clippy & doc check

Goals:
1. don't junk up the owned_buffer_io interface with the
   write_all_borrowed, eliminating one `panic!("should not happen")`
1. make the sandwiching clearer

pageserver/src/tenant/blob_io.rs

@@ -121,7 +121,7 @@ impl<const BUFFERED: bool> BlobWriter<BUFFERED> {
         self.offset
     }
 
-    const CAPACITY: usize = if BUFFERED { PAGE_SZ } else { 0 };
+    const CAPACITY: usize = if BUFFERED { 64 * 1024 } else { 0 };
 
     /// Writes the given buffer directly to the underlying `VirtualFile`.
     /// You need to make sure that the internal buffer is empty, otherwise

pageserver/src/tenant/ephemeral_file.rs

@@ -3,36 +3,27 @@
 
 use crate::config::PageServerConf;
 use crate::context::RequestContext;
-use crate::page_cache::{self, PAGE_SZ};
+use crate::page_cache;
 use crate::tenant::block_io::{BlockCursor, BlockLease, BlockReader};
 use crate::virtual_file::{self, VirtualFile};
-use bytes::BytesMut;
 use camino::Utf8PathBuf;
 use pageserver_api::shard::TenantShardId;
-use std::cmp::min;
 
-use std::io::{self, ErrorKind};
-use std::ops::DerefMut;
+use std::io;
 use std::sync::atomic::AtomicU64;
-use tracing::*;
 use utils::id::TimelineId;
 
 pub struct EphemeralFile {
-    page_cache_file_id: page_cache::FileId,
-
     _tenant_shard_id: TenantShardId,
     _timeline_id: TimelineId,
-    file: VirtualFile,
-    len: u64,
-    /// An ephemeral file is append-only.
-    /// We keep the last page, which can still be modified, in [`Self::mutable_tail`].
-    /// The other pages, which can no longer be modified, are accessed through the page cache.
-    ///
-    /// None <=> IO is ongoing.
-    /// Size is fixed to PAGE_SZ at creation time and must not be changed.
-    mutable_tail: Option<BytesMut>,
+
+    sandwich: page_caching_sandwich::PageCachingSandwich,
 }
 
+mod page_caching_sandwich;
+mod sandwich;
+mod zero_padded_buffer;
+
 impl EphemeralFile {
     pub async fn create(
         conf: &PageServerConf,
@@ -59,21 +50,18 @@ impl EphemeralFile {
         .await?;
 
         Ok(EphemeralFile {
-            page_cache_file_id: page_cache::next_file_id(),
             _tenant_shard_id: tenant_shard_id,
             _timeline_id: timeline_id,
-            file,
-            len: 0,
-            mutable_tail: Some(BytesMut::zeroed(PAGE_SZ)),
+            sandwich: page_caching_sandwich::PageCachingSandwich::new(file),
         })
     }
 
     pub(crate) fn len(&self) -> u64 {
-        self.len
+        self.sandwich.bytes_written()
     }
 
-    pub(crate) fn id(&self) -> page_cache::FileId {
-        self.page_cache_file_id
+    pub(crate) fn page_cache_file_id(&self) -> page_cache::FileId {
+        self.sandwich.page_cache_file_id()
     }
 
     pub(crate) async fn read_blk(
@@ -81,182 +69,30 @@ impl EphemeralFile {
         blknum: u32,
         ctx: &RequestContext,
     ) -> Result<BlockLease, io::Error> {
-        let flushed_blknums = 0..self.len / PAGE_SZ as u64;
-        if flushed_blknums.contains(&(blknum as u64)) {
-            let cache = page_cache::get();
-            match cache
-                .read_immutable_buf(self.page_cache_file_id, blknum, ctx)
-                .await
-                .map_err(|e| {
-                    std::io::Error::new(
-                        std::io::ErrorKind::Other,
-                        // order path before error because error is anyhow::Error => might have many contexts
-                        format!(
-                            "ephemeral file: read immutable page #{}: {}: {:#}",
-                            blknum, self.file.path, e,
-                        ),
-                    )
-                })? {
-                page_cache::ReadBufResult::Found(guard) => {
-                    return Ok(BlockLease::PageReadGuard(guard))
-                }
-                page_cache::ReadBufResult::NotFound(write_guard) => {
-                    let write_guard = self
-                        .file
-                        .read_exact_at_page(write_guard, blknum as u64 * PAGE_SZ as u64)
-                        .await?;
-                    let read_guard = write_guard.mark_valid();
-                    return Ok(BlockLease::PageReadGuard(read_guard));
-                }
-            };
-        } else {
-            debug_assert_eq!(blknum as u64, self.len / PAGE_SZ as u64);
-            Ok(BlockLease::EphemeralFileMutableTail(
-                self.mutable_tail
-                    .as_deref()
-                    .expect("we're not doing IO, it must be Some()")
-                    .try_into()
-                    .expect("we ensure that it's always PAGE_SZ"),
-            ))
-        }
+        self.sandwich.read_blk(blknum, ctx).await
     }
 
