Create backup instead of overwriting log files (#686)

* Create backup instead of overwriting log files

* Clean variable names

* Return string instead of expect on SystemTimeError

* Cargo fmt after merge

lighthouse/environment/src/lib.rs

@@ -12,9 +12,11 @@ use futures::{sync::oneshot, Future};
 use slog::{info, o, Drain, Level, Logger};
 use sloggers::{null::NullLoggerBuilder, Build};
 use std::cell::RefCell;
-use std::fs::OpenOptions;
+use std::ffi::OsStr;
+use std::fs::{rename as FsRename, OpenOptions};
 use std::path::PathBuf;
 use std::sync::Mutex;
+use std::time::{SystemTime, UNIX_EPOCH};
 use tokio::runtime::{Builder as RuntimeBuilder, Runtime, TaskExecutor};
 use types::{EthSpec, InteropEthSpec, MainnetEthSpec, MinimalEthSpec};
 
@@ -229,6 +231,24 @@ impl<E: EthSpec> Environment<E> {
 
     /// Sets the logger (and all child loggers) to log to a file.
     pub fn log_to_json_file(&mut self, path: PathBuf, debug_level: &str) -> Result<(), String> {
+        // Creating a backup if the logfile already exists.
+        if path.exists() {
+            let start = SystemTime::now();
+            let timestamp = start
+                .duration_since(UNIX_EPOCH)
+                .map_err(|e| e.to_string())?
+                .as_secs();
+            let file_stem = path
+                .file_stem()
+                .ok_or_else(|| "Invalid file name".to_string())?
+                .to_str()
+                .ok_or_else(|| "Failed to create str from filename".to_string())?;
+            let file_ext = path.extension().unwrap_or_else(|| OsStr::new(""));
+            let backup_name = format!("{}_backup_{}", file_stem, timestamp);
+            let backup_path = path.with_file_name(backup_name).with_extension(file_ext);
+            FsRename(&path, &backup_path).map_err(|e| e.to_string())?;
+        }
+
         let file = OpenOptions::new()
             .create(true)
             .write(true)