Fix format_volume panic in debug build for small FAT12 volumes

Author: rafalh

CHANGELOG.md

@@ -27,6 +27,7 @@ New features:
 * Fix time encoding during a leap second if using `chrono`.
 * Create directory entry with `VOLUME_ID` attribute when formatting if volume label was set in `FormatVolumeOptions`.
 * Fix creating directory entries when `lfn` feature is enabled and `alloc` feature is disabled
+* Fix `format_volume` function panicking in debug build for FAT12 volumes with size below 1 MB
 * Remove `byteorder` dependency.
 * Bump up minimal Rust compiler version to 1.46.0.
 * Build the crate using the 2018 edition.

src/boot_sector.rs

@@ -513,11 +513,12 @@ fn determine_bytes_per_cluster(total_bytes: u64, bytes_per_sector: u16, fat_type
             }
         }
     };
-    debug_assert!(bytes_per_cluster.is_power_of_two());
-    cmp::min(
+    let bytes_per_cluster_clamped = cmp::min(
         cmp::max(bytes_per_cluster, u32::from(bytes_per_sector)),
         MAX_CLUSTER_SIZE,
-    )
+    );
+    debug_assert!(bytes_per_cluster_clamped.is_power_of_two());
+    bytes_per_cluster_clamped
 }
 
 fn determine_sectors_per_fat(
@@ -817,6 +818,7 @@ mod tests {
 
     #[test]
     fn test_determine_bytes_per_cluster_fat12() {
+        assert_eq!(determine_bytes_per_cluster(128 * KB_64, 512, Some(FatType::Fat12)), 512);
         assert_eq!(determine_bytes_per_cluster(MB_64, 512, Some(FatType::Fat12)), 512);
         assert_eq!(determine_bytes_per_cluster(MB_64 + 1, 512, Some(FatType::Fat12)), 1024);
         assert_eq!(determine_bytes_per_cluster(MB_64, 4096, Some(FatType::Fat12)), 4096);