fix append-ness of new fd affecting old fds (#5162)

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

@@ -163,7 +163,7 @@ pub(crate) fn fd_write_internal<M: MemorySize>(
                                 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();
+                                        offset = fd_entry.inode.stat.read().unwrap().st_size;
                                         fd_entry.offset.store(offset, Ordering::Release);
                                     }
 

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

@@ -237,7 +237,7 @@ pub(crate) fn path_open_internal(
                 let open_options = open_options
                     .write(minimum_rights.write)
                     .create(minimum_rights.create)
-                    .append(minimum_rights.append)
+                    .append(false)
                     .truncate(minimum_rights.truncate);
 
                 if minimum_rights.read {

tests/wasi-fyi/fs_open_append_offset.rs

@@ -0,0 +1,31 @@
+use std::{
+    fs,
+    io::{Seek, SeekFrom, Write},
+};
+
+fn main() {
+    let file = "fyi/fs_open_append_offset.dir/file";
+    let mut f0 = fs::OpenOptions::new()
+        .create(true)
+        .truncate(true)
+        .write(true)
+        .open(file)
+        .unwrap();
+
+    f0.write_all(b"abc").unwrap();
+    f0.seek(SeekFrom::Start(1)).unwrap();
+
+    assert_eq!(fs::read_to_string(file).unwrap(), "abc");
+
+    // This open with append should not affect the offset of f0.
+    let _f1 = fs::OpenOptions::new()
+        .create(true)
+        .write(true)
+        .append(true)
+        .open(file)
+        .unwrap();
+
+    f0.write_all(b"d").unwrap();
+
+    assert_eq!(fs::read_to_string(file).unwrap(), "adc");
+}

tests/wasi-fyi/test_fs/fyi/fs_open_append_offset.dir/.gitignore

@@ -0,0 +1 @@
+file