Issue 4900: Read LAC from EntryMemTable in SortedLedgerStorage if it is missing from the Ledger Cache - #4902
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The internals of getLastAddConfirmed for EntryMemTable are similar to the implementation in InterleavedLedgerStorage. This will eventually be consulted in SortedLedgerStorage to retrieve the LAC.
This method will eventually be used within SortedLedgerStorage as a last-try attempt to read the LAC, with additional logic between checking the ledger cache and reading an entry from the filesystem.
SortedLedgerStorage.getLastAddConfirmed now falls back to the EntryMemTable for LAC if it is not present in the ledger cache of its internal InterleavedLedgerStorage. This fixes an issue where accumulating low-data or empty ledgers can cause ledgers to fall out of the ledger cache, but still be present in the memtable and therefore not flushed, to report the ledger as missing when acquiring the LAC for the ledger.
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Descriptions of the changes in this PR:
Main Issue: #4900
Note that there is a separate issue, #4895, that must be addressed for this to function correctly when ledger metadata is in use.
Motivation
The root cause of #4900 is that
SortedLedgerStorageignoresEntryMemTablewhen getting the LAC. This PR updatesSortedLedgerStorageso that the LAC is read fromEntryMemTableif it is missing in theLedgerCache.Changes
EntryMemTable, expose agetLastAddConfirmedthat functions similar toInterleavedLedgerStorage.getLastAddConfirmed.InterleavedLedgerStorage.getLastAddConfirmedto exposegetLastAddConfirmedFromLedgerEntry, the LAC reading case where the LAC is not present in the Ledger CacheSortedLedgerStorage.getLastAddConfirmed, first explicitly try to read the LAC fromInterleavedLedgerStorage'sLedgerCache. If the LAC isn't present in the ledger cache, read it from theEntryMemTable. If the LAC isn't present in theEntryMemTable, read it fromInterleavedLedgerStoragedirectly, bypassing theLedgerCache.