fix: Make backup boot-sector part of reserved region when formatting volume

Change number of reserved sectors to 8.
Previously backup boot-sector was over-written by FAT formatting code soon
after it was initialized.

src/boot_sector.rs

@@ -513,8 +513,9 @@ fn format_bpb(options: &FormatVolumeOptions) -> io::Result<(BiosParameterBlock,
     let sectors_per_cluster = (bytes_per_cluster / bytes_per_sector as u32) as u8;
 
     // Note: most of implementations use 32 reserved sectors for FAT32 but it's wasting of space
-    // We use 4 because there are two boot sectors and one FS Info sector (1 sector remains unused)
-    let reserved_sectors: u16 = if fat_type == FatType::Fat32 { 4 } else { 1 };
+    // This implementation uses only 8. This is enough to fit in two boot sectors (main and backup) with additional
+    // bootstrap code and one FSInfo sector. It also makes FAT alligned to 4096 which is a nice number.
+    let reserved_sectors: u16 = if fat_type == FatType::Fat32 { 8 } else { 1 };
 
     let fats = 2u8;
     let is_fat32 = fat_type == FatType::Fat32;

src/fs.rs

@@ -913,6 +913,7 @@ pub fn format_volume<T: ReadWriteSeek>(mut disk: T, options: FormatVolumeOptions
         write_zeros_until_end_of_sector(&mut disk, bytes_per_sector)?;
 
         // backup boot sector
+        debug_assert!(boot.bpb.backup_boot_sector() < boot.bpb.reserved_sectors());
         disk.seek(SeekFrom::Start(boot.bpb.bytes_from_sectors(boot.bpb.backup_boot_sector())))?;
         boot.serialize(&mut disk)?;
         write_zeros_until_end_of_sector(&mut disk, bytes_per_sector)?;

tests/format.rs

@@ -14,6 +14,14 @@ const TEST_STR: &str = "Hi there Rust programmer!\n";
 type FileSystem = fatfs::FileSystem<BufStream<io::Cursor<Vec<u8>>>>;
 
 fn basic_fs_test(fs: &FileSystem) {
+    let stats = fs.stats().expect("stats");
+    if fs.fat_type() == fatfs::FatType::Fat32 {
+        // On FAT32 one cluster is allocated for root directory
+        assert_eq!(stats.total_clusters(), stats.free_clusters() + 1);
+    } else {
+        assert_eq!(stats.total_clusters(), stats.free_clusters());
+    }
+
     let root_dir = fs.root_dir();
     let entries = root_dir.iter().map(|r| r.unwrap()).collect::<Vec<_>>();
     assert_eq!(entries.len(), 0);