Persist logs by renaming table (hopefully reduces lock time)

main/DB/DBAppOnly.swift

@@ -33,11 +33,9 @@ extension SQLiteDatabase {
 		do {
 			try run(sql: "SELECT 1 FROM req LIMIT 1;") // fails if req doesnt exist
 			createFunction("domainof") { ($0.first as! String).extractDomain() }
-			try run(sql: """
-				BEGIN TRANSACTION;
+			transaction("""
 				INSERT INTO heap(ts,fqdn,domain,opt) SELECT ts,domain,domainof(domain),nullif(logOpt,0) FROM req;
 				DROP TABLE req;
-				COMMIT;
 				""")
 		} catch { /* no need to migrate */ }
 	}
@@ -119,13 +117,12 @@ extension SQLiteDatabase {
 	/// - Returns: `nil`  in case no entries were transmitted.
 	@discardableResult func dnsLogsPersist() -> SQLiteRowRange? {
 		guard lastRowId(.cache) > 0 else { return nil }
+		transaction("ALTER TABLE cache RENAME TO tmp_cache; \(CreateTable.cache)")
 		let before = lastRowId(.heap) + 1
 		createFunction("domainof") { ($0.first as! String).extractDomain() }
-		try? run(sql:"""
-			BEGIN TRANSACTION;
-			INSERT INTO heap(ts,fqdn,domain,opt) SELECT ts,dns,domainof(dns),nullif(opt&1,0) FROM cache;
-			DELETE FROM cache;
-			COMMIT;
+		transaction("""
+			INSERT INTO heap(ts,fqdn,domain,opt) SELECT ts,dns,domainof(dns),nullif(opt&1,0) FROM tmp_cache;
+			DROP TABLE tmp_cache;
 			""")
 		let after = lastRowId(.heap)
 		return (before > after) ? nil : (before, after)

main/DB/DBCore.swift

@@ -91,15 +91,20 @@ class SQLiteDatabase {
 		}
 	}
 	
+	/// `BEGIN TRANSACTION; \(sql); COMMIT;` on exception rollback.
+	func transaction(_ sql: String) {
+		do { try run(sql: "BEGIN TRANSACTION; \(sql); COMMIT;") }
+		catch { rollback() }
+	}
+	
 	func ifStep(_ stmt: OpaquePointer, _ expected: Int32) throws {
 		guard sqlite3_step(stmt) == expected else {
 			throw SQLiteError.Step(message: errorMessage)
 		}
 	}
 	
-	func vacuum() {
-		try? run(sql: "VACUUM;")
-	}
+	func vacuum() { NSLog("[SQL] VACUUM"); try? run(sql: "VACUUM;"); }
+	func rollback() { NSLog("[SQL] ROLLBACK"); try? run(sql: "ROLLBACK;"); }
 }