From 459c01f93c7b655c4c7d2524d9c1919379b3c97e Mon Sep 17 00:00:00 2001
From: Jussi Saurio <jussi.saurio@gmail.com>
Date: Tue, 21 Oct 2025 11:14:26 +0300
Subject: [PATCH] Add subjournal module

The subjournal is a temporary file where stmt subtransactions write an
'undo log' of pages before modifying them. If a stmt subtransaction
rolls back, the pages are restored from the subjournal.
---
 core/storage/mod.rs        |  1 +
 core/storage/subjournal.rs | 88 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 core/storage/subjournal.rs

diff --git a/core/storage/mod.rs b/core/storage/mod.rs
index ee5029ba0..f64d26385 100644
--- a/core/storage/mod.rs
+++ b/core/storage/mod.rs
@@ -22,6 +22,7 @@ pub(crate) mod pager;
 pub(super) mod slot_bitmap;
 pub(crate) mod sqlite3_ondisk;
 mod state_machines;
+pub(crate) mod subjournal;
 #[allow(clippy::arc_with_non_send_sync)]
 pub(crate) mod wal;
 
diff --git a/core/storage/subjournal.rs b/core/storage/subjournal.rs
new file mode 100644
index 000000000..d174230a5
--- /dev/null
+++ b/core/storage/subjournal.rs
@@ -0,0 +1,88 @@
+use std::sync::Arc;
+
+use crate::{
+    storage::sqlite3_ondisk::finish_read_page, Buffer, Completion, CompletionError, PageRef, Result,
+};
+
+#[derive(Clone)]
+pub struct Subjournal {
+    file: Arc<dyn crate::io::File>,
+}
+
+impl Subjournal {
+    pub fn new(file: Arc<dyn crate::io::File>) -> Self {
+        Self { file }
+    }
+
+    pub fn size(&self) -> Result<u64> {
+        self.file.size()
+    }
+
+    pub fn write_page(
+        &self,
+        offset: u64,
+        page_size: usize,
+        buffer: Arc<Buffer>,
+        c: Completion,
+    ) -> Result<Completion> {
+        assert!(
+            buffer.len() == page_size + 4,
+            "buffer length should be page_size + 4 bytes for page id"
+        );
+        self.file.pwrite(offset, buffer, c)
+    }
+
+    pub fn read_page_number(&self, offset: u64, page_id_buffer: Arc<Buffer>) -> Result<Completion> {
+        assert!(
+            page_id_buffer.len() == 4,
+            "page_id_buffer length should be 4 bytes"
+        );
+        let c = Completion::new_read(
+            page_id_buffer,
+            move |res: Result<(Arc<Buffer>, i32), CompletionError>| {
+                let Ok((_buffer, _bytes_read)) = res else {
+                    return;
+                };
+            },
+        );
+        let c = self.file.pread(offset, c)?;
+        Ok(c)
+    }
+
+    pub fn read_page(
+        &self,
+        offset: u64,
+        buffer: Arc<Buffer>,
+        page: PageRef,
+        page_size: usize,
+    ) -> Result<Completion> {
+        assert!(
+            buffer.len() == page_size,
+            "buffer length should be page_size"
+        );
+        let c = Completion::new_read(
+            buffer,
+            move |res: Result<(Arc<Buffer>, i32), CompletionError>| {
+                let Ok((buffer, bytes_read)) = res else {
+                    return;
+                };
+                assert!(
+                    bytes_read == page_size as i32,
+                    "bytes_read should be page_size"
+                );
+                finish_read_page(page.get().id as usize, buffer, page.clone());
+            },
+        );
+        let c = self.file.pread(offset, c)?;
+        Ok(c)
+    }
+
+    pub fn truncate(&self, offset: u64) -> Result<Completion> {
+        let c = Completion::new_trunc(move |res: Result<i32, CompletionError>| {
+            let Ok(_) = res else {
+                return;
+            };
+        });
+        self.file.truncate(offset, c)
+    }
+}