfg := newBatchingConfig(opts)
-	return &BatchingProcessor{
-		exporter: exporter,
-
-		maxQueueSize:       cfg.maxQSize.Value,
-		exportInterval:     cfg.expInterval.Value,
-		exportTimeout:      cfg.expTimeout.Value,
-		exportMaxBatchSize: cfg.expMaxBatchSize.Value,
+	// Order is important here. Wrap the timeoutExporter with the chunkExporter
+	// to ensure each export completes in timeout (instead of all chuncked
+	// exports).
+	exporter = newTimeoutExporter(exporter, cfg.expTimeout.Value)
+	// Use a chunkExporter to ensure ForceFlush and Shutdown calls are batched
+	// appropriately on export.
+	exporter = newChunkExporter(exporter, cfg.expMaxBatchSize.Value)
+
+	b := &BatchingProcessor{
+		// TODO: explore making the size of this configurable.
+		exporter: newBufferExporter(exporter, 1),
+
+		q:           newQueue(cfg.maxQSize.Value),
+		batchSize:   cfg.expMaxBatchSize.Value,
+		pollTrigger: make(chan struct{}, 1),
+		pollKill:    make(chan struct{}),
 	}
+	b.pollDone = b.poll(cfg.expInterval.Value)
+	return b
+}
+
+// poll spawns a goroutine to handle interval polling and batch exporting. The
+// returned done chan is closed when the spawned goroutine completes.
+func (b *BatchingProcessor) poll(interval time.Duration) (done chan struct{}) {
+	done = make(chan struct{})
+
+	ticker := time.NewTicker(interval)
+	// TODO: investigate using a sync.Pool instead of cloning.
+	buf := make([]Record, b.batchSize)
+	go func() {
+		defer close(done)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-ticker.C:
+			case <-b.pollTrigger:
+				ticker.Reset(interval)
+			case <-b.pollKill:
+				return
+			}
+
+			qLen := b.q.TryDequeue(buf, func(r []Record) bool {
+				ok := b.exporter.EnqueueExport(r)
+				if ok {
+					buf = slices.Clone(buf)
+				}
+				return ok
+			})
+			if qLen >= b.batchSize {
+				// There is another full batch ready. Immediately trigger
+				// another export attempt.
+				select {
+				case b.pollTrigger <- struct{}{}:
+				default:
+					// Another flush signal already received.
+				}
+			}
+		}
+	}()
+	return done
 }
 
 // OnEmit batches provided log record.
-func (b *BatchingProcessor) OnEmit(ctx context.Context, r Record) error {
-	// TODO (#5063): Implement.
+func (b *BatchingProcessor) OnEmit(_ context.Context, r Record) error {
+	if b.stopped.Load() {
+		return nil
+	}
+	if n := b.q.Enqueue(r); n >= b.batchSize {
+		select {
+		case b.pollTrigger <- struct{}{}:
+		default:
+			// Flush chan full. The poll goroutine will handle this by
+			// re-sending any trigger until the queue has less than batchSize
+			// records.
+		}
+	}
 	return nil
 }
 
-// Enabled returns true.
+// Enabled returns if b is enabled.
 func (b *BatchingProcessor) Enabled(context.Context, Record) bool {
-	return true
+	return !b.stopped.Load()
 }
 
 // Shutdown flushes queued log records and shuts down the decorated exporter.
 func (b *BatchingProcessor) Shutdown(ctx context.Context) error {
-	// TODO (#5063): Implement.
-	return nil
+	if b.stopped.Swap(true) {
+		return nil
+	}
+
+	// Stop the poll goroutine.
+	close(b.pollKill)
+	select {
+	case <-b.pollDone:
+	case <-ctx.Done():
+		// Out of time.
+		return errors.Join(ctx.Err(), b.exporter.Shutdown(ctx))
+	}
+
+	// Flush remaining queued before exporter shutdown.
+	err := b.exporter.Export(ctx, b.q.Flush())
+	return errors.Join(err, b.exporter.Shutdown(ctx))
+}
+
+var errPartialFlush = errors.New("partial flush: export buffer full")
+
+// Used for testing.
+var ctxErr = func(ctx context.Context) error {
+	return ctx.Err()
 }
 
 // ForceFlush flushes queued log records and flushes the decorated exporter.
 func (b *BatchingProcessor) ForceFlush(ctx context.Context) error {
-	// TODO (#5063): Implement.
-	return nil
+	if b.stopped.Load() {
+		return nil
+	}
+
+	buf := make([]Record, b.q.cap)
+	notFlushed := func() bool {
+		var flushed bool
+		_ = b.q.TryDequeue(buf, func(r []Record) bool {
+			flushed = b.exporter.EnqueueExport(r)
+			return flushed
+		})
+		return !flushed
+	}
+	var err error
+	// For as long as ctx allows, try to make a single flush of the queue.
+	for notFlushed() {
+		// Use ctxErr instead of calling ctx.Err directly so we can test
+		// the partial error return.
+		if e := ctxErr(ctx); e != nil {
+			err = errors.Join(e, errPartialFlush)
+			break
+		}
+	}
+	return errors.Join(err, b.exporter.ForceFlush(ctx))
 }
 
 // queue holds a queue of logging records.