persist LC data across restarts (#3823)

* persist LC data across restarts

With the Altair spec `LightClientUpdate` structure taking its final form
it is finally possible to persist LC data across restarts without having
to worry about data migration due to spec changes. A separate `lcdataV1`
database is created in the `caches` subdirectory to hold known LC data.
A full database with default settings (129 periods) uses <15 MB disk.

* extend LC data DB rationale

* wording

* add `isSupportedBySQLite` helper and explicit return

* remove redundant `return`

beacon_chain/conf.nim

@@ -1056,6 +1056,9 @@ func outWalletFile*(config: BeaconNodeConf): Option[OutFile] =
 func databaseDir*(config: AnyConf): string =
   config.dataDir / "db"
 
+func cachesDir*(config: AnyConf): string =
+  config.databaseDir / "caches"
+
 func runAsService*(config: BeaconNodeConf): bool =
   config.cmd == noCommand and config.runAsServiceFlag
 

beacon_chain/consensus_object_pools/block_pools_types_light_client.nim

@@ -15,6 +15,7 @@ import
   stew/bitops2,
   # Beacon chain internals
   ../spec/datatypes/altair,
+  ../light_client_data_db,
   ./block_dag
 
 type
@@ -49,15 +50,6 @@ type
       ## Key is the block ID of which the post state was used to get the data.
       ## Data stored for the finalized head block and all non-finalized blocks.
 
-    currentBranches*: Table[Slot, altair.CurrentSyncCommitteeBranch]
-      ## Cached data for creating future `LightClientBootstrap` instances.
-      ## Key is the block slot of which the post state was used to get the data.
-      ## Data stored for all finalized epoch boundary blocks.
-
-    bestUpdates*: Table[SyncCommitteePeriod, altair.LightClientUpdate]
-      ## Stores the `LightClientUpdate` with the most `sync_committee_bits` per
-      ## `SyncCommitteePeriod`. Sync committee finality gives precedence.
-
     pendingBest*:
       Table[(SyncCommitteePeriod, Eth2Digest), altair.LightClientUpdate]
       ## Same as `bestUpdates`, but for `SyncCommitteePeriod` with not yet
@@ -72,6 +64,8 @@ type
       ## The earliest slot for which light client data is imported.
 
   LightClientDataConfig* = object
+    dbDir*: Option[string]
+      ## Directory to store light client data DB in
     serve*: bool
       ## Whether to make local light client data available or not
     importMode*: LightClientDataImportMode
@@ -89,6 +83,8 @@ type
 
     cache*: LightClientDataCache
       ## Cached data to accelerate creating light client data
+    db*: LightClientDataDB
+      ## Persistent light client data to avoid expensive recomputations
 
     # -----------------------------------
     # Config

beacon_chain/consensus_object_pools/blockchain_dag.nim

@@ -581,6 +581,7 @@ proc updateBeaconMetrics(
 import blockchain_dag_light_client
 
 export
+  blockchain_dag_light_client.closeLightClientDataStore,
   blockchain_dag_light_client.getLightClientBootstrap,
   blockchain_dag_light_client.getLightClientUpdateForPeriod,
   blockchain_dag_light_client.getLightClientFinalityUpdate,