Add CLI flag to specify the format of logs written to the logfile (#3839)

## Proposed Changes

Decouple the stdout and logfile formats by adding the `--logfile-format` CLI flag.
This behaves identically to the existing `--log-format` flag, but instead will only affect the logs written to the logfile.
The `--log-format` flag will no longer have any effect on the contents of the logfile.

## Additional Info
This avoids being a breaking change by causing `logfile-format` to default to the value of `--log-format` if it is not provided. 
This means that users who were previously relying on being able to use a JSON formatted logfile will be able to continue to use `--log-format JSON`. 

Users who want to use JSON on stdout and default logs in the logfile, will need to pass the following flags: `--log-format JSON --logfile-format DEFAULT`

lcli/src/main.rs

@@ -827,6 +827,7 @@ fn run<T: EthSpec>(
             debug_level: String::from("trace"),
             logfile_debug_level: String::from("trace"),
             log_format: None,
+            logfile_format: None,
             log_color: false,
             disable_log_timestamp: false,
             max_log_size: 0,

lighthouse/environment/src/lib.rs

@@ -50,6 +50,7 @@ pub struct LoggerConfig {
     pub debug_level: String,
     pub logfile_debug_level: String,
     pub log_format: Option<String>,
+    pub logfile_format: Option<String>,
     pub log_color: bool,
     pub disable_log_timestamp: bool,
     pub max_log_size: u64,
@@ -64,6 +65,7 @@ impl Default for LoggerConfig {
             debug_level: String::from("info"),
             logfile_debug_level: String::from("debug"),
             log_format: None,
+            logfile_format: None,
             log_color: false,
             disable_log_timestamp: false,
             max_log_size: 200,
@@ -252,7 +254,7 @@ impl<E: EthSpec> EnvironmentBuilder<E> {
         let file_logger = FileLoggerBuilder::new(&path)
             .level(logfile_level)
             .channel_size(LOG_CHANNEL_SIZE)
-            .format(match config.log_format.as_deref() {
+            .format(match config.logfile_format.as_deref() {
                 Some("JSON") => Format::Json,
                 _ => Format::default(),
             })

lighthouse/src/main.rs

@@ -99,6 +99,15 @@ fn main() {
                 .default_value("debug")
                 .global(true),
         )
+        .arg(
+            Arg::with_name("logfile-format")
+                .long("logfile-format")
+                .value_name("FORMAT")
+                .help("Specifies the log format used when emitting logs to the logfile.")
+                .possible_values(&["DEFAULT", "JSON"])
+                .takes_value(true)
+                .global(true)
+        )
         .arg(
             Arg::with_name("logfile-max-size")
                 .long("logfile-max-size")
@@ -402,6 +411,11 @@ fn run<E: EthSpec>(
         .value_of("logfile-debug-level")
         .ok_or("Expected --logfile-debug-level flag")?;
 
+    let logfile_format = matches
+        .value_of("logfile-format")
+        // Ensure that `logfile-format` defaults to the value of `log-format`.
+        .or_else(|| matches.value_of("log-format"));
+
     let logfile_max_size: u64 = matches
         .value_of("logfile-max-size")
         .ok_or("Expected --logfile-max-size flag")?
@@ -452,6 +466,7 @@ fn run<E: EthSpec>(
         debug_level: String::from(debug_level),
         logfile_debug_level: String::from(logfile_debug_level),
         log_format: log_format.map(String::from),
+        logfile_format: logfile_format.map(String::from),
         log_color,
         disable_log_timestamp,
         max_log_size: logfile_max_size * 1_024 * 1_024,

lighthouse/tests/beacon_node.rs

@@ -1662,7 +1662,24 @@ fn logfile_no_restricted_perms_flag() {
             assert!(config.logger_config.is_restricted == false);
         });
 }
-
+#[test]
+fn logfile_format_default() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert_eq!(config.logger_config.logfile_format, None));
+}
+#[test]
+fn logfile_format_flag() {
+    CommandLineTest::new()
+        .flag("logfile-format", Some("JSON"))
+        .run_with_zero_port()
+        .with_config(|config| {
+            assert_eq!(
+                config.logger_config.logfile_format,
+                Some("JSON".to_string())
+            )
+        });
+}
 #[test]
 fn sync_eth1_chain_default() {
     CommandLineTest::new()

testing/simulator/src/eth1_sim.rs

@@ -62,6 +62,7 @@ pub fn run_eth1_sim(matches: &ArgMatches) -> Result<(), String> {
             debug_level: String::from("debug"),
             logfile_debug_level: String::from("debug"),
             log_format: None,
+            logfile_format: None,
             log_color: false,
             disable_log_timestamp: false,
             max_log_size: 0,

testing/simulator/src/no_eth1_sim.rs

@@ -47,6 +47,7 @@ pub fn run_no_eth1_sim(matches: &ArgMatches) -> Result<(), String> {
             debug_level: String::from("debug"),
             logfile_debug_level: String::from("debug"),
             log_format: None,
+            logfile_format: None,
             log_color: false,
             disable_log_timestamp: false,
             max_log_size: 0,

testing/simulator/src/sync_sim.rs

@@ -51,6 +51,7 @@ fn syncing_sim(
             debug_level: String::from(log_level),
             logfile_debug_level: String::from("debug"),
             log_format: log_format.map(String::from),
+            logfile_format: None,
             log_color: false,
             disable_log_timestamp: false,
             max_log_size: 0,