Merge 'core: Clean up B-Tree creation code' from Pekka Enberg

Move page allocation to pager so that we don't need to instantiate a
cursor to create a B-Tree.

Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>

Closes #1093

core/storage/btree.rs

@@ -1317,7 +1317,7 @@ impl BTreeCursor {
                         balance_info.pages_to_balance[i].set_dirty();
                         pages_to_balance_new.push(balance_info.pages_to_balance[i].clone());
                     } else {
-                        let page = self.allocate_page(page_type, 0);
+                        let page = self.pager.do_allocate_page(page_type, 0);
                         pages_to_balance_new.push(page);
                         // Since this page didn't exist before, we can set it to cells length as it
                         // marks them as empty since it is a prefix sum of cells.
@@ -1476,7 +1476,7 @@ impl BTreeCursor {
 
         let root = self.stack.top();
         let root_contents = root.get_contents();
-        let child = self.allocate_page(root_contents.page_type(), 0);
+        let child = self.pager.do_allocate_page(root_contents.page_type(), 0);
 
         tracing::debug!(
             "Balancing root. root={}, rightmost={}",
@@ -1533,21 +1533,8 @@ impl BTreeCursor {
         self.stack.push(child.clone());
     }
 
-    /// Allocate a new page to the btree via the pager.
-    /// This marks the page as dirty and writes the page header.
-    fn allocate_page(&self, page_type: PageType, offset: usize) -> PageRef {
-        let page = self.pager.allocate_page().unwrap();
-        btree_init_page(&page, page_type, offset, self.usable_space() as u16);
-        page
-    }
-
-    /// The "usable size" of a database page is the page size specified by the 2-byte integer at offset 16
-    /// in the header, minus the "reserved" space size recorded in the 1-byte integer at offset 20 in the header.
-    /// The usable size of a page might be an odd number. However, the usable size is not allowed to be less than 480.
-    /// In other words, if the page size is 512, then the reserved space size cannot exceed 32.
     fn usable_space(&self) -> usize {
-        let db_header = self.pager.db_header.borrow();
-        (db_header.page_size - db_header.reserved_space as u16) as usize
+        self.pager.usable_space()
     }
 
     /// Find the index of the cell in the page that contains the given rowid.
@@ -1855,20 +1842,6 @@ impl BTreeCursor {
         }
     }
 
-    pub fn btree_create(&mut self, flags: usize) -> u32 {
-        let page_type = match flags {
-            1 => PageType::TableLeaf,
-            2 => PageType::IndexLeaf,
-            _ => unreachable!(
-                "wrong create table flags, should be 1 for table and 2 for index, got {}",
-                flags,
-            ),
-        };
-        let page = self.allocate_page(page_type, 0);
-        let id = page.get().id;
-        id as u32
-    }
-
     fn clear_overflow_pages(&self, cell: &BTreeCell) -> Result<CursorResult<()>> {
         // Get overflow info based on cell type
         let (first_overflow_page, n_overflow) = match cell {

core/storage/pager.rs

@@ -1,7 +1,7 @@
 use crate::result::LimboResult;
 use crate::storage::buffer_pool::BufferPool;
 use crate::storage::database::DatabaseStorage;
-use crate::storage::sqlite3_ondisk::{self, DatabaseHeader, PageContent};
+use crate::storage::sqlite3_ondisk::{self, DatabaseHeader, PageContent, PageType};
 use crate::storage::wal::{CheckpointResult, Wal};
 use crate::{Buffer, LimboError, Result};
 use parking_lot::RwLock;
@@ -203,6 +203,37 @@ impl Pager {
         })
     }
 
+    pub fn btree_create(&self, flags: usize) -> u32 {
+        let page_type = match flags {
+            1 => PageType::TableLeaf,
+            2 => PageType::IndexLeaf,
+            _ => unreachable!(
+                "wrong create table flags, should be 1 for table and 2 for index, got {}",
+                flags,
+            ),
+        };
+        let page = self.do_allocate_page(page_type, 0);
+        let id = page.get().id;
+        id as u32
+    }
+
+    /// Allocate a new page to the btree via the pager.
+    /// This marks the page as dirty and writes the page header.
+    pub fn do_allocate_page(&self, page_type: PageType, offset: usize) -> PageRef {
+        let page = self.allocate_page().unwrap();
+        crate::btree_init_page(&page, page_type, offset, self.usable_space() as u16);
+        page
+    }
+
+    /// The "usable size" of a database page is the page size specified by the 2-byte integer at offset 16
+    /// in the header, minus the "reserved" space size recorded in the 1-byte integer at offset 20 in the header.
+    /// The usable size of a page might be an odd number. However, the usable size is not allowed to be less than 480.
+    /// In other words, if the page size is 512, then the reserved space size cannot exceed 32.
+    pub fn usable_space(&self) -> usize {
+        let db_header = self.db_header.borrow();
+        (db_header.page_size - db_header.reserved_space as u16) as usize
+    }
+
     pub fn begin_read_tx(&self) -> Result<LimboResult> {
         self.wal.borrow_mut().begin_read_tx()
     }

core/vdbe/mod.rs

@@ -2959,9 +2959,7 @@ impl Program {
                         // TODO: implement temp databases
                         todo!("temp databases not implemented yet");
                     }
-                    let mut cursor = Box::new(BTreeCursor::new(pager.clone(), 0));
-
-                    let root_page = cursor.btree_create(*flags);
+                    let root_page = pager.btree_create(*flags);
                     state.registers[*root] = OwnedValue::Integer(root_page as i64);
                     state.pc += 1;
                 }