From 0b8ff8dbe09356b208a3e6857840abd7f7139ea6 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <me@cschwarz.com>
Date: Tue, 29 Aug 2023 16:42:14 +0000
Subject: [PATCH] Revert "switch back to spawn_blocking to make the comparison"

This reverts commit 60971e282e18fdc094c0b034732e9b9a168aaa54.
---
 pageserver/src/virtual_file.rs | 48 ++++++++++++++++++++++++++++------
 1 file changed, 40 insertions(+), 8 deletions(-)

diff --git a/pageserver/src/virtual_file.rs b/pageserver/src/virtual_file.rs
index 6c7e174462..b16ba2b5be 100644
--- a/pageserver/src/virtual_file.rs
+++ b/pageserver/src/virtual_file.rs
@@ -283,7 +283,7 @@ impl VirtualFile {
     // Copied from https://doc.rust-lang.org/1.72.0/src/std/os/unix/fs.rs.html#117-135
     pub async fn read_exact_at_async(
         &self,
-        mut buf: crate::buffer_pool::Buffer,
+        mut write_guard: crate::buffer_pool::Buffer,
         offset: u64,
     ) -> Result<crate::buffer_pool::Buffer, Error> {
         let file = self.handle.lock().unwrap().take().unwrap();
@@ -294,16 +294,48 @@ impl VirtualFile {
                 panic!("mut put self.handle back")
             }
         };
-        let ((file, buf), res) = tokio::task::spawn_blocking(move || {
-            let res = file.read_exact_at(buf.as_mut(), offset);
-            ((file, buf), res)
-        })
-        .await
-        .expect("spawn_blocking");
+        let system = tokio_epoll_uring::thread_local_system().await;
+        struct PageWriteGuardBuf {
+            buf: crate::buffer_pool::Buffer,
+            init_up_to: usize,
+        }
+        unsafe impl tokio_epoll_uring::IoBuf for PageWriteGuardBuf {
+            fn stable_ptr(&self) -> *const u8 {
+                self.buf.as_ptr()
+            }
+            fn bytes_init(&self) -> usize {
+                self.init_up_to
+            }
+            fn bytes_total(&self) -> usize {
+                self.buf.len()
+            }
+        }
+        unsafe impl tokio_epoll_uring::IoBufMut for PageWriteGuardBuf {
+            fn stable_mut_ptr(&mut self) -> *mut u8 {
+                self.buf.as_mut_ptr()
+            }
+
+            unsafe fn set_init(&mut self, pos: usize) {
+                assert!(pos <= self.buf.len());
+                self.init_up_to = pos;
+            }
+        }
+        let buf = PageWriteGuardBuf {
+            buf: write_guard,
+            init_up_to: 0,
+        };
+        let ((file, buf), res) = system.read(file.into(), offset, buf).await;
+        let PageWriteGuardBuf {
+            buf: write_guard,
+            init_up_to,
+        } = buf;
+        if let Ok(num_read) = res {
+            assert!(init_up_to <= num_read);
+        }
         let replaced = self.handle.lock().unwrap().replace(File::from(file));
         assert!(replaced.is_none());
         put_back.store(true, std::sync::atomic::Ordering::Relaxed);
-        res.map(|_| buf)
+        res.map(|_| write_guard)
             .map_err(|e| Error::new(ErrorKind::Other, e))
     }