     pub(crate) async fn write_blob(
         &mut self,
         srcbuf: &[u8],
-        ctx: &RequestContext,
+        _ctx: &RequestContext,
     ) -> Result<u64, io::Error> {
-        struct Writer<'a> {
-            ephemeral_file: &'a mut EphemeralFile,
-            /// The block to which the next [`push_bytes`] will write.
-            blknum: u32,
-            /// The offset inside the block identified by [`blknum`] to which [`push_bytes`] will write.
-            off: usize,
-        }
-        impl<'a> Writer<'a> {
-            fn new(ephemeral_file: &'a mut EphemeralFile) -> io::Result<Writer<'a>> {
-                Ok(Writer {
-                    blknum: (ephemeral_file.len / PAGE_SZ as u64) as u32,
-                    off: (ephemeral_file.len % PAGE_SZ as u64) as usize,
-                    ephemeral_file,
-                })
-            }
-            #[inline(always)]
-            async fn push_bytes(
-                &mut self,
-                src: &[u8],
-                ctx: &RequestContext,
-            ) -> Result<(), io::Error> {
-                let mut src_remaining = src;
-                while !src_remaining.is_empty() {
-                    let dst_remaining = &mut self
-                        .ephemeral_file
-                        .mutable_tail
-                        .as_deref_mut()
-                        .expect("IO is not yet ongoing")[self.off..];
-                    let n = min(dst_remaining.len(), src_remaining.len());
-                    dst_remaining[..n].copy_from_slice(&src_remaining[..n]);
-                    self.off += n;
-                    src_remaining = &src_remaining[n..];
-                    if self.off == PAGE_SZ {
-                        let mutable_tail = std::mem::take(&mut self.ephemeral_file.mutable_tail)
-                            .expect("IO is not yet ongoing");
-                        let (mutable_tail, res) = self
-                            .ephemeral_file
-                            .file
-                            .write_all_at(mutable_tail, self.blknum as u64 * PAGE_SZ as u64)
-                            .await;
-                        // TODO: If we panic before we can put the mutable_tail back, subsequent calls will fail.
-                        // I.e., the IO isn't retryable if we panic.
-                        self.ephemeral_file.mutable_tail = Some(mutable_tail);
-                        match res {
-                            Ok(_) => {
-                                // Pre-warm the page cache with what we just wrote.
-                                // This isn't necessary for coherency/correctness, but it's how we've always done it.
-                                let cache = page_cache::get();
-                                match cache
-                                    .read_immutable_buf(
-                                        self.ephemeral_file.page_cache_file_id,
-                                        self.blknum,
-                                        ctx,
-                                    )
-                                    .await
-                                {
-                                    Ok(page_cache::ReadBufResult::Found(_guard)) => {
-                                        // This function takes &mut self, so, it shouldn't be possible to reach this point.
-                                        unreachable!("we just wrote blknum {} and this function takes &mut self, so, no concurrent read_blk is possible", self.blknum);
-                                    }
-                                    Ok(page_cache::ReadBufResult::NotFound(mut write_guard)) => {
-                                        let buf: &mut [u8] = write_guard.deref_mut();
-                                        debug_assert_eq!(buf.len(), PAGE_SZ);
-                                        buf.copy_from_slice(
-                                            self.ephemeral_file
-                                                .mutable_tail
-                                                .as_deref()
-                                                .expect("IO is not ongoing"),
-                                        );
-                                        let _ = write_guard.mark_valid();
-                                        // pre-warm successful
-                                    }
-                                    Err(e) => {
-                                        error!("ephemeral_file write_blob failed to get immutable buf to pre-warm page cache: {e:?}");
-                                        // fail gracefully, it's not the end of the world if we can't pre-warm the cache here
-                                    }
-                                }
-                                // Zero the buffer for re-use.
-                                // Zeroing is critical for correcntess because the write_blob code below
-                                // and similarly read_blk expect zeroed pages.
-                                self.ephemeral_file
-                                    .mutable_tail
-                                    .as_deref_mut()
-                                    .expect("IO is not ongoing")
-                                    .fill(0);
-                                // This block is done, move to next one.
-                                self.blknum += 1;
-                                self.off = 0;
-                            }
-                            Err(e) => {
-                                return Err(std::io::Error::new(
-                                    ErrorKind::Other,
-                                    // order error before path because path is long and error is short
-                                    format!(
-                                        "ephemeral_file: write_blob: write-back full tail blk #{}: {:#}: {}",
-                                        self.blknum,
-                                        e,
-                                        self.ephemeral_file.file.path,
-                                    ),
-                                ));
-                            }
-                        }
-                    }
-                }
-                Ok(())
-            }
-        }
-
-        let pos = self.len;
-        let mut writer = Writer::new(self)?;
+        let pos = self.sandwich.bytes_written();
 
