From 78a17a70513a54f96504757929e99ccf2dba3579 Mon Sep 17 00:00:00 2001
From: Yuchen Liang <yuchen@neon.tech>
Date: Tue, 19 Nov 2024 18:38:59 +0000
Subject: [PATCH] improve FullSlice semantics

Signed-off-by: Yuchen Liang <yuchen@neon.tech>
---
 pageserver/src/tenant/ephemeral_file.rs         | 17 +++++++++++------
 .../virtual_file/owned_buffers_io/io_buf_ext.rs | 12 +++++++++---
 .../src/virtual_file/owned_buffers_io/write.rs  |  2 +-
 .../owned_buffers_io/write/flush.rs             |  4 ++--
 4 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/pageserver/src/tenant/ephemeral_file.rs b/pageserver/src/tenant/ephemeral_file.rs
index 5f7e86f364..9aecfff384 100644
--- a/pageserver/src/tenant/ephemeral_file.rs
+++ b/pageserver/src/tenant/ephemeral_file.rs
@@ -210,13 +210,13 @@ impl super::storage_layer::inmemory_layer::vectored_dio_read::File for Ephemeral
             }
         }
 
-        // [     written     ][ maybe_flushed ][    mutable    ]
-        // 				      |-   TAIL_SZ   -||-   TAIL_SZ   -|
-        //                                     ^
-        //                                `flushed_offset`
-        //
         let (written_range, maybe_flushed_range) = {
             if maybe_flushed.is_some() {
+                // [       written       ][ maybe_flushed ][    mutable    ]
+                //                        <-   TAIL_SZ   -><-   TAIL_SZ   ->
+                //                                         ^
+                //                                 `submitted_offset`
+                // <++++++ on disk +++++++????????????????>
                 (
                     Range(
                         start,
@@ -228,6 +228,11 @@ impl super::storage_layer::inmemory_layer::vectored_dio_read::File for Ephemeral
                     ),
                 )
             } else {
+                // [       written                        ][    mutable    ]
+                //                                         <-   TAIL_SZ   ->
+                //                                         ^
+                //                                 `submitted_offset`
+                // <++++++ on disk +++++++++++++++++++++++>
                 (
                     Range(start, std::cmp::min(end, submitted_offset)),
                     // zero len
@@ -434,7 +439,7 @@ mod tests {
         assert!(file_contents == content[0..cap * 2]);
 
         let maybe_flushed_buffer_contents = file.buffered_writer.inspect_maybe_flushed().unwrap();
-        assert_eq!(maybe_flushed_buffer_contents, &content[cap..cap * 2]);
+        assert_eq!(&maybe_flushed_buffer_contents[..], &content[cap..cap * 2]);
 
         let mutable_buffer_contents = file.buffered_writer.inspect_mutable();
         assert_eq!(mutable_buffer_contents, &content[cap * 2..write_nbytes]);
diff --git a/pageserver/src/virtual_file/owned_buffers_io/io_buf_ext.rs b/pageserver/src/virtual_file/owned_buffers_io/io_buf_ext.rs
index ede04d4957..70d33d59bf 100644
--- a/pageserver/src/virtual_file/owned_buffers_io/io_buf_ext.rs
+++ b/pageserver/src/virtual_file/owned_buffers_io/io_buf_ext.rs
@@ -27,10 +27,16 @@ where
         let FullSlice { slice: s } = self;
         s
     }
+}
 
-    pub(crate) fn as_raw_slice(&self) -> &Slice<B> {
-        let FullSlice { slice: s } = &self;
-        s
+impl<B> Clone for FullSlice<B>
+where
+    B: IoBuf + Clone,
+{
+    fn clone(&self) -> Self {
+        Self {
+            slice: self.slice.get_ref().clone().slice_full(),
+        }
     }
 }
 
diff --git a/pageserver/src/virtual_file/owned_buffers_io/write.rs b/pageserver/src/virtual_file/owned_buffers_io/write.rs
index 84bf32b684..0f2e8a635c 100644
--- a/pageserver/src/virtual_file/owned_buffers_io/write.rs
+++ b/pageserver/src/virtual_file/owned_buffers_io/write.rs
@@ -92,7 +92,7 @@ where
 
     /// Gets a reference to the maybe flushed read-only buffer.
     /// Returns `None` if the writer has not submitted any flush request.
-    pub fn inspect_maybe_flushed(&self) -> Option<&Buf> {
+    pub fn inspect_maybe_flushed(&self) -> Option<&FullSlice<Buf>> {
         self.flush_handle.maybe_flushed.as_ref()
     }
 
diff --git a/pageserver/src/virtual_file/owned_buffers_io/write/flush.rs b/pageserver/src/virtual_file/owned_buffers_io/write/flush.rs
index 585c70218b..4c2019076e 100644
--- a/pageserver/src/virtual_file/owned_buffers_io/write/flush.rs
+++ b/pageserver/src/virtual_file/owned_buffers_io/write/flush.rs
@@ -14,7 +14,7 @@ pub struct FlushHandle<Buf, W> {
     inner: Option<FlushHandleInner<Buf, W>>,
     /// Immutable buffer for serving tail reads.
     /// `None` if no flush request has been submitted.
-    pub(super) maybe_flushed: Option<Buf>,
+    pub(super) maybe_flushed: Option<FullSlice<Buf>>,
 }
 
 // TODO(yuchen): special actions in drop to clean up the join handle?
@@ -162,7 +162,7 @@ where
 
         // Saves a buffer for read while flushing. This also removes reference to the old buffer.
         self.maybe_flushed = if save_buf_for_read {
-            Some(slice.as_raw_slice().get_ref().clone())
+            Some(slice.clone())
         } else {
             None
         };