Add open method for legacy savefile, including shitty error handling

backend/src/api/handler.rs

@@ -23,7 +23,7 @@ pub enum ApiMessage {
     OnChargeChange(f64), // battery fill amount: 0 = empty, 1 = full
     PowerVibeCheck,
     WaitForEmptyQueue(Callback<()>),
-    LoadSettings(u64, String, u64, String), // (path, name, variant, variant name)
+    LoadSettings(Option<(u64, String)>, u64, String, u64, String), // ((legacy game_id, legacy name), path, name, variant, variant name)
     LoadVariant(u64, String), // (variant, variant name) -- path and name assumed to be for current profile
     LoadMainSettings,
     LoadSystemSettings,
@@ -504,7 +504,7 @@ impl ApiMessageHandler {
                 self.on_empty.push(callback);
                 false
             }
-            ApiMessage::LoadSettings(id, name, variant_id, variant_name) => {
+            ApiMessage::LoadSettings(legacy_settings, id, name, variant_id, variant_name) => {
                 /* Migration steps:
                 1. Modify to the frontend to send the game ID in this message (`id` here is the app
                    ID).
@@ -514,6 +514,21 @@ impl ApiMessageHandler {
                    save in the new format yet.
                 4. Let the rest of the code below work its magic.
                 */
+
+                // If `legacy_settings` is `Some`, there is a legacy savefile present.
+                if let Some((legacy_game_id, legacy_name)) = legacy_settings {
+                    // Create path to the legacy settings file.
+                    let legacy_file_path = format!("{}.json", legacy_game_id);
+
+                    // Load the file from disk and deserialize it.
+                    let file = crate::persist::OldSettingsJson::open(legacy_file_path);
+
+                    // If a file in the modern format exists, add it as a new variant to
+                    // `FileJson` using `FileJson::merge_legacy_settings()`. Else, create a new
+                    // FileJson with this as the only variant, using
+                    // `FileJson::from<OldSettingsJson>()`.
+                }
+
                 let path = format!("{}.ron", id);
                 if let Err(e) = settings.on_unload() {
                     print_errors("LoadSettings on_unload()", e);

backend/src/persist/error.rs

@@ -36,6 +36,14 @@ impl From<ron::error::Error> for RonError {
     }
 }
 
+impl From<serde_json::Error> for RonError {
+    fn from(_value: serde_json::Error) -> Self {
+        RonError::General(ron::Error::Message(String::from(
+            "TODO: make error handling in migration logic good. Specifically, make it so a json error can be used as a RonError instead of just returning this string",
+        )))
+    }
+}
+
 impl From<ron::error::SpannedError> for RonError {
     fn from(value: ron::error::SpannedError) -> Self {
         Self::Spanned(value)

backend/src/persist/migration/settings.rs

@@ -2,7 +2,9 @@ use std::collections::HashMap;
 
 use serde::{Deserialize, Serialize};
 
-use crate::persist::{BatteryJson, CpuJson, DriverJson, FileJson, SettingsJson, LATEST_VERSION};
+use crate::persist::{
+    BatteryJson, CpuJson, DriverJson, FileJson, SerdeError, SettingsJson, LATEST_VERSION,
+};
 
 use super::gpu::OldGpuJson;
 
@@ -56,3 +58,10 @@ impl From<OldSettingsJson> for FileJson {
         }
     }
 }
+
+impl OldSettingsJson {
+    pub fn open<P: AsRef<std::path::Path>>(path: P) -> Result<Self, SerdeError> {
+        let mut file = std::fs::File::open(path).map_err(SerdeError::Io)?;
+        serde_json::from_reader(&mut file).map_err(|e| SerdeError::Serde(e.into()))
+    }
+}