Turn BlockLease associated type into an enum (neondatabase#4982)

## Problem

The `BlockReader` trait is not ready to be asyncified, as associated
types are not supported by asyncification strategies like via the
`async_trait` macro, or via adopting enums.

## Summary of changes

Remove the `BlockLease` associated type from the `BlockReader` trait and
turn it into an enum instead, bearing the same name. The enum has two
variants, one of which is gated by `#[cfg(test)]`. Therefore, outside of
test settings, the enum has zero overhead over just having the
`PageReadGuard`. Using the enum allows us to impl `BlockReader` without
needing the page cache.

Part of neondatabase#4743

pageserver/src/tenant/block_io.rs

@@ -2,8 +2,7 @@
 //! Low-level Block-oriented I/O functions
 //!
 
-use crate::page_cache;
-use crate::page_cache::{ReadBufResult, PAGE_SZ};
+use crate::page_cache::{self, PageReadGuard, ReadBufResult, PAGE_SZ};
 use bytes::Bytes;
 use std::ops::{Deref, DerefMut};
 use std::os::unix::fs::FileExt;
@@ -15,14 +14,12 @@ use std::sync::atomic::AtomicU64;
 /// There are currently two implementations: EphemeralFile, and FileBlockReader
 /// below.
 pub trait BlockReader {
-    type BlockLease: Deref<Target = [u8; PAGE_SZ]> + 'static;
-
     ///
     /// Read a block. Returns a "lease" object that can be used to
     /// access to the contents of the page. (For the page cache, the
     /// lease object represents a lock on the buffer.)
     ///
-    fn read_blk(&self, blknum: u32) -> Result<Self::BlockLease, std::io::Error>;
+    fn read_blk(&self, blknum: u32) -> Result<BlockLease, std::io::Error>;
 
     ///
     /// Create a new "cursor" for reading from this reader.
@@ -41,13 +38,48 @@ impl<B> BlockReader for &B
 where
     B: BlockReader,
 {
-    type BlockLease = B::BlockLease;
-
-    fn read_blk(&self, blknum: u32) -> Result<Self::BlockLease, std::io::Error> {
+    fn read_blk(&self, blknum: u32) -> Result<BlockLease, std::io::Error> {
         (*self).read_blk(blknum)
     }
 }
 
+/// A block accessible for reading
+///
+/// During builds with `#[cfg(test)]`, this is a proper enum
+/// with two variants to support testing code. During normal
+/// builds, it just has one variant and is thus a cheap newtype
+/// wrapper of [`PageReadGuard`]
+pub enum BlockLease {
+    PageReadGuard(PageReadGuard<'static>),
+    #[cfg(test)]
+    Rc(std::rc::Rc<[u8; PAGE_SZ]>),
+}
+
+impl From<PageReadGuard<'static>> for BlockLease {
+    fn from(value: PageReadGuard<'static>) -> Self {
+        BlockLease::PageReadGuard(value)
+    }
+}
+
+#[cfg(test)]
+impl From<std::rc::Rc<[u8; PAGE_SZ]>> for BlockLease {
+    fn from(value: std::rc::Rc<[u8; PAGE_SZ]>) -> Self {
+        BlockLease::Rc(value)
+    }
+}
+
+impl Deref for BlockLease {
+    type Target = [u8; PAGE_SZ];
+
+    fn deref(&self) -> &Self::Target {
+        match self {
+            BlockLease::PageReadGuard(v) => v.deref(),
+            #[cfg(test)]
+            BlockLease::Rc(v) => v.deref(),
+        }
+    }
+}
+
 ///
 /// A "cursor" for efficiently reading multiple pages from a BlockReader
 ///
@@ -80,7 +112,7 @@ where
         BlockCursor { reader }
     }
 
-    pub fn read_blk(&self, blknum: u32) -> Result<R::BlockLease, std::io::Error> {
+    pub fn read_blk(&self, blknum: u32) -> Result<BlockLease, std::io::Error> {
         self.reader.read_blk(blknum)
     }
 }
@@ -118,9 +150,7 @@ impl<F> BlockReader for FileBlockReader<F>
 where
     F: FileExt,
 {
-    type BlockLease = page_cache::PageReadGuard<'static>;
-
-    fn read_blk(&self, blknum: u32) -> Result<Self::BlockLease, std::io::Error> {
+    fn read_blk(&self, blknum: u32) -> Result<BlockLease, std::io::Error> {
         // Look up the right page
         let cache = page_cache::get();
         loop {
@@ -132,7 +162,7 @@ where
                         format!("Failed to read immutable buf: {e:#}"),
                     )
                 })? {
-                ReadBufResult::Found(guard) => break Ok(guard),
+                ReadBufResult::Found(guard) => break Ok(guard.into()),
                 ReadBufResult::NotFound(mut write_guard) => {
                     // Read the page from disk into the buffer
                     self.fill_buffer(write_guard.deref_mut(), blknum)?;

