[StreamTracker] allow rollback to start-1 (#294)

Related issue: eBay/HomeStore#857

Co-authored-by: yawzhang <yawzhang@ebay.com>

Author: Besroy

conanfile.py

@@ -9,7 +9,7 @@
 
 class SISLConan(ConanFile):
     name = "sisl"
-    version = "13.2.1"
+    version = "13.2.2"
 
     homepage = "https://github.com/eBay/sisl"
     description = "Library for fast data structures, utilities"

include/sisl/fds/stream_tracker.hpp

@@ -90,6 +90,12 @@ class StreamTracker {
 
     void rollback(int64_t new_end_idx) {
         auto holder = std::shared_lock< folly::SharedMutex >(m_lock);
+        // Special case: allow rollback exactly to (start_idx - 1), which clears all slots >= start
+        if (new_end_idx + 1 == m_slot_ref_idx) {
+            m_active_slot_bits.reset_bits(0, m_active_slot_bits.size());
+            m_comp_slot_bits.reset_bits(0, m_comp_slot_bits.size());
+            return;
+        }
         if ((new_end_idx < m_slot_ref_idx) ||
             (new_end_idx >= (m_slot_ref_idx + int64_cast(m_active_slot_bits.size())))) {
             throw std::out_of_range("Slot idx is not in range");