From 59f50252c202b6cffb14088f64136b8ad597d6db Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Mon, 29 Jul 2024 11:23:16 +0000
Subject: [PATCH] WIP

---
 pageserver/src/tenant/ephemeral_file.rs       |  4 +--
 .../src/tenant/ephemeral_file/page_caching.rs |  2 +-
 .../ephemeral_file/zero_padded_read_write.rs  | 25 ++++++++++---------
 3 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/pageserver/src/tenant/ephemeral_file.rs b/pageserver/src/tenant/ephemeral_file.rs
index bb65ae24fc5e..7a544c318553 100644
--- a/pageserver/src/tenant/ephemeral_file.rs
+++ b/pageserver/src/tenant/ephemeral_file.rs
@@ -58,7 +58,7 @@ impl EphemeralFile {
         })
     }
 
-    pub(crate) fn len(&self) -> u64 {
+    pub(crate) fn len(&self) -> u32 {
         self.rw.bytes_written()
     }
 
@@ -83,7 +83,7 @@ impl EphemeralFile {
         &mut self,
         srcbuf: &[u8],
         ctx: &RequestContext,
-    ) -> Result<u64, io::Error> {
+    ) -> Result<u32, io::Error> {
         let pos = self.rw.bytes_written();
 
         // Write the length field
diff --git a/pageserver/src/tenant/ephemeral_file/page_caching.rs b/pageserver/src/tenant/ephemeral_file/page_caching.rs
index 43b9fff28d98..31054b9888be 100644
--- a/pageserver/src/tenant/ephemeral_file/page_caching.rs
+++ b/pageserver/src/tenant/ephemeral_file/page_caching.rs
@@ -55,7 +55,7 @@ impl RW {
         self.rw.write_all_borrowed(srcbuf, ctx).await
     }
 
-    pub(crate) fn bytes_written(&self) -> u64 {
+    pub(crate) fn bytes_written(&self) -> u32 {
         self.rw.bytes_written()
     }
 
diff --git a/pageserver/src/tenant/ephemeral_file/zero_padded_read_write.rs b/pageserver/src/tenant/ephemeral_file/zero_padded_read_write.rs
index fe310acab888..85c207cc16f1 100644
--- a/pageserver/src/tenant/ephemeral_file/zero_padded_read_write.rs
+++ b/pageserver/src/tenant/ephemeral_file/zero_padded_read_write.rs
@@ -69,10 +69,10 @@ where
         self.buffered_writer.write_buffered_borrowed(buf, ctx).await
     }
 
-    pub fn bytes_written(&self) -> u64 {
+    pub fn bytes_written(&self) -> u32 {
         let flushed_offset = self.buffered_writer.as_inner().bytes_written();
         let buffer: &zero_padded::Buffer<TAIL_SZ> = self.buffered_writer.inspect_buffer();
-        flushed_offset + u64::try_from(buffer.pending()).unwrap()
+        flushed_offset + u32::try_from(buffer.pending()).unwrap()
     }
 
     /// Get a slice of all blocks that [`Self::read_blk`] would return as [`ReadResult::ServedFromZeroPaddedMutableTail`].
@@ -91,10 +91,11 @@ where
     }
 
     pub(crate) async fn read_blk(&self, blknum: u32) -> Result<ReadResult<'_, W>, std::io::Error> {
-        let flushed_offset = self.buffered_writer.as_inner().bytes_written();
+        let flushed_offset = u32::try_from(self.buffered_writer.as_inner().bytes_written()).expect("");
         let buffer: &zero_padded::Buffer<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);
+        let buffered_offset = flushed_offset + u32::try_from(buffer.pending()).unwrap();
+        let page_sz = u32::try_from(PAGE_SZ).unwrap();
+        let read_offset = blknum.checked_mul(page_sz).unwrap();
 
         // The trailing page ("block") might only be partially filled,
         // yet the blob_io code relies on us to return a full PAGE_SZed slice anyway.
@@ -103,28 +104,28 @@ where
         // DeltaLayer probably has the same issue, not sure why it needs no special treatment.
         // => check here that the read doesn't go beyond this potentially trailing
         // => the zero-padding is done in the `else` branch below
-        let blocks_written = if buffered_offset % (PAGE_SZ as u64) == 0 {
-            buffered_offset / (PAGE_SZ as u64)
+        let blocks_written = if buffered_offset % page_sz == 0 {
+            buffered_offset / page_sz
         } else {
-            (buffered_offset / (PAGE_SZ as u64)) + 1
+            (buffered_offset / page_sz) + 1
         };
-        if (blknum as u64) >= blocks_written {
+        if blknum >= blocks_written {
             return Err(std::io::Error::new(std::io::ErrorKind::Other, anyhow::anyhow!("read past end of ephemeral_file: read=0x{read_offset:x} buffered=0x{buffered_offset:x} flushed=0x{flushed_offset}")));
         }
 
         // assertions for the `if-else` below
         assert_eq!(
-            flushed_offset % (TAIL_SZ as u64), 0,
+            flushed_offset % (u32::try_from(TAIL_SZ).unwrap()), 0,
             "we only use write_buffered_borrowed to write to the buffered writer, so it's guaranteed that flushes happen buffer.cap()-sized chunks"
         );
         assert_eq!(
-            flushed_offset % (PAGE_SZ as u64),
+            flushed_offset % page_sz,
             0,
             "the logic below can't handle if the page is spread across the flushed part and the buffer"
         );
 
         if read_offset < flushed_offset {
-            assert!(read_offset + (PAGE_SZ as u64) <= flushed_offset);
+            assert!(read_offset + page_sz <= flushed_offset);
             Ok(ReadResult::NeedsReadFromWriter {
                 writer: self.as_writer(),
             })