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, 8 insertions, 0 deletions

diff --git a/src/msg.fbs b/src/msg.fbs
index fcf29ad58..9776bb893 100644
--- a/src/msg.fbs
+++ b/src/msg.fbs
@@ -65,6 +65,7 @@ union Any {
   NowRes,
   IsTTY,
   IsTTYRes,
+  Seek,
 }
 
 enum ErrorKind: byte {
@@ -117,6 +118,7 @@ enum ErrorKind: byte {
 
   // custom errors
   InvalidUri,
+  InvalidSeekMode,
 }
 
 table Cwd {}
@@ -496,4 +498,10 @@ table IsTTYRes {
   stderr: bool;
 }
 
+table Seek {
+  rid: uint32;
+  offset: int;
+  whence: uint;
+}
+
 root_type Base;