batch: add external batching engine for bbolt operations

batch/batch.go

@@ -0,0 +1,102 @@
+package batch
+
+import (
+	"errors"
+	"sync"
+
+	"github.com/lightningnetwork/lnd/channeldb/kvdb"
+)
+
+// errSolo is a sentinel error indicating that the requester should re-run the
+// operation in isolation.
+var errSolo = errors.New(
+	"batch function returned an error and should be re-run solo",
+)
+
+type request struct {
+	*Request
+	errChan chan error
+}
+
+type batch struct {
+	db     kvdb.Backend
+	start  sync.Once
+	reqs   []*request
+	clear  func(b *batch)
+	locker sync.Locker
+}
+
+// trigger is the entry point for the batch and ensures that run is started at
+// most once.
+func (b *batch) trigger() {
+	b.start.Do(b.run)
+}
+
+// run executes the current batch of requests. If any individual requests fail
+// alongside others they will be retried by the caller.
+func (b *batch) run() {
+	// Clear the batch from its scheduler, ensuring that no new requests are
+	// added to this batch.
+	b.clear(b)
+
+	// If a cache lock was provided, hold it until the this method returns.
+	// This is critical for ensuring external consistency of the operation,
+	// so that caches don't get out of sync with the on disk state.
+	if b.locker != nil {
+		b.locker.Lock()
+		defer b.locker.Unlock()
+	}
+
+	// Apply the batch until a subset succeeds or all of them fail. Requests
+	// that fail will be retried individually.
+	for len(b.reqs) > 0 {
+		var failIdx = -1
+		err := kvdb.Update(b.db, func(tx kvdb.RwTx) error {
+			for i, req := range b.reqs {
+				err := req.Update(tx)
+				if err != nil {
+					failIdx = i
+					return err
+				}
+			}
+			return nil
+		}, func() {
+			for _, req := range b.reqs {
+				if req.Reset != nil {
+					req.Reset()
+				}
+			}
+		})
+
+		// If a request's Update failed, extract it and re-run the
+		// batch. The removed request will be retried individually by
+		// the caller.
+		if failIdx >= 0 {
+			req := b.reqs[failIdx]
+
+			// It's safe to shorten b.reqs here because the
+			// scheduler's batch no longer points to us.
+			b.reqs[failIdx] = b.reqs[len(b.reqs)-1]
+			b.reqs = b.reqs[:len(b.reqs)-1]
+
+			// Tell the submitter re-run it solo, continue with the
+			// rest of the batch.
+			req.errChan <- errSolo
+			continue
+		}
+
+		// None of the remaining requests failed, process the errors
+		// using each request's OnCommit closure and return the error
+		// to the requester. If no OnCommit closure is provided, simply
+		// return the error directly.
+		for _, req := range b.reqs {
+			if req.OnCommit != nil {
+				req.errChan <- req.OnCommit(err)
+			} else {
+				req.errChan <- err
+			}
+		}
+
+		return
+	}
+}