         // Write the length field
         if srcbuf.len() < 0x80 {
             // short one-byte length header
             let len_buf = [srcbuf.len() as u8];
-            writer.push_bytes(&len_buf, ctx).await?;
+
+            self.sandwich.write_all_borrowed(&len_buf).await?;
         } else {
             let mut len_buf = u32::to_be_bytes(srcbuf.len() as u32);
             len_buf[0] |= 0x80;
-            writer.push_bytes(&len_buf, ctx).await?;
+            self.sandwich.write_all_borrowed(&len_buf).await?;
         }
 
         // Write the payload
-        writer.push_bytes(srcbuf, ctx).await?;
-
-        if srcbuf.len() < 0x80 {
-            self.len += 1;
-        } else {
-            self.len += 4;
-        }
-        self.len += srcbuf.len() as u64;
+        self.sandwich.write_all_borrowed(srcbuf).await?;
 
         Ok(pos)
     }
@@ -271,28 +107,6 @@ pub fn is_ephemeral_file(filename: &str) -> bool {
     }
 }
 
-impl Drop for EphemeralFile {
-    fn drop(&mut self) {
-        // There might still be pages in the [`crate::page_cache`] for this file.
-        // We leave them there, [`crate::page_cache::PageCache::find_victim`] will evict them when needed.
-
-        // unlink the file
-        let res = std::fs::remove_file(&self.file.path);
-        if let Err(e) = res {
-            if e.kind() != std::io::ErrorKind::NotFound {
-                // just never log the not found errors, we cannot do anything for them; on detach
-                // the tenant directory is already gone.
-                //
-                // not found files might also be related to https://github.com/neondatabase/neon/issues/2442
-                error!(
-                    "could not remove ephemeral file '{}': {}",
-                    self.file.path, e
-                );
-            }
-        }
-    }
-}
-
 impl BlockReader for EphemeralFile {
     fn block_cursor(&self) -> super::block_io::BlockCursor<'_> {
         BlockCursor::new(super::block_io::BlockReaderRef::EphemeralFile(self))

pageserver/src/tenant/ephemeral_file/page_caching_sandwich.rs

@@ -0,0 +1,101 @@
+use crate::context::RequestContext;
+use crate::page_cache::{self, PAGE_SZ};
+use crate::tenant::block_io::BlockLease;
+use crate::virtual_file::VirtualFile;
+
+use std::io;
+use tracing::*;
+
+use super::sandwich;
+
+pub struct PageCachingSandwich {
+    page_cache_file_id: page_cache::FileId,
+    sandwich: super::sandwich::Sandwich,
+}
+
+impl PageCachingSandwich {
+    pub fn new(file: VirtualFile) -> Self {
+        Self {
+            page_cache_file_id: page_cache::next_file_id(),
+            sandwich: super::sandwich::Sandwich::new(file),
+        }
+    }
+
+    pub fn page_cache_file_id(&self) -> page_cache::FileId {
+        self.page_cache_file_id
+    }
+
+    pub(crate) async fn write_all_borrowed(&mut self, srcbuf: &[u8]) -> Result<usize, io::Error> {
+        // It doesn't make sense to proactively fill the page cache on the Pageserver write path
+        // because Compute is unlikely to access recently written data.
+        self.sandwich.write_all_borrowed(srcbuf).await
+    }
+
+    pub(crate) fn bytes_written(&self) -> u64 {
+        self.sandwich.bytes_written()
+    }
+
+    pub(crate) async fn read_blk(
+        &self,
+        blknum: u32,
+        ctx: &RequestContext,
+    ) -> Result<BlockLease, io::Error> {
+        match self.sandwich.read_blk(blknum).await? {
+            sandwich::ReadResult::NeedsReadFromVirtualFile { virtual_file } => {
+                let cache = page_cache::get();
+                match cache
+                    .read_immutable_buf(self.page_cache_file_id, blknum, ctx)
+                    .await
+                    .map_err(|e| {
+                        std::io::Error::new(
+                            std::io::ErrorKind::Other,
+                            // order path before error because error is anyhow::Error => might have many contexts
+                            format!(
+                                "ephemeral file: read immutable page #{}: {}: {:#}",
+                                blknum,
+                                self.sandwich.as_inner_virtual_file().path,
+                                e,
+                            ),
+                        )
+                    })? {
+                    page_cache::ReadBufResult::Found(guard) => {
+                        return Ok(BlockLease::PageReadGuard(guard))
+                    }
+                    page_cache::ReadBufResult::NotFound(write_guard) => {
+                        let write_guard = virtual_file
+                            .read_exact_at_page(write_guard, blknum as u64 * PAGE_SZ as u64)
+                            .await?;
+                        let read_guard = write_guard.mark_valid();
+                        return Ok(BlockLease::PageReadGuard(read_guard));
+                    }
+                }
+            }
+            sandwich::ReadResult::ServedFromZeroPaddedMutableTail { buffer } => {
+                Ok(BlockLease::EphemeralFileMutableTail(buffer))
+            }
+        }
+    }
+}
+
+impl Drop for PageCachingSandwich {
+    fn drop(&mut self) {
+        // There might still be pages in the [`crate::page_cache`] for this file.
+        // We leave them there, [`crate::page_cache::PageCache::find_victim`] will evict them when needed.
+
+        // unlink the file
+        let res = std::fs::remove_file(&self.sandwich.as_inner_virtual_file().path);
+        if let Err(e) = res {
+            if e.kind() != std::io::ErrorKind::NotFound {
+                // just never log the not found errors, we cannot do anything for them; on detach
+                // the tenant directory is already gone.
+                //
+                // not found files might also be related to https://github.com/neondatabase/neon/issues/2442
+                error!(
+                    "could not remove ephemeral file '{}': {}",
+                    self.sandwich.as_inner_virtual_file().path,
+                    e
+                );
+            }
+        }
+    }
+}

