wasi: Handle read(0) with file streams (#8611)

* Test that wasi file streams can handle read(0)

* Zero-sized reads don't fail for file streams

* Accidentally removed the `read(0)` when refactoring the test

crates/test-programs/src/bin/preview2_file_read_write.rs

@@ -22,7 +22,31 @@ fn main() {
     let stream = file.read_via_stream(0).unwrap();
     let contents = stream.blocking_read(100).unwrap();
     assert_eq!(contents, b"\0\0\0\0\0Hello, World!");
-    drop((stream, file));
+    drop(stream);
+
+    // Test that file read streams behave like other read streams.
+    let mut buf = Vec::new();
+    let stream = file.read_via_stream(0).unwrap();
+    let ready = stream.subscribe();
+    loop {
+        ready.block();
+
+        match stream.read(0) {
+            Ok(chunk) => assert!(chunk.is_empty()),
+            Err(wasi::io::streams::StreamError::Closed) => break,
+            Err(e) => panic!("Failed checking stream state: {e:?}"),
+        }
+
+        match stream.read(4) {
+            Ok(chunk) => buf.extend(chunk),
+            Err(wasi::io::streams::StreamError::Closed) => break,
+            Err(e) => panic!("Failed reading stream: {e:?}"),
+        }
+    }
+    assert_eq!(buf, b"\0\0\0\0\0Hello, World!");
+    drop(ready);
+    drop(stream);
+    drop(file);
 
     dir.unlink_file_at(filename).unwrap();
 }

crates/wasi/src/filesystem.rs

@@ -248,6 +248,7 @@ impl FileInputStream {
     pub async fn read(&mut self, size: usize) -> Result<Bytes, StreamError> {
         use system_interface::fs::FileIoExt;
         let p = self.position;
+
         let (r, mut buf) = self
             .file
             .spawn_blocking(move |f| {
@@ -256,7 +257,7 @@ impl FileInputStream {
                 (r, buf)
             })
             .await;
-        let n = read_result(r)?;
+        let n = read_result(r, size)?;
         buf.truncate(n);
         self.position += n as u64;
         Ok(buf.freeze())
@@ -268,9 +269,9 @@ impl FileInputStream {
     }
 }
 
-fn read_result(r: io::Result<usize>) -> Result<usize, StreamError> {
+fn read_result(r: io::Result<usize>, size: usize) -> Result<usize, StreamError> {
     match r {
-        Ok(0) => Err(StreamError::Closed),
+        Ok(0) if size > 0 => Err(StreamError::Closed),
         Ok(n) => Ok(n),
         Err(e) if e.kind() == std::io::ErrorKind::Interrupted => Ok(0),
         Err(e) => Err(StreamError::LastOperationFailed(e.into())),