blockstore: atomize slot clearing, relax parent slot meta check (#35124)

* blockstore: atomize slot clearing, relax parent slot meta check

clear_unconfirmed_slot can leave blockstore in an irrecoverable state
if it panics in the middle. write batch this function, so that any
errors can be recovered after restart.

additionally relax the constraint that the parent slot meta must exist,
as it could have been cleaned up if outdated.

* pr feedback: use PurgeType, don't pass slot_meta

* pr feedback: add unit test

* pr feedback: refactor into separate function

* pr feedback: add special columns to helper, err msg, comments

* pr feedback: reword comments and write batch error message

* pr feedback: bubble write_batch error to caller

* pr feedback: reword comments

Co-authored-by: steviez <[email protected]>

---------

Co-authored-by: steviez <[email protected]>
(cherry picked from commit cc4072b)

# Conflicts:
#	ledger/src/blockstore.rs
#	ledger/src/blockstore/blockstore_purge.rs

ledger/src/blockstore.rs

@@ -1111,16 +1111,16 @@ impl Blockstore {
         self.completed_slots_senders.lock().unwrap().clear();
     }
 
-    /// Range-delete all entries which prefix matches the specified `slot`,
-    /// remove `slot` its' parents SlotMeta next_slots list, and
-    /// clear `slot`'s SlotMeta (except for next_slots).
+    /// Clear `slot` from the Blockstore, see ``Blockstore::purge_slot_cleanup_chaining`
+    /// for more details.
     ///
     /// This function currently requires `insert_shreds_lock`, as both
     /// `clear_unconfirmed_slot()` and `insert_shreds_handle_duplicate()`
     /// try to perform read-modify-write operation on [`cf::SlotMeta`] column
     /// family.
     pub fn clear_unconfirmed_slot(&self, slot: Slot) {
         let _lock = self.insert_shreds_lock.lock().unwrap();
+<<<<<<< HEAD
         if let Some(mut slot_meta) = self
             .meta(slot)
             .expect("Couldn't fetch from SlotMeta column family")
@@ -1152,9 +1152,21 @@ impl Blockstore {
                 .expect("Couldn't insert into SlotMeta column family");
         } else {
             error!(
+=======
+        // Purge the slot and insert an empty `SlotMeta` with only the `next_slots` field preserved.
+        // Shreds inherently know their parent slot, and a parent's SlotMeta `next_slots` list
+        // will be updated when the child is inserted (see `Blockstore::handle_chaining()`).
+        // However, we are only purging and repairing the parent slot here. Since the child will not be
+        // reinserted the chaining will be lost. In order for bank forks discovery to ingest the child,
+        // we must retain the chain by preserving `next_slots`.
+        match self.purge_slot_cleanup_chaining(slot) {
+            Ok(_) => {}
+            Err(BlockstoreError::SlotUnavailable) => error!(
+>>>>>>> cc4072bce8 (blockstore: atomize slot clearing, relax parent slot meta check  (#35124))
                 "clear_unconfirmed_slot() called on slot {} with no SlotMeta",
                 slot
-            );
+            ),
+            Err(e) => panic!("Purge database operations failed {}", e),
         }
     }