feat(NIP-26): allow searches for delegated public keys

Implements core NIP-26 delegated event functionality.  Events can
include a `delegation` tag that provides a signature and restrictions
on which events can be delegated.

Notable points on the implementation so far:

* Schema has been upgraded to include an index and new column.
* Basic rune parsing/evaluation to implement the example event in the
  NIP, but no more.
* No special logic for deletion.
* No migration logic for determining delegated authors for
  already-stored events.

src/schema.rs

@@ -20,7 +20,7 @@ pragma mmap_size = 536870912; -- 512MB of mmap
 "##;
 
 /// Latest database version
-pub const DB_VERSION: usize = 6;
+pub const DB_VERSION: usize = 7;
 
 /// Schema definition
 const INIT_SQL: &str = formatcp!(
@@ -40,6 +40,7 @@ event_hash BLOB NOT NULL, -- 4-byte hash
 first_seen INTEGER NOT NULL, -- when the event was first seen (not authored!) (seconds since 1970)
 created_at INTEGER NOT NULL, -- when the event was authored
 author BLOB NOT NULL, -- author pubkey
+delegated_by BLOB, -- delegator pubkey (NIP-26)
 kind INTEGER NOT NULL, -- event kind
 hidden INTEGER, -- relevant for queries
 content TEXT NOT NULL -- serialized json of event object
@@ -49,6 +50,7 @@ content TEXT NOT NULL -- serialized json of event object
 CREATE UNIQUE INDEX IF NOT EXISTS event_hash_index ON event(event_hash);
 CREATE INDEX IF NOT EXISTS created_at_index ON event(created_at);
 CREATE INDEX IF NOT EXISTS author_index ON event(author);
+CREATE INDEX IF NOT EXISTS delegated_by_index ON event(delegated_by);
 CREATE INDEX IF NOT EXISTS kind_index ON event(kind);
 
 -- Tag Table
@@ -152,6 +154,9 @@ pub fn upgrade_db(conn: &mut PooledConnection) -> Result<()> {
             if curr_version == 5 {
                 curr_version = mig_5_to_6(conn)?;
             }
+            if curr_version == 6 {
+                curr_version = mig_6_to_7(conn)?;
+            }
             if curr_version == DB_VERSION {
                 info!(
                     "All migration scripts completed successfully.  Welcome to v{}.",
@@ -348,3 +353,23 @@ fn mig_5_to_6(conn: &mut PooledConnection) -> Result<usize> {
     info!("vacuumed DB after tags rebuild in {:?}", start.elapsed());
     Ok(6)
 }
+
+fn mig_6_to_7(conn: &mut PooledConnection) -> Result<usize> {
+    info!("database schema needs update from 6->7");
+    // only change is adding a hidden column to events.
+    let upgrade_sql = r##"
+ALTER TABLE event ADD delegated_by BLOB;
+CREATE INDEX IF NOT EXISTS delegated_by_index ON event(delegated_by);
+PRAGMA user_version = 7;
+"##;
+    match conn.execute_batch(upgrade_sql) {
+        Ok(()) => {
+            info!("database schema upgraded v6 -> v7");
+        }
+        Err(err) => {
+            error!("update failed: {}", err);
+            panic!("database could not be upgraded");
+        }
+    }
+    Ok(7)
+}