Add `seek` and implement `Seeker` on `File` (#1797)

This patch contains a special hack that circumvents the current tokio
seek problem.

tokio `seek` is implemented to take ownership of the original File and
emit a new one in its future, which conflicts with the design of
ResourceTable.

To avoid the problem, the current hack makes the FsFile resource
an Option which we could `take` the value ownership out of it. We then
convert the tokio File into a Rust std File, perform the seek, and then
put it back into the resource.

This might be able to drop this hack after
https://github.com/tokio-rs/tokio/pull/785 lands.

1 files changed, 9 insertions, 1 deletions

diff --git a/js/io.ts b/js/io.ts
index f7dbf3318..7f56187c1 100644
--- a/js/io.ts
+++ b/js/io.ts
@@ -9,6 +9,14 @@ export interface ReadResult {
   eof: boolean;
 }
 
+// Seek whence values.
+// https://golang.org/pkg/io/#pkg-constants
+export enum SeekMode {
+  SEEK_START = 0,
+  SEEK_CURRENT = 1,
+  SEEK_END = 2
+}
+
 // Reader is the interface that wraps the basic read() method.
 // https://golang.org/pkg/io/#Reader
 export interface Reader {
@@ -74,7 +82,7 @@ export interface Seeker {
    * any positive offset is legal, but the behavior of subsequent I/O operations
    * on the underlying object is implementation-dependent.
    */
-  seek(offset: number, whence: number): Promise<void>;
+  seek(offset: number, whence: SeekMode): Promise<void>;
 }
 
 // https://golang.org/pkg/io/#ReadCloser