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, 23 insertions, 2 deletions

diff --git a/js/files.ts b/js/files.ts
index e2c7123e6..a77f788df 100644
--- a/js/files.ts
+++ b/js/files.ts
@@ -1,12 +1,12 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
-import { Reader, Writer, Closer, ReadResult } from "./io";
+import { Reader, Writer, Seeker, Closer, ReadResult, SeekMode } from "./io";
 import * as dispatch from "./dispatch";
 import * as msg from "gen/msg_generated";
 import { assert } from "./util";
 import * as flatbuffers from "./flatbuffers";
 
 /** The Deno abstraction for reading and writing files. */
-export class File implements Reader, Writer, Closer {
+export class File implements Reader, Writer, Seeker, Closer {
   constructor(readonly rid: number) {}
 
   write(p: Uint8Array): Promise<number> {
@@ -17,6 +17,10 @@ export class File implements Reader, Writer, Closer {
     return read(this.rid, p);
   }
 
+  seek(offset: number, whence: SeekMode): Promise<void> {
+    return seek(this.rid, offset, whence);
+  }
+
   close(): void {
     close(this.rid);
   }
@@ -123,6 +127,23 @@ export async function write(rid: number, p: Uint8Array): Promise<number> {
   return res.nbyte();
 }
 
+/** Seek a file ID to the given offset under mode given by `whence`.
+ *
+ */
+export async function seek(
+  rid: number,
+  offset: number,
+  whence: SeekMode
+): Promise<void> {
+  const builder = flatbuffers.createBuilder();
+  msg.Seek.startSeek(builder);
+  msg.Seek.addRid(builder, rid);
+  msg.Seek.addOffset(builder, offset);
+  msg.Seek.addWhence(builder, whence);
+  const inner = msg.Seek.endSeek(builder);
+  await dispatch.sendAsync(builder, msg.Any.Seek, inner);
+}
+
 /** Close the file ID. */
 export function close(rid: number): void {
   const builder = flatbuffers.createBuilder();