[yuxi-liu-wired]

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Problem

find_central_directory_end checks that the central directory starts before the EOCD, but it
returns early for an archive with zero entries, before that check. So a 22-byte EOCD that says
"0 entries, central directory at 0x7fffffff" opens as a valid empty archive, with
central_directory_start() = 2147483647 in a 22-byte file.

ZipWriter::new_append then takes that offset as the place to write the next entry:

// 22-byte EOCD: 0 entries, cd size 0, cd offset 0x7fffffff
let mut w = ZipWriter::new_append(file)?;
w.start_file("hello.txt", SimpleFileOptions::default())?;
w.write_all(b"hello")?;
w.finish()?; // Ok

finish returns Ok, and the file is now 2,147,483,770 bytes, with the new local header at
0x7fffffff. With offset 0xffffff00 it is 4 GiB. On ext4 the file is sparse, so it takes only a few
KiB on disk, but anything that copies or uploads it moves 2–4 GiB. With a Cursor<Vec<u8>>,
new_append + finish tries to allocate the whole 2–4 GiB. Any program that appends to an
archive it got from somewhere else is affected.

Info-ZIP unzip rejects such a file. The ZIP64 path already rejects the same shape, because it
checks the file count against the EOCD64 position.

Change

In the zero-entry branch, apply the same consistency check as for non-empty archives: the
central directory offset must not be past the EOCD. An offset equal to the EOCD position is still
accepted, because that is exactly where an empty central directory sits (APPNOTE 4.4.24: the
offset is where the central directory starts; with no entries it ends where it starts, right
before the EOCD). On failure the search goes on to the next EOCD candidate, as for the other
checks.

Test

tests/empty_archive_cd_offset.rs:

• empty_archive_with_cd_offset_past_eocd_is_rejected: offsets 23, 0x7fffffff and 0xffffff00 in
  a 22-byte EOCD. Both ZipArchive::new and ZipWriter::new_append must fail. Fails on master
  (offset 23 is accepted).
• empty_archive_still_opens_and_appends: the normal empty archive (offset 0), with and without a
  prepended prefix, still opens, takes an appended entry, and the result is small and reads back.

The suite passes on 1.88, stable and nightly with default features, --no-default-features,
--all-features, and the two single-deflate feature sets; cargo clippy --all-targets -- -D warnings and cargo doc --no-deps are clean with default, --no-default-features and
--all-features; cargo fmt --check --all is clean.

Found with a fuzzer that appends to and merges generated archives and checks that existing
entries keep their bytes.