fix fd_read and fd_write for wasm32-wasi-threads (#7750)

Previously, `first_non_empty_{c}io_vec` always returned `Ok(None)` for buffers
residing in shared memories since they cannot, in general, safely be represented
as slices.  That caused e.g. `wasi-libc` to spin forever when trying to write to
stdout using `fd_write` since it always got `Ok(0)` and never made progress.

This commit changes the return type of both functions to use `GuestPtr` instead
of `GuestSlice{Mut}`, allowing safe access to shared guest memory.

Big thanks to Alex Crichton for narrowing this down and suggesting the fix.

Fixes #7745

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

crates/wasi/src/preview2/preview1.rs

@@ -19,7 +19,7 @@ use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::Arc;
 use wasmtime::component::Resource;
 use wiggle::tracing::instrument;
-use wiggle::{GuestError, GuestPtr, GuestSlice, GuestSliceMut, GuestStrCow, GuestType};
+use wiggle::{GuestError, GuestPtr, GuestStrCow, GuestType};
 
 #[derive(Debug)]
 struct File {
@@ -81,10 +81,13 @@ impl BlockingMode {
         &self,
         host: &mut (impl streams::Host + poll::Host),
         output_stream: Resource<streams::OutputStream>,
-        mut bytes: &[u8],
+        bytes: GuestPtr<'_, [u8]>,
     ) -> StreamResult<usize> {
         use streams::HostOutputStream as Streams;
 
+        let bytes = bytes.as_cow().map_err(|e| StreamError::Trap(e.into()))?;
+        let mut bytes = &bytes[..];
+
         match self {
             BlockingMode::Blocking => {
                 let total = bytes.len();
@@ -814,29 +817,27 @@ fn read_string<'a>(ptr: impl Borrow<GuestPtr<'a, str>>) -> Result<String> {
 // Find first non-empty buffer.
 fn first_non_empty_ciovec<'a, 'b>(
     ciovs: &'a types::CiovecArray<'b>,
-) -> Result<Option<GuestSlice<'a, u8>>> {
+) -> Result<Option<GuestPtr<'a, [u8]>>> {
     for iov in ciovs.iter() {
         let iov = iov?.read()?;
         if iov.buf_len == 0 {
             continue;
         }
-        return Ok(iov.buf.as_array(iov.buf_len).as_slice()?);
+        return Ok(Some(iov.buf.as_array(iov.buf_len)));
     }
     Ok(None)
 }
 
 // Find first non-empty buffer.
-fn first_non_empty_iovec<'a>(
-    iovs: &types::IovecArray<'a>,
-) -> Result<Option<GuestSliceMut<'a, u8>>> {
+fn first_non_empty_iovec<'a>(iovs: &types::IovecArray<'a>) -> Result<Option<GuestPtr<'a, [u8]>>> {
     iovs.iter()
         .map(|iov| {
             let iov = iov?.read()?;
             if iov.buf_len == 0 {
                 return Ok(None);
             }
-            let slice = iov.buf.as_array(iov.buf_len).as_slice_mut()?;
-            Ok(slice)
+            let slice = iov.buf.as_array(iov.buf_len);
+            Ok(Some(slice))
         })
         .find_map(Result::transpose)
         .transpose()
@@ -1317,7 +1318,7 @@ impl<
     ) -> Result<types::Size, types::Error> {
         let t = self.transact()?;
         let desc = t.get_descriptor(fd)?;
-        let (mut buf, read) = match desc {
+        let (buf, read) = match desc {
             Descriptor::File(File {
                 fd,
                 blocking_mode,
@@ -1338,7 +1339,9 @@ impl<
                         .context("failed to call `read-via-stream`")
                         .unwrap_or_else(types::Error::trap)
                 })?;
-                let read = blocking_mode.read(self, stream, buf.len()).await?;
+                let read = blocking_mode
+                    .read(self, stream, buf.len().try_into()?)
+                    .await?;
                 let n = read.len().try_into()?;
                 let pos = pos.checked_add(n).ok_or(types::Errno::Overflow)?;
                 position.store(pos, Ordering::Relaxed);
@@ -1351,16 +1354,18 @@ impl<
                 let Some(buf) = first_non_empty_iovec(iovs)? else {
                     return Ok(0);
                 };
-                let read = BlockingMode::Blocking.read(self, stream, buf.len()).await?;
+                let read = BlockingMode::Blocking
+                    .read(self, stream, buf.len().try_into()?)
+                    .await?;
                 (buf, read)
             }
             _ => return Err(types::Errno::Badf.into()),
         };
-        if read.len() > buf.len() {
+        if read.len() > buf.len().try_into()? {
             return Err(types::Errno::Range.into());
         }
-        let (buf, _) = buf.split_at_mut(read.len());
-        buf.copy_from_slice(&read);
+        let buf = buf.get_range(0..u32::try_from(read.len())?).unwrap();
+        buf.copy_from_slice(&read)?;
         let n = read.len().try_into()?;
         Ok(n)
     }
@@ -1376,7 +1381,7 @@ impl<
     ) -> Result<types::Size, types::Error> {
         let t = self.transact()?;
         let desc = t.get_descriptor(fd)?;
-        let (mut buf, read) = match desc {
+        let (buf, read) = match desc {
             Descriptor::File(File {
                 fd, blocking_mode, ..
             }) if t.view.table().get(fd)?.is_file() => {
@@ -1392,7 +1397,9 @@ impl<
                         .context("failed to call `read-via-stream`")
                         .unwrap_or_else(types::Error::trap)
                 })?;
-                let read = blocking_mode.read(self, stream, buf.len()).await?;
+                let read = blocking_mode
+                    .read(self, stream, buf.len().try_into()?)
+                    .await?;
                 (buf, read)
             }
             Descriptor::Stdin { .. } => {
@@ -1401,11 +1408,11 @@ impl<
             }
             _ => return Err(types::Errno::Badf.into()),
         };
-        if read.len() > buf.len() {
+        if read.len() > buf.len().try_into()? {
             return Err(types::Errno::Range.into());
         }
-        let (buf, _) = buf.split_at_mut(read.len());
-        buf.copy_from_slice(&read);
+        let buf = buf.get_range(0..u32::try_from(read.len())?).unwrap();
+        buf.copy_from_slice(&read)?;
         let n = read.len().try_into()?;
         Ok(n)
     }
@@ -1452,7 +1459,7 @@ impl<
                     })?;
                     (stream, pos)
                 };
-                let n = blocking_mode.write(self, stream, &buf).await?;
+                let n = blocking_mode.write(self, stream, buf).await?;
                 if append {
                     let len = self.stat(fd2).await?;
                     position.store(len.size, Ordering::Relaxed);
@@ -1470,7 +1477,7 @@ impl<
                     return Ok(0);
                 };
                 let n = BlockingMode::Blocking
-                    .write(self, stream, &buf)
+                    .write(self, stream, buf)
                     .await?
                     .try_into()?;
                 Ok(n)
@@ -1505,7 +1512,7 @@ impl<
                         .context("failed to call `write-via-stream`")
                         .unwrap_or_else(types::Error::trap)
                 })?;
-                blocking_mode.write(self, stream, &buf).await?
+                blocking_mode.write(self, stream, buf).await?
             }
             Descriptor::Stdout { .. } | Descriptor::Stderr { .. } => {
                 // NOTE: legacy implementation returns SPIPE here