Backup old map.

src/io.rs

@@ -1,7 +1,7 @@
 use std::fs::{File, OpenOptions};
 use std::io::{self, Seek, SeekFrom};
-use std::ops::{Index, Range, RangeFrom};
-use std::slice::SliceIndex;
+use std::ops::Index;
+use std::path::Path;
 
 use crate::cli::CONFIG;
 
@@ -66,10 +66,21 @@ pub fn load_map_read() -> std::io::Result<File> {
 }
 
 pub fn load_map_write() -> anyhow::Result<File> {
+    // Attempt to check if a map exists on the disk.
+    // If so, make a backup of it.
+    //
+    // This should be recoverable by just skipping over this error and logging a warning,
+    // but for now it will be an error condition.
+    if std::fs::exists(crate::path::MAP_PATH.clone())
+        .context("Could not check if map exists in fs to make a backup.")?
+    {
+        backup(crate::path::MAP_PATH.clone())?;
+    }
+
     OpenOptions::new()
         .write(true)
         .create(true)
-        .truncate(true) // Wipe old map. Should really make a backup first.
+        .truncate(true) // Wipe old map, in case we skip over backing up the old one.
         .open(crate::path::MAP_PATH.clone())
         .with_context(|| {
             format!(
@@ -79,6 +90,13 @@ pub fn load_map_write() -> anyhow::Result<File> {
         })
 }
 
+fn backup<P: AsRef<Path>>(path: P) -> std::io::Result<()> {
+    std::fs::rename(
+        &path,
+        format!("{}.bak", path.as_ref().to_path_buf().display()),
+    )
+}
+
 #[repr(C, align(512))]
 pub struct DirectIOBuffer(pub [u8; crate::MAX_BUFFER_SIZE]);