pageserver/src/tenant/ephemeral_file/sandwich.rs

@@ -0,0 +1,99 @@
+use crate::{
+    page_cache::PAGE_SZ,
+    tenant::ephemeral_file::zero_padded_buffer,
+    virtual_file::{
+        owned_buffers_io::{self, write::Buffer},
+        VirtualFile,
+    },
+};
+
+pub struct Sandwich {
+    buffered_writer: owned_buffers_io::write::BufferedWriter<
+        zero_padded_buffer::Buf<{ Self::TAIL_SZ }>,
+        owned_buffers_io::util::size_tracking_writer::Writer<VirtualFile>,
+    >,
+}
+
+pub enum ReadResult<'a> {
+    NeedsReadFromVirtualFile { virtual_file: &'a VirtualFile },
+    ServedFromZeroPaddedMutableTail { buffer: &'a [u8; PAGE_SZ] },
+}
+
+impl Sandwich {
+    const TAIL_SZ: usize = 64 * 1024;
+
+    pub fn new(file: VirtualFile) -> Self {
+        let bytes_flushed_tracker = owned_buffers_io::util::size_tracking_writer::Writer::new(file);
+        let buffered_writer = owned_buffers_io::write::BufferedWriter::new(
+            bytes_flushed_tracker,
+            zero_padded_buffer::Buf::default(),
+        );
+        Self { buffered_writer }
+    }
+
+    pub(crate) fn as_inner_virtual_file(&self) -> &VirtualFile {
+        self.buffered_writer.as_inner().as_inner()
+    }
+
+    pub async fn write_all_borrowed(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.buffered_writer.write_buffered_borrowed(buf).await
+    }
+
+    pub fn bytes_written(&self) -> u64 {
+        let flushed_offset = self.buffered_writer.as_inner().bytes_written();
+        let buffer: &zero_padded_buffer::Buf<{ Self::TAIL_SZ }> =
+            self.buffered_writer.inspect_buffer();
+        flushed_offset + u64::try_from(buffer.pending()).unwrap()
+    }
+
+    pub(crate) async fn read_blk(&self, blknum: u32) -> Result<ReadResult, std::io::Error> {
+        let flushed_offset = self.buffered_writer.as_inner().bytes_written();
+        let buffer: &zero_padded_buffer::Buf<{ Self::TAIL_SZ }> =
+            self.buffered_writer.inspect_buffer();
+        let buffered_offset = flushed_offset + u64::try_from(buffer.pending()).unwrap();
+        let read_offset = (blknum as u64) * (PAGE_SZ as u64);
+
+        assert_eq!(
+            flushed_offset % (PAGE_SZ as u64),
+            0,
+            "we need this in the logic below, because it assumes the page isn't spread across flushed part and in-memory buffer"
+        );
+
+        if read_offset < flushed_offset {
+            assert!(
+                read_offset + (PAGE_SZ as u64) <= flushed_offset,
+                "this impl can't deal with pages spread across flushed & buffered part"
+            );
+            Ok(ReadResult::NeedsReadFromVirtualFile {
+                virtual_file: self.as_inner_virtual_file(),
+            })
+        } else {
+            let read_until_offset = read_offset + (PAGE_SZ as u64);
+            if !(0..buffered_offset).contains(&read_until_offset) {
+                // The blob_io code relies on the reader allowing reads past
+                // the end of what was written, up to end of the current PAGE_SZ chunk.
+                // This is a relict of the past where we would get a pre-zeroed page from the page cache.
+                //
+                // DeltaLayer probably has the same issue, not sure why it needs no special treatment.
+                let nbytes_past_end = read_until_offset.checked_sub(buffered_offset).unwrap();
+                if nbytes_past_end >= (PAGE_SZ as u64) {
+                    // TODO: treat this as error. Pre-existing issue before this patch.
+                    panic!(
+                        "return IO error: read past end of file: read=0x{read_offset:x} buffered=0x{buffered_offset:x} flushed=0x{flushed_offset}"
+                    )
+                }
+            }
+            let read_offset_in_buffer = read_offset
+                .checked_sub(flushed_offset)
+                .expect("would have taken `if` branch instead of this one");
+            let read_offset_in_buffer = usize::try_from(read_offset_in_buffer).unwrap();
+            let zero_padded_slice = buffer.as_zero_padded_slice();
+            let page = &zero_padded_slice[read_offset_in_buffer..(read_offset_in_buffer + PAGE_SZ)];
+            Ok(ReadResult::ServedFromZeroPaddedMutableTail {
+                buffer: page
+                    .try_into()
+                    .expect("the slice above got it as page-size slice"),
+            })
+        }
+    }
+}

