make BasicBatch handle concurrent puts/deletes

batch.go

 package datastore
 
+type verb int
+
+type op struct {
+	delete bool
+	value  []byte
+}
+
 // basicBatch implements the transaction interface for datastores who do
 // not have any sort of underlying transactional support
 type basicBatch struct {
-	puts    map[Key][]byte
-	deletes map[Key]struct{}
+	ops map[Key]op
 
 	target Datastore
 }
 
 func NewBasicBatch(ds Datastore) Batch {
 	return &basicBatch{
-		puts:    make(map[Key][]byte),
-		deletes: make(map[Key]struct{}),
+		ops:    make(map[Key]op),
 		target: ds,
 	}
 }
 
 func (bt *basicBatch) Put(key Key, val []byte) error {
-	bt.puts[key] = val
+	bt.ops[key] = op{value: val}
 	return nil
 }
 
 func (bt *basicBatch) Delete(key Key) error {
-	bt.deletes[key] = struct{}{}
+	bt.ops[key] = op{delete: true}
 	return nil
 }
 
 func (bt *basicBatch) Commit() error {
-	for k, val := range bt.puts {
-		if err := bt.target.Put(k, val); err != nil {
-			return err
+	var err error
+	for k, op := range bt.ops {
+		if op.delete {
+			err = bt.target.Delete(k)
+			// We could try to do something smarter but I really
+			// don't care. Delete should be idempotent anyways.
+			if err == ErrNotFound {
+				err = nil
 			}
+		} else {
+			err = bt.target.Put(k, op.value)
 		}
-
-	for k, _ := range bt.deletes {
-		if err := bt.target.Delete(k); err != nil {
-			return err
+		if err != nil {
+			break
 		}
 	}
 
-	return nil
+	return err
 }