From bb88e5bf57e2bb7f5bccb4517f24ce04f5063240 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <me@cschwarz.com>
Date: Tue, 29 Aug 2023 09:53:01 +0000
Subject: [PATCH] try to convert to async, now lifetime errors because buffer
 lifetime is insufficient

---
 pageserver/src/virtual_file.rs | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/pageserver/src/virtual_file.rs b/pageserver/src/virtual_file.rs
index 7b12cc89ac..1b28ef036c 100644
--- a/pageserver/src/virtual_file.rs
+++ b/pageserver/src/virtual_file.rs
@@ -15,8 +15,10 @@ use crate::metrics::{STORAGE_IO_SIZE, STORAGE_IO_TIME};
 use std::fs::{self, File, OpenOptions};
 use std::io::{Error, ErrorKind, Seek, SeekFrom, Write};
 
+use std::os::fd::AsRawFd;
 use std::os::unix::fs::FileExt;
 use std::path::{Path, PathBuf};
+use std::sync::{Arc, Mutex};
 
 ///
 /// A virtual file descriptor. You can use this just like std::fs::File, but internally
@@ -38,7 +40,7 @@ pub struct VirtualFile {
     /// Lazy handle to the global file descriptor cache. The slot that this points to
     /// might contain our File, or it may be empty, or it may contain a File that
     /// belongs to a different VirtualFile.
-    handle: File,
+    handle: Arc<Mutex<Option<File>>>, // only transiently None
 
     /// Current file position
     pos: u64,
@@ -108,7 +110,7 @@ impl VirtualFile {
         reopen_options.truncate(false);
 
         let vfile = VirtualFile {
-            handle: file,
+            handle: Arc::new(Mutex::new(Some(file))),
             pos: 0,
             path: path.to_path_buf(),
             tenant_id,
@@ -138,7 +140,7 @@ impl VirtualFile {
     {
         return Ok(STORAGE_IO_TIME
             .with_label_values(&[op])
-            .observe_closure_duration(|| func(&self.handle)));
+            .observe_closure_duration(|| func(&*self.handle.lock().unwrap().as_ref().unwrap())));
     }
 
     pub fn remove(self) {
@@ -210,6 +212,28 @@ impl VirtualFile {
         Ok(())
     }
 
+    // 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: &mut [u8],
+        mut offset: u64,
+    ) -> Result<(), Error> {
+        let file = self.handle.lock().unwrap().take().unwrap();
+        let mut put_back = false;
+        scopeguard::defer! {
+            if !put_back {
+                panic!("mut put self.handle back")
+            }
+        };
+        let res = tokio::task::spawn_blocking(|| file.read_exact_at(buf, offset))
+            .await
+            .expect("spawn_blocking");
+        let replaced = self.handle.lock().unwrap().replace(file);
+        assert!(replaced.is_none());
+        put_back = true;
+        res
+    }
+
     // Copied from https://doc.rust-lang.org/1.72.0/src/std/os/unix/fs.rs.html#219-235
     pub fn write_all_at(&self, mut buf: &[u8], mut offset: u64) -> Result<(), Error> {
         while !buf.is_empty() {