Use wiggle "trappable error" to implement wasi-common (#5279)

* convert wasi-common from defining its own error to using wiggle trappable error

* wasi-common impl crates: switch error strategy

* wasmtime-wasi: error is trappable, and no longer requires UserErrorConversion

* docs

* typo

* readdir: windows fixes

* fix windows scheduler errors

fun fact! the Send and Recv errors here that just had a `.context` on
them were previously not being captured in the downcasting either. They
need to be traps, and would have ended up that way by ommission, but
you'd never actually know that by reading the code!

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

@@ -138,6 +138,19 @@ impl WasiDir for Dir {
         &self,
         cursor: ReaddirCursor,
     ) -> Result<Box<dyn Iterator<Item = Result<ReaddirEntity, Error>> + Send>, Error> {
+        // We need to keep a full-fidelity io Error around to check for a special failure mode
+        // on windows, but also this function can fail due to an illegal byte sequence in a
+        // filename, which we can't construct an io Error to represent.
+        enum ReaddirError {
+            Io(std::io::Error),
+            IllegalSequence,
+        }
+        impl From<std::io::Error> for ReaddirError {
+            fn from(e: std::io::Error) -> ReaddirError {
+                ReaddirError::Io(e)
+            }
+        }
+
         // cap_std's read_dir does not include . and .., we should prepend these.
         // Why does the Ok contain a tuple? We can't construct a cap_std::fs::DirEntry, and we don't
         // have enough info to make a ReaddirEntity yet.
@@ -145,7 +158,7 @@ impl WasiDir for Dir {
         let rd = vec![
             {
                 let name = ".".to_owned();
-                Ok((FileType::Directory, dir_meta.ino(), name))
+                Ok::<_, ReaddirError>((FileType::Directory, dir_meta.ino(), name))
             },
             {
                 let name = "..".to_owned();
@@ -163,24 +176,22 @@ impl WasiDir for Dir {
                 let name = entry
                     .file_name()
                     .into_string()
-                    .map_err(|_| Error::illegal_byte_sequence().context("filename"))?;
+                    .map_err(|_| ReaddirError::IllegalSequence)?;
                 Ok((filetype, inode, name))
             });
 
             // On Windows, filter out files like `C:\DumpStack.log.tmp` which we
             // can't get a full metadata for.
             #[cfg(windows)]
-            let entries = entries.filter(|entry: &Result<_, wasi_common::Error>| {
+            let entries = entries.filter(|entry| {
                 use windows_sys::Win32::Foundation::{
                     ERROR_ACCESS_DENIED, ERROR_SHARING_VIOLATION,
                 };
-                if let Err(err) = entry {
-                    if let Some(err) = err.downcast_ref::<std::io::Error>() {
-                        if err.raw_os_error() == Some(ERROR_SHARING_VIOLATION as i32)
-                            || err.raw_os_error() == Some(ERROR_ACCESS_DENIED as i32)
-                        {
-                            return false;
-                        }
+                if let Err(ReaddirError::Io(err)) = entry {
+                    if err.raw_os_error() == Some(ERROR_SHARING_VIOLATION as i32)
+                        || err.raw_os_error() == Some(ERROR_ACCESS_DENIED as i32)
+                    {
+                        return false;
                     }
                 }
                 true
@@ -197,7 +208,8 @@ impl WasiDir for Dir {
                 inode,
                 name,
             }),
-            Err(e) => Err(e),
+            Err(ReaddirError::Io(e)) => Err(e.into()),
+            Err(ReaddirError::IllegalSequence) => Err(Error::illegal_byte_sequence()),
         })
         .skip(u64::from(cursor) as usize);