Fix panic when a non-zip64 local header is written after 0xFFFFFFFF

The zip spec allows non-zip64 local headers to be written at offset
>=2^32 as long as the corresponding central directory entry has a zip64
extra field for the local header offset. However, when writing a
non-zip64 small file after the 2^32 boundary, `local_header_size()`
would return a zip64 size while writing a non-zip64 local header to
disk. This would cause `self.stats.start` to get out of sync, leading to
a panic in `finish_file()` when doing a checked subtraction for
`self.compressed_size`. This would only manifest when performing
streaming writes.

This commit fixes the problem by updating `local_header_size()` to use a
new `zip64_sizes()` method that only considers the compressed and
uncompressed size fields since only those two matter for determining the
size of a local header.

Signed-off-by: Andrew Gunnerson <[email protected]>

src/types.rs

@@ -422,9 +422,13 @@ impl ZipFileData {
         }
     }
 
-    pub fn zip64_extension(&self) -> bool {
+    pub(crate) fn zip64_sizes(&self) -> bool {
         self.uncompressed_size > 0xFFFFFFFF
             || self.compressed_size > 0xFFFFFFFF
+    }
+
+    pub fn zip64_extension(&self) -> bool {
+        self.zip64_sizes()
             || self.header_start > 0xFFFFFFFF
     }
 
@@ -449,7 +453,9 @@ impl ZipFileData {
         } else {
             size += self.file_name_raw.len() as u64;
         };
-        if self.zip64_extension() || self.large_file {
+        // The local header does not necessarily need to be zip64 when its
+        // offset exceeds the 2^32 boundary.
+        if self.zip64_sizes() || self.large_file {
             size += 20;
         }
         size