fix(do) child listens on ctx too

util/do.go

 package util
 
-import "code.google.com/p/go.net/context"
+import "github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
 
 func Do(ctx context.Context, f func() error) error {
+
 	ch := make(chan error)
-	go func() { ch <- f() }()
+
+	go func() {
+		select {
+		case <-ctx.Done():
+		case ch <- f():
+		}
+	}()
 	select {
 	case <-ctx.Done():
 		return ctx.Err()

util/do_test.go

@@ -4,7 +4,7 @@ import (
 	"errors"
 	"testing"
 
-	"code.google.com/p/go.net/context"
+	"github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
 )
 
 func TestDoReturnsContextErr(t *testing.T) {