set_fd_flags is only good for append and nonblock

crates/wasi-common/cap-std-sync/src/dir.rs

@@ -65,21 +65,15 @@ impl WasiDir for Dir {
         } else {
             opts.follow(FollowSymlinks::No);
         }
-
-        let mut f = self.0.open_with(Path::new(path), &opts)?;
-        // This takes care of setting the fdflags that can't be handled by OpenOptions (yet)
-        // (DSYNC, SYNC, NONBLOCK, RSYNC)
+        // the DSYNC, SYNC, and RSYNC flags are ignored! We do not
+        // have support for them in cap-std yet.
         // ideally OpenOptions would just support this though:
         // https://github.com/bytecodealliance/cap-std/issues/146
-        {
-            // Even worse - set_fd_flags panicks when given DSYNC, SYNC, RSYNC!
-            // it only supports NONBLOCK
-            let fdflags = if fdflags.contains(wasi_common::file::FdFlags::NONBLOCK) {
-                system_interface::fs::FdFlags::NONBLOCK
-            } else {
-                system_interface::fs::FdFlags::empty()
-            };
-            f.set_fd_flags(fdflags)?;
+
+        let mut f = self.0.open_with(Path::new(path), &opts)?;
+        // NONBLOCK does not have an OpenOption either, but we can patch that on with set_fd_flags:
+        if fdflags.contains(wasi_common::file::FdFlags::NONBLOCK) {
+            f.set_fd_flags(system_interface::fs::FdFlags::NONBLOCK)?;
         }
         Ok(Box::new(File::from_cap_std(f)))
     }

crates/wasi-common/cap-std-sync/src/file.rs

@@ -9,7 +9,7 @@ use system_interface::{
 };
 use wasi_common::{
     file::{Advice, FdFlags, FileType, Filestat, WasiFile},
-    Error,
+    Error, ErrorExt,
 };
 
 pub struct File(cap_std::fs::File);
@@ -41,6 +41,13 @@ impl WasiFile for File {
         Ok(from_sysif_fdflags(fdflags))
     }
     fn set_fdflags(&mut self, fdflags: FdFlags) -> Result<(), Error> {
+        if fdflags.intersects(
+            wasi_common::file::FdFlags::DSYNC
+                | wasi_common::file::FdFlags::SYNC
+                | wasi_common::file::FdFlags::RSYNC,
+        ) {
+            return Err(Error::invalid_argument().context("cannot set DSYNC, SYNC, or RSYNC flag"));
+        }
         Ok(self.0.set_fd_flags(to_sysif_fdflags(fdflags))?)
     }
     fn get_filestat(&self) -> Result<Filestat, Error> {