pageserver/src/tenant/ephemeral_file/zero_padded_buffer.rs

@@ -0,0 +1,98 @@
+use std::mem::MaybeUninit;
+
+pub struct Buf<const N: usize> {
+    allocation: Box<[u8; N]>,
+    written: usize,
+}
+
+impl<const N: usize> Default for Buf<N> {
+    fn default() -> Self {
+        Self {
+            allocation: Box::new(
+                // SAFETY: zeroed memory is a valid [u8; N]
+                unsafe { MaybeUninit::zeroed().assume_init() },
+            ),
+            written: 0,
+        }
+    }
+}
+
+impl<const N: usize> Buf<N> {
+    #[inline(always)]
+    fn invariants(&self) {
+        // don't check by default, unoptimized is too expensive even for debug mode
+        if false {
+            debug_assert!(self.written <= N, "{}", self.written);
+            debug_assert!(self.allocation[self.written..N].iter().all(|v| *v == 0));
+        }
+    }
+
+    pub fn as_zero_padded_slice(&self) -> &[u8; N] {
+        &self.allocation
+    }
+}
+
+impl<const N: usize> crate::virtual_file::owned_buffers_io::write::Buffer for Buf<N> {
+    type IoBuf = Self;
+
+    fn cap(&self) -> usize {
+        self.allocation.len()
+    }
+
+    fn extend_from_slice(&mut self, other: &[u8]) {
+        self.invariants();
+        let remaining = self.cap() - other.len();
+        if other.len() > remaining {
+            panic!("calling extend_from_slice() with insufficient remaining capacity");
+        }
+        self.allocation[self.written..(self.written + other.len())].copy_from_slice(other);
+        self.written += other.len();
+        self.invariants();
+    }
+
+    fn pending(&self) -> usize {
+        self.written
+    }
+
+    fn flush(self) -> tokio_epoll_uring::Slice<Self> {
+        self.invariants();
+        let written = self.written;
+        tokio_epoll_uring::BoundedBuf::slice(self, 0..written)
+    }
+
+    fn reconstruct_after_flush(this: Self::IoBuf) -> Self {
+        let Self {
+            mut allocation,
+            written,
+        } = this;
+        allocation[0..written].fill(0);
+        let new = Self {
+            allocation,
+            written: 0,
+        };
+        new.invariants();
+        new
+    }
+}
+
+/// We have this implementation so that [`Buf<N>`] can be used with [`crate::virtual_file::owned_buffers_io::write::BufferedWriter`].
+///
+///
+/// SAFETY:
+///
+/// The [`Self::allocation`] is stable becauses boxes are stable.
+/// The memory is zero-initialized, so, bytes_init is always N.
+///
+unsafe impl<const N: usize> tokio_epoll_uring::IoBuf for Buf<N> {
+    fn stable_ptr(&self) -> *const u8 {
+        self.allocation.as_ptr()
+    }
+
+    fn bytes_init(&self) -> usize {
+        N
+    }
+
+    fn bytes_total(&self) -> usize {
+        N
+    }
+}

pageserver/src/tenant/remote_timeline_client/download.rs

@@ -7,6 +7,7 @@ use std::collections::HashSet;
 use std::future::Future;
 
