Improve: loosen validity check with RaftState.snapshot_last_log_id()

A application may not persist snapshot. And when it restarted, the
last-purged-log-id is not `None` but `snapshot_last_log_id()` is None.
This is a valid state and should not emit error.

openraft/src/lib.rs

@@ -71,6 +71,7 @@ mod runtime;
 mod try_as_ref;
 
 #[cfg(test)] mod raft_state_test;
+#[cfg(test)] mod rust_state_validate_test;
 
 pub use anyerror;
 pub use anyerror::AnyError;

openraft/src/raft_state.rs

@@ -218,7 +218,14 @@ where
         }
 
         less_equal!(self.last_purged_log_id(), self.purge_upto());
-        less_equal!(self.purge_upto(), self.snapshot_last_log_id());
+        if self.snapshot_last_log_id().is_none() {
+            // There is no snapshot, it is possible the application does not store snapshot, and
+            // just restarted. it is just ok.
+            // In such a case, we assert the monotonic relation without  snapshot-last-log-id
+            less_equal!(self.purge_upto(), self.committed());
+        } else {
+            less_equal!(self.purge_upto(), self.snapshot_last_log_id());
+        }
         less_equal!(self.snapshot_last_log_id(), self.committed());
         less_equal!(self.committed(), self.last_log_id());
 

openraft/src/rust_state_validate_test.rs

@@ -0,0 +1,32 @@
+use crate::engine::LogIdList;
+use crate::validate::Validate;
+use crate::CommittedLeaderId;
+use crate::LogId;
+use crate::RaftState;
+use crate::SnapshotMeta;
+
+fn log_id(term: u64, index: u64) -> LogId<u64> {
+    LogId::<u64> {
+        leader_id: CommittedLeaderId::new(term, 0),
+        index,
+    }
+}
+
+#[test]
+fn test_raft_state_validate_snapshot_is_none() -> anyhow::Result<()> {
+    // Some app does not persist snapshot, when restarted, purged is not None but snapshot_last_log_id
+    // is None. This is a valid state and should not emit error.
+
+    let rs = RaftState::<u64, ()> {
+        log_ids: LogIdList::new(vec![log_id(1, 1), log_id(3, 4)]),
+        purged_next: 2,
+        purge_upto: Some(log_id(1, 1)),
+        committed: Some(log_id(1, 1)),
+        snapshot_meta: SnapshotMeta::default(),
+        ..Default::default()
+    };
+
+    assert!(rs.validate().is_ok());
+
+    Ok(())
+}