Fix slasher database schema migration to v2 (#2125)

## Issue Addressed

Closes #2119

## Proposed Changes

Update the slasher schema version to v2 for the breaking changes to the config introduced in #2079. Implement a migration from v1 to v2 so that users can seamlessly upgrade from any version of Lighthouse <=1.0.5.

Users who deleted their database for v1.0.5 can upgrade to a release including this patch without any manual intervention. Similarly, any users still on v1.0.4 or earlier can now upgrade without having to drop their database.

slasher/src/migrate.rs

@@ -0,0 +1,74 @@
+use crate::{database::CURRENT_SCHEMA_VERSION, Config, Error, SlasherDB};
+use lmdb::RwTransaction;
+use serde_derive::{Deserialize, Serialize};
+use std::path::PathBuf;
+use types::EthSpec;
+
+/// Config from schema version 1, for migration to version 2+.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ConfigV1 {
+    database_path: PathBuf,
+    chunk_size: usize,
+    validator_chunk_size: usize,
+    history_length: usize,
+    update_period: u64,
+    max_db_size_mbs: usize,
+}
+
+type ConfigV2 = Config;
+
+impl Into<ConfigV2> for ConfigV1 {
+    fn into(self) -> ConfigV2 {
+        Config {
+            database_path: self.database_path,
+            chunk_size: self.chunk_size,
+            validator_chunk_size: self.validator_chunk_size,
+            history_length: self.history_length,
+            update_period: self.update_period,
+            max_db_size_mbs: self.max_db_size_mbs,
+            broadcast: false,
+        }
+    }
+}
+
+impl<E: EthSpec> SlasherDB<E> {
+    /// If the database exists, and has a schema, attempt to migrate it to the current version.
+    pub fn migrate(&self, txn: &mut RwTransaction<'_>) -> Result<(), Error> {
+        if let Some(schema_version) = self.load_schema_version(txn)? {
+            match (schema_version, CURRENT_SCHEMA_VERSION) {
+                // The migration from v1 to v2 is a bit messy because v1.0.5 silently
+                // changed the schema to v2, so a v1 schema could have either a v1 or v2
+                // config.
+                (1, 2) => {
+                    match self.load_config::<ConfigV1>(txn) {
+                        Ok(Some(config_v1)) => {
+                            // Upgrade to v2 config and store on disk.
+                            let config_v2 = config_v1.into();
+                            self.store_config(&config_v2, txn)?;
+                        }
+                        Ok(None) => {
+                            // Impossible to have schema version and no config.
+                            return Err(Error::ConfigMissing);
+                        }
+                        Err(_) => {
+                            // If loading v1 config failed, ensure loading v2 config succeeds.
+                            // No further action is needed.
+                            let _config_v2 = self.load_config::<ConfigV2>(txn)?;
+                        }
+                    }
+                }
+                (x, y) if x == y => {}
+                (_, _) => {
+                    return Err(Error::IncompatibleSchemaVersion {
+                        database_schema_version: schema_version,
+                        software_schema_version: CURRENT_SCHEMA_VERSION,
+                    });
+                }
+            }
+        }
+
+        // If the migration succeeded, update the schema version on-disk.
+        self.store_schema_version(txn)?;
+        Ok(())
+    }
+}