 use anyhow::{anyhow, Context};
+use bytes::BytesMut;
 use camino::{Utf8Path, Utf8PathBuf};
 use pageserver_api::shard::TenantShardId;
 use tokio::fs::{self, File, OpenOptions};
@@ -194,10 +195,10 @@ async fn download_object<'a>(
                 // There's chunks_vectored() on the stream.
                 let (bytes_amount, destination_file) = async {
                     let size_tracking = size_tracking_writer::Writer::new(destination_file);
-                    let mut buffered = owned_buffers_io::write::BufferedWriter::<
-                        { super::BUFFER_SIZE },
-                        _,
-                    >::new(size_tracking);
+                    let mut buffered = owned_buffers_io::write::BufferedWriter::<BytesMut, _>::new(
+                        size_tracking,
+                        BytesMut::with_capacity(super::BUFFER_SIZE),
+                    );
                     while let Some(res) =
                         futures::StreamExt::next(&mut download.download_stream).await
                     {

pageserver/src/tenant/storage_layer/inmemory_layer.rs

@@ -454,7 +454,7 @@ impl InMemoryLayer {
         trace!("initializing new empty InMemoryLayer for writing on timeline {timeline_id} at {start_lsn}");
 
         let file = EphemeralFile::create(conf, tenant_shard_id, timeline_id).await?;
-        let key = InMemoryLayerFileId(file.id());
+        let key = InMemoryLayerFileId(file.page_cache_file_id());
 
         Ok(InMemoryLayer {
             file_id: key,

pageserver/src/virtual_file.rs

@@ -36,7 +36,8 @@ pub(crate) use io_engine::IoEngineKind;
 pub(crate) use metadata::Metadata;
 pub(crate) use open_options::*;
 
-#[cfg_attr(not(target_os = "linux"), allow(dead_code))]
+use self::owned_buffers_io::write::OwnedAsyncWriter;
+
 pub(crate) mod owned_buffers_io {
     //! Abstractions for IO with owned buffers.
     //!
@@ -1083,6 +1084,17 @@ impl Drop for VirtualFile {
     }
 }
 
+impl OwnedAsyncWriter for VirtualFile {
+    #[inline(always)]
+    async fn write_all<B: BoundedBuf<Buf = Buf>, Buf: IoBuf + Send>(
+        &mut self,
+        buf: B,
+    ) -> std::io::Result<(usize, B::Buf)> {
+        let (buf, res) = VirtualFile::write_all(self, buf).await;
+        res.map(move |v| (v, buf))
+    }
+}
+
 impl OpenFiles {
     fn new(num_slots: usize) -> OpenFiles {
         let mut slots = Box::new(Vec::with_capacity(num_slots));

pageserver/src/virtual_file/owned_buffers_io/util/size_tracking_writer.rs

@@ -1,33 +1,41 @@
-use crate::virtual_file::{owned_buffers_io::write::OwnedAsyncWriter, VirtualFile};
+use crate::virtual_file::owned_buffers_io::write::OwnedAsyncWriter;
 use tokio_epoll_uring::{BoundedBuf, IoBuf};
 
-pub struct Writer {
-    dst: VirtualFile,
+pub struct Writer<W> {
+    dst: W,
     bytes_amount: u64,
 }
 
-impl Writer {
-    pub fn new(dst: VirtualFile) -> Self {
+impl<W> Writer<W> {
+    pub fn new(dst: W) -> Self {
         Self {
             dst,
             bytes_amount: 0,
         }
     }
+    pub fn bytes_written(&self) -> u64 {
+        self.bytes_amount
+    }
+    pub fn as_inner(&self) -> &W {
+        &self.dst
+    }
     /// Returns the wrapped `VirtualFile` object as well as the number
     /// of bytes that were written to it through this object.
-    pub fn into_inner(self) -> (u64, VirtualFile) {
+    pub fn into_inner(self) -> (u64, W) {
         (self.bytes_amount, self.dst)
     }
 }
 
-impl OwnedAsyncWriter for Writer {
+impl<W> OwnedAsyncWriter for Writer<W>
+where
+    W: OwnedAsyncWriter,
+{
     #[inline(always)]
     async fn write_all<B: BoundedBuf<Buf = Buf>, Buf: IoBuf + Send>(
         &mut self,
         buf: B,
     ) -> std::io::Result<(usize, B::Buf)> {
-        let (buf, res) = self.dst.write_all(buf).await;
-        let nwritten = res?;
+        let (nwritten, buf) = self.dst.write_all(buf).await?;
         self.bytes_amount += u64::try_from(nwritten).unwrap();
         Ok((nwritten, buf))
     }

pageserver/src/virtual_file/owned_buffers_io/write.rs

@@ -10,6 +10,47 @@ pub trait OwnedAsyncWriter {
     ) -> std::io::Result<(usize, B::Buf)>;
 }
 