pageserver/src/tenant/disk_btree.rs

@@ -685,6 +685,7 @@ impl<const L: usize> BuildNode<L> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::tenant::block_io::BlockLease;
     use rand::Rng;
     use std::collections::BTreeMap;
     use std::sync::atomic::{AtomicUsize, Ordering};
@@ -699,12 +700,10 @@ mod tests {
         }
     }
     impl BlockReader for TestDisk {
-        type BlockLease = std::rc::Rc<[u8; PAGE_SZ]>;
-
-        fn read_blk(&self, blknum: u32) -> io::Result<Self::BlockLease> {
+        fn read_blk(&self, blknum: u32) -> io::Result<BlockLease> {
             let mut buf = [0u8; PAGE_SZ];
             buf.copy_from_slice(&self.blocks[blknum as usize]);
-            Ok(std::rc::Rc::new(buf))
+            Ok(std::rc::Rc::new(buf).into())
         }
     }
     impl BlockWriter for &mut TestDisk {

pageserver/src/tenant/ephemeral_file.rs

@@ -4,7 +4,7 @@
 use crate::config::PageServerConf;
 use crate::page_cache::{self, ReadBufResult, WriteBufResult, PAGE_SZ};
 use crate::tenant::blob_io::BlobWriter;
-use crate::tenant::block_io::BlockReader;
+use crate::tenant::block_io::{BlockLease, BlockReader};
 use crate::virtual_file::VirtualFile;
 use once_cell::sync::Lazy;
 use std::cmp::min;
@@ -303,17 +303,15 @@ pub fn writeback(file_id: u64, blkno: u32, buf: &[u8]) -> Result<(), io::Error>
 }
 
 impl BlockReader for EphemeralFile {
-    type BlockLease = page_cache::PageReadGuard<'static>;
-
-    fn read_blk(&self, blknum: u32) -> Result<Self::BlockLease, io::Error> {
+    fn read_blk(&self, blknum: u32) -> Result<BlockLease, io::Error> {
         // Look up the right page
         let cache = page_cache::get();
         loop {
             match cache
                 .read_ephemeral_buf(self.file_id, blknum)
                 .map_err(|e| to_io_error(e, "Failed to read ephemeral buf"))?
             {
-                ReadBufResult::Found(guard) => return Ok(guard),
+                ReadBufResult::Found(guard) => return Ok(guard.into()),
                 ReadBufResult::NotFound(mut write_guard) => {
                     // Read the page from disk into the buffer
                     self.fill_buffer(write_guard.deref_mut(), blknum)?;

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -29,10 +29,10 @@
 //!
 use crate::config::PageServerConf;
 use crate::context::RequestContext;
-use crate::page_cache::{PageReadGuard, PAGE_SZ};
+use crate::page_cache::PAGE_SZ;
 use crate::repository::{Key, Value, KEY_SIZE};
 use crate::tenant::blob_io::{BlobWriter, WriteBlobWriter};
-use crate::tenant::block_io::{BlockBuf, BlockCursor, BlockReader, FileBlockReader};
+use crate::tenant::block_io::{BlockBuf, BlockCursor, BlockLease, BlockReader, FileBlockReader};
 use crate::tenant::disk_btree::{DiskBtreeBuilder, DiskBtreeReader, VisitDirection};
 use crate::tenant::storage_layer::{
     PersistentLayer, ValueReconstructResult, ValueReconstructState,
@@ -1041,9 +1041,7 @@ impl<T: AsRef<DeltaLayerInner>> ValueRef<T> {
 struct Adapter<T: AsRef<DeltaLayerInner>>(T);
 
 impl<T: AsRef<DeltaLayerInner>> BlockReader for Adapter<T> {
-    type BlockLease = PageReadGuard<'static>;
-
-    fn read_blk(&self, blknum: u32) -> Result<Self::BlockLease, std::io::Error> {
+    fn read_blk(&self, blknum: u32) -> Result<BlockLease, std::io::Error> {
         self.0.as_ref().file.read_blk(blknum)
     }
 }