Unify logfile handling in environment crate.

lighthouse/environment/Cargo.toml

@@ -17,3 +17,4 @@ slog-async = "^2.3.0"
 ctrlc = { version = "3.1.1", features = ["termination"] }
 futures = "0.1.25"
 parking_lot = "0.7"
+slog-json = "2.3.0"

lighthouse/environment/src/lib.rs

@@ -9,9 +9,12 @@
 
 use eth2_config::Eth2Config;
 use futures::{sync::oneshot, Future};
-use slog::{o, Drain, Level, Logger};
+use slog::{info, o, Drain, Level, Logger};
 use sloggers::{null::NullLoggerBuilder, Build};
 use std::cell::RefCell;
+use std::fs::OpenOptions;
+use std::path::PathBuf;
+use std::sync::Mutex;
 use tokio::runtime::{Builder as RuntimeBuilder, Runtime, TaskExecutor};
 use types::{EthSpec, InteropEthSpec, MainnetEthSpec, MinimalEthSpec};
 
@@ -224,6 +227,28 @@ impl<E: EthSpec> Environment<E> {
             .map_err(|e| format!("Tokio runtime shutdown returned an error: {:?}", e))
     }
 
+    /// Sets the logger (and all child loggers) to log to a file.
+    pub fn log_to_json_file(&mut self, path: PathBuf) -> Result<(), String> {
+        let file = OpenOptions::new()
+            .create(true)
+            .write(true)
+            .truncate(true)
+            .open(&path)
+            .map_err(|e| format!("Unable to open logfile: {:?}", e))?;
+
+        let drain = Mutex::new(slog_json::Json::default(file)).fuse();
+        let drain = slog_async::Async::new(drain).build().fuse();
+        self.log = slog::Logger::root(drain, o!());
+
+        info!(
+            self.log,
+            "Logging to JSON file";
+            "path" => format!("{:?}", path)
+        );
+
+        Ok(())
+    }
+
     pub fn eth_spec_instance(&self) -> &E {
         &self.eth_spec_instance
     }

lighthouse/src/main.rs

@@ -6,6 +6,7 @@ use clap::{App, Arg, ArgMatches};
 use env_logger::{Builder, Env};
 use environment::EnvironmentBuilder;
 use slog::{crit, info, warn};
+use std::path::PathBuf;
 use std::process::exit;
 use types::EthSpec;
 use validator_client::ProductionValidatorClient;
@@ -94,6 +95,13 @@ fn run<E: EthSpec>(
 
     let log = environment.core_context().log;
 
+    if let Some(log_path) = matches.value_of("logfile") {
+        let path = log_path
+            .parse::<PathBuf>()
+            .map_err(|e| format!("Failed to parse log path: {:?}", e))?;
+        environment.log_to_json_file(path)?;
+    }
+
     if std::mem::size_of::<usize>() != 8 {
         crit!(
             log,