feat: sparse block header storage for wallet scanner

base_layer/wallet/src/storage/database.rs

@@ -63,6 +63,11 @@ pub trait WalletBackend: Send + Sync + Clone {
         exclude_recovered: bool,
     ) -> Result<(), WalletStorageError>;
 
+    /// Apply a sparse storage schedule to scanned blocks, keeping all recent blocks
+    /// and progressively sparser checkpoints for older blocks. Blocks containing
+    /// recovered outputs are always preserved.
+    fn apply_sparse_scanned_blocks_schedule(&self, tip_height: u64) -> Result<(), WalletStorageError>;
+
     /// Change the passphrase used to encrypt the database
     fn change_passphrase(&self, existing: &SafePassword, new: &SafePassword) -> Result<(), WalletStorageError>;
 
@@ -337,6 +342,11 @@ where T: WalletBackend + 'static
         Ok(())
     }
 
+    pub fn apply_sparse_scanned_blocks_schedule(&self, tip_height: u64) -> Result<(), WalletStorageError> {
+        self.db.apply_sparse_scanned_blocks_schedule(tip_height)?;
+        Ok(())
+    }
+
     pub fn get_all_burn_proofs(&self) -> Result<Vec<DbBurnProof>, WalletStorageError> {
         self.db.fetch_burn_proofs()
     }

base_layer/wallet/src/storage/sqlite_db/scanned_blocks.rs

@@ -107,6 +107,42 @@ impl ScannedBlockSql {
         query.execute(conn)?;
         Ok(())
     }
+
+    /// Apply a sparse storage schedule to scanned blocks, keeping:
+    /// - All blocks within `tip - 720` to `tip`
+    /// - Every 100th block from `tip - 10,000` to `tip - 720`
+    /// - Every 1,000th block from `tip - 100,000` to `tip - 10,000`
+    /// - Every 5,000th block from genesis to `tip - 100,000`
+    /// Blocks containing recovered outputs (num_outputs > 0) are always preserved.
+    pub fn apply_sparse_schedule(tip_height: u64, conn: &mut SqliteConnection) -> Result<(), WalletStorageError> {
+        let blocks = Self::index(conn)?;
+        for block in blocks {
+            let height = block.height as u64;
+            if let Some(outputs) = block.num_outputs {
+                if outputs > 0 {
+                    continue;
+                }
+            }
+            if !Self::should_keep_at_height(height, tip_height) {
+                diesel::delete(scanned_blocks::table.filter(scanned_blocks::height.eq(block.height))).execute(conn)?;
+            }
+        }
+        Ok(())
+    }
+
+    /// Determine whether a block at `height` should be kept given the current `tip`.
+    fn should_keep_at_height(height: u64, tip: u64) -> bool {
+        let depth = tip.saturating_sub(height);
+        if depth <= 720 {
+            true
+        } else if depth <= 10_000 {
+            height % 100 == 0
+        } else if depth <= 100_000 {
+            height % 1_000 == 0
+        } else {
+            height % 5_000 == 0
+        }
+    }
 }
 
 impl From<ScannedBlock> for ScannedBlockSql {

base_layer/wallet/src/storage/sqlite_db/wallet.rs

@@ -551,6 +551,11 @@ impl WalletBackend for WalletSqliteDatabase {
         ScannedBlockSql::clear_before_height(height, exclude_recovered, &mut conn)
     }
 
+    fn apply_sparse_scanned_blocks_schedule(&self, tip_height: u64) -> Result<(), WalletStorageError> {
+        let mut conn = self.database_connection.get_pooled_connection()?;
+        ScannedBlockSql::apply_sparse_schedule(tip_height, &mut conn)
+    }
+
     fn change_passphrase(&self, existing: &SafePassword, new: &SafePassword) -> Result<(), WalletStorageError> {
         let mut conn = self.database_connection.get_pooled_connection()?;
 

base_layer/wallet/src/utxo_scanner_service/service.rs

@@ -44,7 +44,7 @@ use crate::{
 
 pub const LOG_TARGET: &str = "wallet::utxo_scanning";
 
-// Cache 1 days worth of headers.
+// Keep all blocks within the most recent window (approximately 1 day of blocks).
 pub const SCANNED_BLOCK_CACHE_SIZE: u64 = 720;
 
 pub struct UtxoScannerService<

base_layer/wallet/src/utxo_scanner_service/utxo_scanner_task.rs

@@ -66,7 +66,7 @@ use crate::{
     utxo_scanner_service::{
         RECOVERY_KEY,
         handle::UtxoScannerEvent,
-        service::{SCANNED_BLOCK_CACHE_SIZE, ScannedBlock, UtxoScannerResources},
+        service::{ScannedBlock, UtxoScannerResources},
         uxto_scanner_service_builder::UtxoScannerMode,
     },
 };
@@ -576,10 +576,9 @@ where
             }
             // We need to update the last one
             if let Some(scanned_block) = prev_scanned_block.clone() {
-                self.resources.db.clear_scanned_blocks_before_height(
-                    scanned_block.height.saturating_sub(SCANNED_BLOCK_CACHE_SIZE),
-                    true,
-                )?;
+                self.resources
+                    .db
+                    .apply_sparse_scanned_blocks_schedule(scanned_block.height)?;
                 if last_saved_hash != Some(scanned_block.header_hash) {
                     self.resources.db.save_scanned_block(scanned_block)?;
                 }