Support File::seek for Hermit

Author: thaliaarchi

library/std/src/sys/fs/hermit.rs

@@ -417,12 +417,12 @@ impl File {
         Ok(())
     }
 
-    pub fn seek(&self, _pos: SeekFrom) -> io::Result<u64> {
-        Err(Error::from_raw_os_error(22))
+    pub fn seek(&self, pos: SeekFrom) -> io::Result<u64> {
+        self.0.seek(pos)
     }
 
     pub fn tell(&self) -> io::Result<u64> {
-        self.seek(SeekFrom::Current(0))
+        self.0.tell()
     }
 
     pub fn duplicate(&self) -> io::Result<File> {

library/std/src/sys/pal/hermit/fd.rs

@@ -2,8 +2,8 @@
 
 use super::hermit_abi;
 use crate::cmp;
-use crate::io::{self, IoSlice, IoSliceMut, Read};
-use crate::os::hermit::io::{FromRawFd, OwnedFd, RawFd, *};
+use crate::io::{self, IoSlice, IoSliceMut, Read, SeekFrom};
+use crate::os::hermit::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
 use crate::sys::{cvt, unsupported};
 use crate::sys_common::{AsInner, FromInner, IntoInner};
 
@@ -66,9 +66,26 @@ impl FileDesc {
         true
     }
 
+    pub fn seek(&self, pos: SeekFrom) -> io::Result<u64> {
+        let (whence, pos) = match pos {
+            // Casting to `i64` is fine, too large values will end up as
+            // negative which will cause an error in `lseek`.
+            SeekFrom::Start(off) => (hermit_abi::SEEK_SET, off as i64),
+            SeekFrom::End(off) => (hermit_abi::SEEK_END, off),
+            SeekFrom::Current(off) => (hermit_abi::SEEK_CUR, off),
+        };
+        let n = cvt(unsafe { hermit_abi::lseek(self.as_raw_fd(), pos as isize, whence) })?;
+        Ok(n as u64)
+    }
+
+    pub fn tell(&self) -> io::Result<u64> {
+        self.seek(SeekFrom::Current(0))
+    }
+
     pub fn duplicate(&self) -> io::Result<FileDesc> {
         self.duplicate_path(&[])
     }
+
     pub fn duplicate_path(&self, _path: &[u8]) -> io::Result<FileDesc> {
         unsupported()
     }