Add a regression test for empty config files

See the previous commit for a reason why!

Also, no longer create an empty config file by default. Initially I
thought this was a good idea so that if users wanted to customize the
file the name would be obvious (since the file existed). However, the
name isn't obvious because you need to know the directory anyways (e.g.
I _just_ had to look this up on my Windows machine). So it feels better
to not make empty files for no real reason. There are probably other
ways of self-describing config (e.g. render the location in --help).

Cargo.toml

@@ -29,6 +29,9 @@ pest = "2.1.3"
 pest_derive = "2.1.0"
 tracing = { version = "0.1.25", features = ["log"] }
 
+[dev-dependencies]
+tempdir = "0.3"
+
 [[bin]]
 name = "qldb"
 path = "src/main.rs"

src/settings.rs

@@ -4,10 +4,7 @@ use pest::Parser;
 use pest_derive::Parser;
 use serde::{Deserialize, Serialize};
 use std::path::Path;
-use std::{
-    collections::HashMap,
-    fs::{self, File},
-};
+use std::{collections::HashMap, fs};
 use thiserror::Error;
 use toml;
 
@@ -158,9 +155,10 @@ impl Config {
         fs::create_dir_all(&shell_dir)?;
         let config_file = shell_dir.join("default_config.toml");
         if !config_file.exists() {
-            File::create(&config_file)?;
+            Ok(Config::default())
+        } else {
+            Config::load(&config_file)
         }
-        Config::load(&config_file)
     }
 }
 
@@ -284,3 +282,21 @@ impl FromStr for ExecuteStatementOpt {
         })
     }
 }
+
+#[cfg(test)]
+mod settings_tests {
+    use super::*;
+    use fs::File;
+    use tempdir::TempDir;
+
+    /// Tests that an empty config is valid. This makes sure we don't forget an
+    /// `Optional` in any new fields we add.
+    #[test]
+    fn load_empty_config() -> Result<()> {
+        let tmp = TempDir::new("config")?;
+        let path = tmp.path().join("empty.toml");
+        File::create(&path)?;
+        let _ = Config::load(&path)?;
+        Ok(())
+    }
+}