+pub trait Buffer {
+    type IoBuf: IoBuf;
+
+    fn cap(&self) -> usize;
+    fn extend_from_slice(&mut self, other: &[u8]);
+    fn pending(&self) -> usize;
+    fn flush(self) -> Slice<Self::IoBuf>;
+    fn reconstruct_after_flush(iobuf: Self::IoBuf) -> Self;
+}
+
+impl Buffer for BytesMut {
+    type IoBuf = BytesMut;
+
+    #[inline(always)]
+    fn cap(&self) -> usize {
+        self.capacity()
+    }
+
+    fn extend_from_slice(&mut self, other: &[u8]) {
+        BytesMut::extend_from_slice(self, other)
+    }
+
+    #[inline(always)]
+    fn pending(&self) -> usize {
+        self.len()
+    }
+
+    fn flush(self) -> Slice<BytesMut> {
+        if self.is_empty() {
+            return self.slice_full();
+        }
+        let len = self.len();
+        self.slice(0..len)
+    }
+
+    fn reconstruct_after_flush(mut iobuf: BytesMut) -> Self {
+        iobuf.clear();
+        iobuf
+    }
+}
+
 /// A wrapper aorund an [`OwnedAsyncWriter`] that batches smaller writers
 /// into `BUFFER_SIZE`-sized writes.
 ///
@@ -25,27 +66,38 @@ pub trait OwnedAsyncWriter {
 ///
 /// In such cases, a different implementation that always buffers in memory
 /// may be preferable.
-pub struct BufferedWriter<const BUFFER_SIZE: usize, W> {
+pub struct BufferedWriter<B, W> {
     writer: W,
     // invariant: always remains Some(buf)
     // with buf.capacity() == BUFFER_SIZE except
     // - while IO is ongoing => goes back to Some() once the IO completed successfully
     // - after an IO error => stays `None` forever
     // In these exceptional cases, it's `None`.
-    buf: Option<BytesMut>,
+    buf: Option<B>,
 }
 
-impl<const BUFFER_SIZE: usize, W> BufferedWriter<BUFFER_SIZE, W>
+impl<B, Buf, W> BufferedWriter<B, W>
 where
+    B: Buffer<IoBuf = Buf> + Send,
+    Buf: IoBuf + Send,
     W: OwnedAsyncWriter,
 {
-    pub fn new(writer: W) -> Self {
+    pub fn new(writer: W, buf: B) -> Self {
         Self {
             writer,
-            buf: Some(BytesMut::with_capacity(BUFFER_SIZE)),
+            buf: Some(buf),
         }
     }
 
+    pub fn as_inner(&self) -> &W {
+        &self.writer
+    }
+
+    /// Panics if used after any of the write paths returned an error
+    pub fn inspect_buffer(&self) -> &B {
+        self.buf()
+    }
+
     pub async fn flush_and_into_inner(mut self) -> std::io::Result<W> {
         self.flush().await?;
         let Self { buf, writer } = self;
@@ -53,57 +105,84 @@ where
         Ok(writer)
     }
 
-    pub async fn write_buffered<B: IoBuf>(&mut self, chunk: Slice<B>) -> std::io::Result<()>
+    #[inline(always)]
+    fn buf(&self) -> &B {
+        self.buf
+            .as_ref()
+            .expect("must not use after we returned an error")
+    }
+
+    pub async fn write_buffered<S: IoBuf>(&mut self, chunk: Slice<S>) -> std::io::Result<(usize, S)>
     where
-        B: IoBuf + Send,
+        S: IoBuf + Send,
     {
+        let chunk_len = chunk.len();
         // avoid memcpy for the middle of the chunk
-        if chunk.len() >= BUFFER_SIZE {
+        if chunk.len() >= self.buf().cap() {
             self.flush().await?;
             // do a big write, bypassing `buf`
             assert_eq!(
                 self.buf
                     .as_ref()
                     .expect("must not use after an error")
-                    .len(),
+                    .pending(),
                 0
             );
-            let chunk_len = chunk.len();
             let (nwritten, chunk) = self.writer.write_all(chunk).await?;
             assert_eq!(nwritten, chunk_len);
-            drop(chunk);
-            return Ok(());
+            return Ok((nwritten, chunk));
         }
         // in-memory copy the < BUFFER_SIZED tail of the chunk
-        assert!(chunk.len() < BUFFER_SIZE);
-        let mut chunk = &chunk[..];
+        assert!(chunk.len() < self.buf().cap());
+        let mut slice = &chunk[..];
+        while !slice.is_empty() {
+            let buf = self.buf.as_mut().expect("must not use after an error");
+            let need = buf.cap() - buf.pending();
+            let have = slice.len();
+            let n = std::cmp::min(need, have);
+            buf.extend_from_slice(&slice[..n]);
+            slice = &slice[n..];
+            if buf.pending() >= buf.cap() {
+                assert_eq!(buf.pending(), buf.cap());
+                self.flush().await?;
+            }
+        }
+        assert!(slice.is_empty(), "by now we should have drained the chunk");
+        Ok((chunk_len, chunk.into_inner()))
+    }
+
+    /// Strictly less performant variant of [`Self::write_buffered`] that allows writing borrowed data.
+    ///
+    /// It is less performant because we always have to copy the borrowed data into the internal buffer
+    /// before we can do the IO. The [`Self::write_buffered`] can avoid this, which is more performant
+    /// for large writes.
+    pub async fn write_buffered_borrowed(&mut self, mut chunk: &[u8]) -> std::io::Result<usize> {
+        let chunk_len = chunk.len();
         while !chunk.is_empty() {
             let buf = self.buf.as_mut().expect("must not use after an error");
-            let need = BUFFER_SIZE - buf.len();
+            let need = buf.cap() - buf.pending();
             let have = chunk.len();
             let n = std::cmp::min(need, have);
             buf.extend_from_slice(&chunk[..n]);
             chunk = &chunk[n..];
-            if buf.len() >= BUFFER_SIZE {
-                assert_eq!(buf.len(), BUFFER_SIZE);
+            if buf.pending() >= buf.cap() {
+                assert_eq!(buf.pending(), buf.cap());
                 self.flush().await?;
             }
         }
-        assert!(chunk.is_empty(), "by now we should have drained the chunk");
-        Ok(())
+        Ok(chunk_len)
     }
 
