refactor: get_file_guard & base with_file on it

pageserver/src/virtual_file.rs

@@ -14,10 +14,11 @@ use crate::metrics::{STORAGE_IO_SIZE, STORAGE_IO_TIME};
 use once_cell::sync::OnceCell;
 use std::fs::{self, File, OpenOptions};
 use std::io::{Error, ErrorKind, Seek, SeekFrom, Write};
+use std::ops::Deref;
 use std::os::unix::fs::FileExt;
 use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
-use std::sync::{RwLock, RwLockWriteGuard};
+use std::sync::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
 ///
 /// A virtual file descriptor. You can use this just like std::fs::File, but internally
@@ -244,7 +245,26 @@ impl VirtualFile {
     pub fn metadata(&self) -> Result<fs::Metadata, Error> {
         self.with_file("metadata", |file| file.metadata())?
     }
+}
 
+/// The function that constructs the guard guarantees that the respective [`SlotInner::file`] is `Some`.
+enum FileSlotGuard {
+    Write(RwLockWriteGuard<'static, SlotInner>),
+    Read(RwLockReadGuard<'static, SlotInner>),
+}
+
+impl Deref for FileSlotGuard {
+    type Target = File;
+
+    fn deref(&self) -> &Self::Target {
+        match self {
+            FileSlotGuard::Write(guard) => guard.file.as_ref().unwrap(),
+            FileSlotGuard::Read(guard) => guard.file.as_ref().unwrap(),
+        }
+    }
+}
+
+impl VirtualFile {
     /// Helper function that looks up the underlying File for this VirtualFile,
     /// opening it and evicting some other File if necessary. It calls 'func'
     /// with the physical File.
@@ -252,6 +272,14 @@ impl VirtualFile {
     where
         F: FnMut(&File) -> R,
     {
+        let guard = self.get_file_guard()?;
+
+        return Ok(STORAGE_IO_TIME
+            .with_label_values(&[op])
+            .observe_closure_duration(|| func(&guard)));
+    }
+
+    fn get_file_guard(&self) -> Result<FileSlotGuard, Error> {
         let open_files = get_open_files();
 
         let mut handle_guard = {
@@ -268,12 +296,10 @@ impl VirtualFile {
                     let slot = &open_files.slots[handle.index];
                     let slot_guard = slot.inner.read().unwrap();
                     if slot_guard.tag == handle.tag {
-                        if let Some(file) = &slot_guard.file {
+                        if let Some(_) = &slot_guard.file {
                             // Found a cached file descriptor.
                             slot.recently_used.store(true, Ordering::Relaxed);
-                            return Ok(STORAGE_IO_TIME
-                                .with_label_values(&[op])
-                                .observe_closure_duration(|| func(file)));
+                            return Ok(FileSlotGuard::Read(slot_guard));
                         }
                     }
                 }
@@ -302,18 +328,13 @@ impl VirtualFile {
             .with_label_values(&["open"])
             .observe_closure_duration(|| self.open_options.open(&self.path))?;
 
-        // Perform the requested operation on it
-        let result = STORAGE_IO_TIME
-            .with_label_values(&[op])
-            .observe_closure_duration(|| func(&file));
-
         // Store the File in the slot and update the handle in the VirtualFile
         // to point to it.
         slot_guard.file.replace(file);
 
         *handle_guard = handle;
 
-        Ok(result)
+        Ok(FileSlotGuard::Write(slot_guard))
     }
 
     pub fn remove(self) {
@@ -359,7 +380,6 @@ impl Write for VirtualFile {
 }
 
 impl VirtualFile {
-
     pub fn seek(&mut self, pos: SeekFrom) -> Result<u64, Error> {
         match pos {
             SeekFrom::Start(offset) => {
@@ -407,7 +427,11 @@ 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: &mut [u8], mut offset: u64) -> Result<(), Error> {
+    pub async fn read_exact_at_async(
+        &self,
+        mut buf: &mut [u8],
+        mut offset: u64,
+    ) -> Result<(), Error> {
         while !buf.is_empty() {
             match self.read_at_async(buf, offset).await {
                 Ok(0) => {