storage: Don't check completion state on every write

storage/wrappers.go

@@ -1,7 +1,6 @@
 package storage
 
 import (
-	"errors"
 	"io"
 	"os"
 
@@ -37,11 +36,15 @@ type Piece struct {
 }
 
 func (p Piece) WriteAt(b []byte, off int64) (n int, err error) {
-	c := p.Completion()
-	if c.Ok && c.Complete {
-		err = errors.New("piece already completed")
-		return
-	}
+	// Callers should not be writing to completed pieces, but it's too
+	// expensive to be checking this on every single write using uncached
+	// completions.
+
+	// c := p.Completion()
+	// if c.Ok && c.Complete {
+	// 	err = errors.New("piece already completed")
+	// 	return
+	// }
 	if off+int64(len(b)) > p.mip.Length() {
 		panic("write overflows piece")
 	}