Improve freezer DB efficiency with periodic restore points (#649)

* Draft of checkpoint freezer DB

* Fix bugs

* Adjust root iterators for checkpoint database

* Fix freezer state lookups with no slot hint

* Fix split comment

* Use "restore point" to refer to frozen states

* Resolve some FIXMEs

* Configurable slots per restore point

* Document new freezer DB functions

* Fix up StoreConfig

* Fix new test for merge

* Document SPRP default CLI flag, clarify tests

beacon_node/src/cli.rs

@@ -1,4 +1,5 @@
 use clap::{App, Arg, SubCommand};
+use store::StoreConfig;
 
 pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
     App::new("beacon_node")
@@ -188,6 +189,20 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .takes_value(true)
                 .default_value("http://localhost:8545")
         )
+        .arg(
+            Arg::with_name("slots-per-restore-point")
+                .long("slots-per-restore-point")
+                .value_name("SLOT_COUNT")
+                .help("Specifies how often a freezer DB restore point should be stored. \
+                       DO NOT CHANGE AFTER INITIALIZATION.")
+                .takes_value(true)
+                .default_value(
+                    Box::leak(
+                        format!("{}", StoreConfig::default().slots_per_restore_point)
+                            .into_boxed_str()
+                    )
+                )
+        )
         /*
          * The "testnet" sub-command.
          *

beacon_node/src/config.rs

@@ -233,7 +233,13 @@ pub fn get_configs<E: EthSpec>(
     };
 
     if let Some(freezer_dir) = cli_args.value_of("freezer-dir") {
-        client_config.freezer_db_path = Some(PathBuf::from(freezer_dir));
+        client_config.store.freezer_db_path = Some(PathBuf::from(freezer_dir));
+    }
+
+    if let Some(slots_per_restore_point) = cli_args.value_of("slots-per-restore-point") {
+        client_config.store.slots_per_restore_point = slots_per_restore_point
+            .parse()
+            .map_err(|_| "slots-per-restore-point is not a valid integer".to_string())?;
     }
 
     if eth2_config.spec_constants != client_config.spec_constants {

beacon_node/src/lib.rs

@@ -79,6 +79,7 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
         let spec = context.eth2_config().spec.clone();
         let genesis_eth1_config = client_config.eth1.clone();
         let client_genesis = client_config.genesis.clone();
+        let store_config = client_config.store.clone();
         let log = context.log.clone();
 
         let db_path_res = client_config.create_db_path();
@@ -90,7 +91,11 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
                 Ok(ClientBuilder::new(context.eth_spec_instance.clone())
                     .runtime_context(context)
                     .chain_spec(spec)
-                    .disk_store(&db_path, &freezer_db_path_res?)?
+                    .disk_store(
+                        &db_path,
+                        &freezer_db_path_res?,
+                        store_config.slots_per_restore_point,
+                    )?
                     .background_migrator()?)
             })
             .and_then(move |builder| {