     async fn flush(&mut self) -> std::io::Result<()> {
         let buf = self.buf.take().expect("must not use after an error");
-        if buf.is_empty() {
+        let buf_len = buf.pending();
+        if buf_len == 0 {
             self.buf = Some(buf);
-            return std::io::Result::Ok(());
+            return Ok(());
         }
-        let buf_len = buf.len();
-        let (nwritten, mut buf) = self.writer.write_all(buf).await?;
+        let (nwritten, io_buf) = self.writer.write_all(buf.flush()).await?;
         assert_eq!(nwritten, buf_len);
-        buf.clear();
-        self.buf = Some(buf);
+        self.buf = Some(Buffer::reconstruct_after_flush(io_buf));
         Ok(())
     }
 }
@@ -125,6 +204,8 @@ impl OwnedAsyncWriter for Vec<u8> {
 
 #[cfg(test)]
 mod tests {
+    use bytes::BytesMut;
+
     use super::*;
 
     #[derive(Default)]
@@ -158,7 +239,7 @@ mod tests {
     #[tokio::test]
     async fn test_buffered_writes_only() -> std::io::Result<()> {
         let recorder = RecorderWriter::default();
-        let mut writer = BufferedWriter::<2, _>::new(recorder);
+        let mut writer = BufferedWriter::new(recorder, BytesMut::with_capacity(2));
         write!(writer, b"a");
         write!(writer, b"b");
         write!(writer, b"c");
@@ -175,7 +256,7 @@ mod tests {
     #[tokio::test]
     async fn test_passthrough_writes_only() -> std::io::Result<()> {
         let recorder = RecorderWriter::default();
-        let mut writer = BufferedWriter::<2, _>::new(recorder);
+        let mut writer = BufferedWriter::new(recorder, BytesMut::with_capacity(2));
         write!(writer, b"abc");
         write!(writer, b"de");
         write!(writer, b"");
@@ -191,7 +272,7 @@ mod tests {
     #[tokio::test]
     async fn test_passthrough_write_with_nonempty_buffer() -> std::io::Result<()> {
         let recorder = RecorderWriter::default();
-        let mut writer = BufferedWriter::<2, _>::new(recorder);
+        let mut writer = BufferedWriter::new(recorder, BytesMut::with_capacity(2));
         write!(writer, b"a");
         write!(writer, b"bc");
         write!(writer, b"d");
@@ -203,4 +284,31 @@ mod tests {
         );
         Ok(())
     }
+
+    #[tokio::test]
+    async fn test_write_all_borrowed_always_goes_through_buffer() -> std::io::Result<()> {
+        let recorder = RecorderWriter::default();
+        let mut writer = BufferedWriter::new(recorder, BytesMut::with_capacity(2));
+
+        writer.write_buffered_borrowed(b"abc").await?;
+        writer.write_buffered_borrowed(b"d").await?;
+        writer.write_buffered_borrowed(b"e").await?;
+        writer.write_buffered_borrowed(b"fg").await?;
+        writer.write_buffered_borrowed(b"hi").await?;
+        writer.write_buffered_borrowed(b"j").await?;
+        writer.write_buffered_borrowed(b"klmno").await?;
+
+        let recorder = writer.flush_and_into_inner().await?;
+        assert_eq!(
+            recorder.writes,
+            {
+                let expect: &[&[u8]] = &[b"ab", b"cd", b"ef", b"gh", b"ij", b"kl", b"mn", b"o"];
+                expect
+            }
+            .iter()
+            .map(|v| v[..].to_vec())
+            .collect::<Vec<_>>()
+        );
+        Ok(())
+    }
 }