RATIS-1369. When there is no snapshot, the return snapshotIndex should be -1, not 0.

[GlenGeng-awx]

What changes were proposed in this pull request?

For now, if there is no snapshot existed, the returned snapshotIndex is 0.
It is not correctly, since the raft log starts from index 0 (BTW, in raft paper, the first valid log index is 1, but in ratis, it is 0), a snapshot taken at snapshotIndex 0 should contain the log entry 0. The snapshotIndex indicating an empty/fake snapshot should be -1.

diff --git a/ratis-server/src/main/java/org/apache/ratis/server/impl/ServerState.java b/ratis-server/src/main/java/org/apache/ratis/server/impl/ServerState.java
index 728f7e9c..6307ca79 100644
--- a/ratis-server/src/main/java/org/apache/ratis/server/impl/ServerState.java
+++ b/ratis-server/src/main/java/org/apache/ratis/server/impl/ServerState.java
@@ -431,7 +431,7 @@ class ServerState implements Closeable {
 
   long getLatestInstalledSnapshotIndex() {
     final TermIndex ti = latestInstalledSnapshot.get();
-    return ti != null? ti.getIndex(): 0L;
+    return ti != null? ti.getIndex(): -1L;
   }
 
   /**
@@ -440,7 +440,7 @@ class ServerState implements Closeable {
    */
   long getSnapshotIndex() {
     final SnapshotInfo s = getLatestSnapshot();
-    final long latestSnapshotIndex = s != null ? s.getIndex() : 0;
+    final long latestSnapshotIndex = s != null ? s.getIndex() : -1;
     return Math.max(latestSnapshotIndex, getLatestInstalledSnapshotIndex());
   }

This issue is found in the test of bootstrap SCM in SCM HA. For details, please check the jira.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-1369

How was this patch tested?

CI

[GlenGeng-awx]

@bshashikant @bharatviswa504 @szetszwo Can you please take a look ?

[bshashikant]

@szetszwo , can you please check this one?

[szetszwo]

+1
The change looks good. Good catch on the bug.

Let's use RaftLog.INVALID_LOG_INDEX instead of hard coding it. Since it is very minor, I will change it before merging.