Implementing backup of broken config files

This addresses issue fxbox#390.

src/config_store.rs

@@ -95,7 +95,20 @@ impl ConfigStore {
             Err(error) => {
                 error!("Unable to generate JSON from config file {}: {}",
                     file_name, error.to_string());
-                    empty_config
+                let mut backup_name = file_name.to_owned();
+                backup_name.push_str("_BROKEN");
+                match fs::rename(file_name, &backup_name) {
+                    Ok(_) => {
+                        warn!("Moved broken config file to {}", backup_name);
+                    },
+                    Err(error) => {
+                        error!("Error while writing config backup to {}: {}",
+                            backup_name, error.to_string());
+                        panic!("Unrecoverable error: unable to backup broken config");
+                    }
+                }
+                warn!("Continuing with empty config file");
+                empty_config
             }
         };
 
@@ -270,5 +283,28 @@ describe! config {
                 assert_eq!(foo_bar, "baz");
             };
         }
+
+        it "ConfigService should back-up broken config files" {
+            use std::io::{ Read, Write };
+            use std::path::Path;
+            // Create broken config file
+            let broken_conf = "{\"foo\":\"bar\",}".to_owned();
+            let mut file = fs::File::create(&Path::new(&config_file_name)).ok().unwrap();
+            let _ = file.write_all(&broken_conf.as_bytes());
+            // Block to make `config` go out of scope
+            {
+                let config = ConfigService::new(&config_file_name);
+                config.set("foo", "bar", "baz");
+            }
+            // `config` should now be out of scope and dropped
+            {
+                let mut backup_name = config_file_name.clone();
+                backup_name.push_str("_BROKEN");
+                let mut file = fs::File::open(&Path::new(&backup_name)).ok().unwrap();
+                let mut moved_conf = String::new();
+                let _ = file.read_to_string(&mut moved_conf);
+                assert_eq!(moved_conf, broken_conf);
+            };
+        }
     }
 }