From 235baffbf4465794a31f2ab30ca34439dc989bf0 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Tue, 29 Aug 2023 10:50:21 +0200
Subject: [PATCH] make the read path async, except the read_at impl

---
 pageserver/src/tenant/ephemeral_file.rs |  3 ++-
 pageserver/src/virtual_file.rs          | 32 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/pageserver/src/tenant/ephemeral_file.rs b/pageserver/src/tenant/ephemeral_file.rs
index 02ef7166e5..fbb1b6a0bf 100644
--- a/pageserver/src/tenant/ephemeral_file.rs
+++ b/pageserver/src/tenant/ephemeral_file.rs
@@ -87,7 +87,8 @@ impl EphemeralFile {
                         let buf: &mut [u8] = write_guard.deref_mut();
                         debug_assert_eq!(buf.len(), PAGE_SZ);
                         self.file
-                            .read_exact_at(&mut buf[..], blknum as u64 * PAGE_SZ as u64)?;
+                            .read_exact_at_async(&mut buf[..], blknum as u64 * PAGE_SZ as u64)
+                            .await?;
                         write_guard.mark_valid();
 
                         // Swap for read lock
diff --git a/pageserver/src/virtual_file.rs b/pageserver/src/virtual_file.rs
index 836b3ef17d..edc8329e63 100644
--- a/pageserver/src/virtual_file.rs
+++ b/pageserver/src/virtual_file.rs
@@ -406,6 +406,27 @@ impl VirtualFile {
         Ok(())
     }
 
+    // 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> {
+        while !buf.is_empty() {
+            match self.read_at_async(buf, offset).await {
+                Ok(0) => {
+                    return Err(Error::new(
+                        std::io::ErrorKind::UnexpectedEof,
+                        "failed to fill whole buffer",
+                    ))
+                }
+                Ok(n) => {
+                    buf = &mut buf[n..];
+                    offset += n as u64;
+                }
+                Err(ref e) if e.kind() == std::io::ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(())
+    }
+
     // Copied from https://doc.rust-lang.org/1.72.0/src/std/os/unix/fs.rs.html#219-235
     pub fn write_all_at(&self, mut buf: &[u8], mut offset: u64) -> Result<(), Error> {
         while !buf.is_empty() {
@@ -437,6 +458,17 @@ impl VirtualFile {
         result
     }
 
+    async fn read_at_async(&self, buf: &mut [u8], offset: u64) -> Result<usize, Error> {
+        // TODO: don't block here
+        let result = self.with_file("read", |file| file.read_at(buf, offset))?;
+        if let Ok(size) = result {
+            STORAGE_IO_SIZE
+                .with_label_values(&["read", &self.tenant_id, &self.timeline_id])
+                .add(size as i64);
+        }
+        result
+    }
+
     pub fn write_at(&self, buf: &[u8], offset: u64) -> Result<usize, Error> {
         let result = self.with_file("write", |file| file.write_at(buf, offset))?;
         if let Ok(size) = result {