fix: don't overwrite config file

So far we have written a default config file if we encountered any error
when trying to read the one that should exist. That included:
* missing key-value-pairs,
* empty file, and
* a non-existant file.

Therefore, when a field is added to the Config struct, the entire config
file was overwritten with a default config. This occured during #20.

Instead, we now provide default values in form of generator functions to
serde. Any value that is present in the config is then parsed and
replaces the default value. Only if the config file does not exist we
create an empty one.

Closes #26.

crates/main/main.rs

@@ -18,10 +18,10 @@ Main program for `pacdef`. All internal logic happens in [`pacdef_core`].
 use std::path::Path;
 use std::process::{ExitCode, Termination};
 
-use anyhow::{Context, Result};
+use anyhow::{bail, Context, Result};
 
 use pacdef_core::path::{get_config_path, get_config_path_old_version, get_group_dir};
-use pacdef_core::{get_args, Config, Group, Pacdef};
+use pacdef_core::{get_args, Config, Error as PacdefError, Group, Pacdef};
 
 const MAJOR_UPDATE_MESSAGE: &str = "VERSION UPGRADE
 You seem to have used version 0.x of pacdef before.
@@ -41,7 +41,7 @@ fn handle_final_result(result: Result<()>) -> ExitCode {
     match result {
         Ok(_) => ExitCode::SUCCESS,
         Err(ref e) => {
-            if let Some(root_error) = e.root_cause().downcast_ref::<pacdef_core::Error>() {
+            if let Some(root_error) = e.root_cause().downcast_ref::<PacdefError>() {
                 eprintln!("{root_error}");
                 ExitCode::FAILURE
             } else {
@@ -56,9 +56,23 @@ fn main_inner() -> Result<()> {
 
     let config_file = get_config_path().context("getting config file")?;
 
-    let config = Config::load(&config_file)
-        .or_else(|_| load_default_config(&config_file))
-        .context("loading config")?;
+    let config = match Config::load(&config_file).context("loading config file") {
+        Ok(config) => config,
+        Err(e) => {
+            if let Some(crate_error) = e.downcast_ref::<PacdefError>() {
+                match crate_error {
+                    PacdefError::ConfigFileNotFound => load_default_config(&config_file)?,
+                    _ => bail!("unexpected error: {crate_error}"),
+                }
+            } else {
+                bail!("unexpected error: {e:?}");
+            }
+        }
+    };
+
+    dbg!(&config);
+    // .or_else(|_| load_default_config(&config_file))
+    // .context("loading config")?;
 
     let group_dir = get_group_dir().context("resolving group dir")?;
     let groups = Group::load(&group_dir, config.warn_not_symlinks)
@@ -73,8 +87,14 @@ fn load_default_config(config_file: &Path) -> Result<Config> {
         println!("{MAJOR_UPDATE_MESSAGE}");
     }
 
-    let default = Config::default();
-    default.save(config_file)?;
+    if !config_file.exists() {
+        create_empty_config_file(config_file)?;
+    }
 
-    Ok(default)
+    Ok(Config::default())
+}
+
+fn create_empty_config_file(config_file: &Path) -> Result<()> {
+    std::fs::File::create(config_file).context("creating empty config file")?;
+    Ok(())
 }