Fix fd with append flag can't seek correctly

lib/wasix/src/syscalls/wasi/fd_seek.rs

@@ -62,9 +62,6 @@ pub(crate) fn fd_seek_internal(
     if !fd_entry.rights.contains(Rights::FD_SEEK) {
         return Ok(Err(Errno::Access));
     }
-    if fd_entry.flags.contains(Fdflags::APPEND) {
-        return Ok(Ok(fd_entry.offset.load(Ordering::Acquire)));
-    }
 
     // TODO: handle case if fd is a dir?
     let new_offset = match whence {

lib/wasix/src/syscalls/wasi/fd_write.rs

@@ -127,6 +127,7 @@ pub(crate) fn fd_write_internal<M: MemorySize>(
     should_update_cursor: bool,
     should_snapshot: bool,
 ) -> Result<Result<usize, Errno>, WasiError> {
+    let mut offset = offset;
     let mut env = ctx.data();
     let state = env.state.clone();
 
@@ -160,6 +161,12 @@ pub(crate) fn fd_write_internal<M: MemorySize>(
                             async {
                                 let mut handle = handle.write().unwrap();
                                 if !is_stdio {
+                                    if fd_entry.flags.contains(Fdflags::APPEND) {
+                                        // `fdflags::append` means we need to seek to the end before writing.
+                                        offset = handle.size();
+                                        fd_entry.offset.store(offset, Ordering::Release);
+                                    }
+
                                     handle
                                         .seek(std::io::SeekFrom::Start(offset))
                                         .await

tests/wasi-fyi/fd_seek_append.rs

@@ -0,0 +1,15 @@
+use std::io::{Seek as _, SeekFrom};
+
+fn main() {
+    let offset = 100u64;
+    let mut f = std::fs::OpenOptions::new()
+        .read(true)
+        .write(true)
+        .create(true)
+        .append(true)
+        .open("/fyi/fs_seek_append.dir/file")
+        .unwrap();
+    let new_offset = f.seek(SeekFrom::Start(offset)).unwrap();
+
+    assert_eq!(offset, new_offset);
+}

tests/wasi-fyi/fs_seek_append_mode.rs

@@ -13,8 +13,9 @@ fn main() {
     // file offset must be 1 now
     write!(file, "{}", "a").unwrap();
 
-    // rewind should not work on file in append mode
-    // since the offset must always be at the end of the file
+    // rewind should not work on file in append mode.
+    // It changes the offset, which is immediately set to the end of the file
+    // with a write.
     let _ = file.rewind();
 
     // file offset must be 2 now