async closure workaround for virtual file

pageserver/src/virtual_file.rs

@@ -594,10 +594,8 @@ impl VirtualFile {
     where
         B: IoBufMut + Send,
     {
-        let (buf, res) = read_exact_at_impl(buf, offset, None, |buf, offset| {
-            self.read_at(buf, offset, ctx)
-        })
-        .await;
+        let (buf, res) =
+            read_exact_at_impl(buf, offset, None, VirtualFileReadAt { file: self, ctx }).await;
         res.map(|()| buf)
     }
 
@@ -611,9 +609,12 @@ impl VirtualFile {
     where
         B: IoBufMut + Send,
     {
-        let (buf, res) = read_exact_at_impl(buf, offset, Some(count), |buf, offset| {
-            self.read_at(buf, offset, ctx)
-        })
+        let (buf, res) = read_exact_at_impl(
+            buf,
+            offset,
+            Some(count),
+            VirtualFileReadAt { file: self, ctx },
+        )
         .await;
         res.map(|()| buf)
     }
@@ -780,8 +781,35 @@ impl VirtualFile {
     }
 }
 
+trait AsyncClosureThatDoesReadAt {
+    async fn call<B>(
+        &mut self,
+        buf: tokio_epoll_uring::Slice<B>,
+        offset: u64,
+    ) -> (tokio_epoll_uring::Slice<B>, std::io::Result<usize>)
+    where
+        B: IoBufMut + Send;
+}
+
+struct VirtualFileReadAt<'a> {
+    file: &'a VirtualFile,
+    ctx: &'a mut RequestContext,
+}
+impl<'a> AsyncClosureThatDoesReadAt for VirtualFileReadAt<'a> {
+    async fn call<B>(
+        &mut self,
+        buf: tokio_epoll_uring::Slice<B>,
+        offset: u64,
+    ) -> (tokio_epoll_uring::Slice<B>, std::io::Result<usize>)
+    where
+        B: IoBufMut + Send,
+    {
+        self.file.read_at(buf, offset, self.ctx).await
+    }
+}
+
 // Adapted from https://doc.rust-lang.org/1.72.0/src/std/os/unix/fs.rs.html#117-135
-pub async fn read_exact_at_impl<B, F, Fut>(
+pub async fn read_exact_at_impl<B, F>(
     buf: B,
     mut offset: u64,
     count: Option<usize>,
@@ -789,8 +817,7 @@ pub async fn read_exact_at_impl<B, F, Fut>(
 ) -> (B, std::io::Result<()>)
 where
     B: IoBufMut + Send,
-    F: FnMut(tokio_epoll_uring::Slice<B>, u64) -> Fut,
-    Fut: std::future::Future<Output = (tokio_epoll_uring::Slice<B>, std::io::Result<usize>)>,
+    F: AsyncClosureThatDoesReadAt,
 {
     let mut buf: tokio_epoll_uring::Slice<B> = match count {
         Some(count) => {
@@ -802,8 +829,8 @@ where
     };
 
     while buf.bytes_total() != 0 {
-        let res;
-        (buf, res) = read_at(buf, offset).await;
+        let res: std::io::Result<usize>;
+        (buf, res) = read_at.call(buf, offset).await;
         match res {
             Ok(0) => break,
             Ok(n) => {
@@ -1058,7 +1085,11 @@ impl VirtualFile {
         Ok(crate::tenant::block_io::BlockLease::Vec(buf))
     }
 
-    async fn read_to_end(&mut self, buf: &mut Vec<u8>, ctx: &mut RequestContext) -> Result<(), Error> {
+    async fn read_to_end(
+        &mut self,
+        buf: &mut Vec<u8>,
+        ctx: &mut RequestContext,
+    ) -> Result<(), Error> {
         let mut tmp = vec![0; 128];
         loop {
             let res;