Feature: Add RaftLogReader::limited_get_log_entries()

This commit adds the `RaftLogReader::limited_get_log_entries()` method,
which enables applications to fetch log entries that are equal to or
smaller than a specified range. This functionality is particularly
useful for customizing the size of AppendEntries requests at the storage
API level.

- Applications can now decide the number of log entries to return based
  on the input range. If the application determines that the requested
  log entries range is too large for a single RPC, it can opt to return
  only the first several requested log entries instead of the full
  range.

- The method provides a default implementation that delegates the
  operation to `RaftLogReader::try_get_log_entries`.

This enhancement allows for more flexible and efficient handling of log
entries, particularly in scenarios where network constraints or
performance considerations require smaller data transfers.

openraft/src/replication/mod.rs

@@ -394,21 +394,23 @@ where
                 let r = LogIdRange::new(rng.prev, rng.prev);
                 (vec![], r)
             } else {
-                let logs = self.log_reader.try_get_log_entries(start..end).await?;
-                debug_assert_eq!(
-                    logs.len(),
-                    (end - start) as usize,
-                    "expect logs {}..{} but got only {} entries, first: {}, last: {}",
+                // limited_get_log_entries will return logs smaller than the range [start, end).
+                let logs = self.log_reader.limited_get_log_entries(start, end).await?;
+
+                let first = *logs.first().map(|x| x.get_log_id()).unwrap();
+                let last = *logs.last().map(|x| x.get_log_id()).unwrap();
+
+                debug_assert!(
+                    !logs.is_empty() && logs.len() <= (end - start) as usize,
+                    "expect logs ⊆ [{}..{}) but got {} entries, first: {}, last: {}",
                     start,
                     end,
                     logs.len(),
-                    logs.first().map(|ent| ent.get_log_id()).display(),
-                    logs.last().map(|ent| ent.get_log_id()).display()
+                    first,
+                    last
                 );
 
-                let last_log_id = logs.last().map(|ent| *ent.get_log_id());
-
-                let r = LogIdRange::new(rng.prev, last_log_id);
+                let r = LogIdRange::new(rng.prev, Some(last));
                 (logs, r)
             }
         };

openraft/src/storage/mod.rs

@@ -167,6 +167,23 @@ where C: RaftTypeConfig
         &mut self,
         range: RB,
     ) -> Result<Vec<C::Entry>, StorageError<C::NodeId>>;
+
+    /// Returns log entries within range `[start, end)`, `end` is exclusive,
+    /// potentially limited by implementation-defined constraints.
+    ///
+    /// If the specified range is too large, the implementation may return only the first few log
+    /// entries to ensure the result is not excessively large.
+    ///
+    /// It must not return empty result if the input range is not empty.
+    ///
+    /// The default implementation just returns the full range of log entries.
+    async fn limited_get_log_entries(
+        &mut self,
+        start: u64,
+        end: u64,
+    ) -> Result<Vec<C::Entry>, StorageError<C::NodeId>> {
+        self.try_get_log_entries(start..end).await
+    }
 }
 
 /// A trait defining the interface for a Raft state machine snapshot subsystem.

openraft/src/testing/suite.rs

@@ -100,6 +100,7 @@ where
         run_fut(run_test(builder, Self::get_initial_state_re_apply_committed))?;
         run_fut(run_test(builder, Self::save_vote))?;
         run_fut(run_test(builder, Self::get_log_entries))?;
+        run_fut(run_test(builder, Self::limited_get_log_entries))?;
         run_fut(run_test(builder, Self::try_get_log_entry))?;
         run_fut(run_test(builder, Self::initial_logs))?;
         run_fut(run_test(builder, Self::get_log_state))?;
@@ -720,6 +721,27 @@ where
         Ok(())
     }
 
+    pub async fn limited_get_log_entries(mut store: LS, mut sm: SM) -> Result<(), StorageError<C::NodeId>> {
+        Self::feed_10_logs_vote_self(&mut store).await?;
+
+        tracing::info!("--- get start == stop");
+        {
+            let logs = store.limited_get_log_entries(3, 3).await?;
+            assert_eq!(logs.len(), 0, "expected no logs to be returned");
+        }
+
+        tracing::info!("--- get start < stop");
+        {
+            let logs = store.limited_get_log_entries(5, 7).await?;
+
+            assert!(!logs.is_empty());
+            assert!(logs.len() <= 2);
+            assert_eq!(*logs[0].get_log_id(), log_id_0(1, 5));
+        }
+
+        Ok(())
+    }
+
     pub async fn try_get_log_entry(mut store: LS, mut sm: SM) -> Result<(), StorageError<C::NodeId>> {
         Self::feed_10_logs_vote_self(&mut store).await?;