refactor: use read transaction in rollback

nomt/src/lib.rs

@@ -255,7 +255,7 @@ impl<T: HashAlgorithm> Nomt<T> {
         assert_eq!(prev, 0, "only one session could be active at a time");
         let store = self.store.clone();
         let rollback_delta = if allow_rollback {
-            self.store.rollback().map(|r| r.delta_builder())
+            self.store.rollback().map(|r| r.delta_builder(&store))
         } else {
             None
         };
@@ -329,7 +329,7 @@ impl<T: HashAlgorithm> Nomt<T> {
         if let Some(delta_builder) = session.rollback_delta.take() {
             // UNWRAP: if rollback_delta is `Some``, then rollback must be also `Some`.
             let rollback = self.store.rollback().unwrap();
-            rollback.commit(self.store.clone(), &actuals, delta_builder)?;
+            rollback.commit(&actuals, delta_builder)?;
         }
 
         let mut compact_actuals = Vec::with_capacity(actuals.len());
@@ -465,7 +465,7 @@ impl Session {
     /// this call is issued, the better for efficiency.
     pub fn preserve_prior_value(&self, path: KeyPath) {
         if let Some(rollback) = &self.rollback_delta {
-            rollback.tentative_preserve_prior(self.store.clone(), path);
+            rollback.tentative_preserve_prior(path);
         }
     }
 }

nomt/src/options.rs

@@ -17,8 +17,6 @@ pub struct Options {
     /// The maximum number of commits that can be rolled back.
     pub(crate) max_rollback_log_len: u32,
     pub(crate) warm_up: bool,
-    /// The number of threads to use for fetching prior values.
-    pub(crate) rollback_tp_size: usize,
     /// Whether to preallocate the hashtable file.
     pub(crate) preallocate_ht: bool,
 }
@@ -41,7 +39,6 @@ impl Options {
             rollback: false,
             max_rollback_log_len: 100,
             warm_up: false,
-            rollback_tp_size: 4,
             preallocate_ht: true,
         }
     }
@@ -116,15 +113,6 @@ impl Options {
         self.warm_up = warm_up;
     }
 
-    /// Set the number of threads to use for fetching prior values.
-    ///
-    /// Only relevant if rollback is enabled.
-    ///
-    /// Default: 4.
-    pub fn rollback_tp_size(&mut self, rollback_tp_size: usize) {
-        self.rollback_tp_size = rollback_tp_size;
-    }
-
     /// Sets whether to preallocate the hashtable file.
     ///
     /// Many filesystems don't handle sparse files well. If the `preallocate_ht` option is set to