GH-128131: Completely support random read access of uncompressed unencrypted files in ZipFile (#128143)
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
diff --git a/Lib/test/test_zipfile/test_core.py b/Lib/test/test_zipfile/test_core.py
index 79e7337..02e28d4 100644
--- a/Lib/test/test_zipfile/test_core.py
+++ b/Lib/test/test_zipfile/test_core.py
@@ -1,3 +1,4 @@
+import _pyio
import array
import contextlib
import importlib.util
@@ -3491,5 +3492,87 @@ def test_too_short(self):
b"zzz", zipfile._Extra.strip(b"zzz", (self.ZIP64_EXTRA,)))
+class StatIO(_pyio.BytesIO):
+ """Buffer which remembers the number of bytes that were read."""
+
+ def __init__(self):
+ super().__init__()
+ self.bytes_read = 0
+
+ def read(self, size=-1):
+ bs = super().read(size)
+ self.bytes_read += len(bs)
+ return bs
+
+
+class StoredZipExtFileRandomReadTest(unittest.TestCase):
+ """Tests whether an uncompressed, unencrypted zip entry can be randomly
+ seek and read without reading redundant bytes."""
+ def test_stored_seek_and_read(self):
+
+ sio = StatIO()
+ # 20000 bytes
+ txt = b'0123456789' * 2000
+
+ # The seek length must be greater than ZipExtFile.MIN_READ_SIZE
+ # as `ZipExtFile._read2()` reads in blocks of this size and we
+ # need to seek out of the buffered data
+ read_buffer_size = zipfile.ZipExtFile.MIN_READ_SIZE
+ self.assertGreaterEqual(10002, read_buffer_size) # for forward seek test
+ self.assertGreaterEqual(5003, read_buffer_size) # for backward seek test
+ # The read length must be less than MIN_READ_SIZE, since we assume that
+ # only 1 block is read in the test.
+ read_length = 100
+ self.assertGreaterEqual(read_buffer_size, read_length) # for read() calls
+
+ with zipfile.ZipFile(sio, "w", compression=zipfile.ZIP_STORED) as zipf:
+ zipf.writestr("foo.txt", txt)
+
+ # check random seek and read on a file
+ with zipfile.ZipFile(sio, "r") as zipf:
+ with zipf.open("foo.txt", "r") as fp:
+ # Test this optimized read hasn't rewound and read from the
+ # start of the file (as in the case of the unoptimized path)
+
+ # forward seek
+ old_count = sio.bytes_read
+ forward_seek_len = 10002
+ current_pos = 0
+ fp.seek(forward_seek_len, os.SEEK_CUR)
+ current_pos += forward_seek_len
+ self.assertEqual(fp.tell(), current_pos)
+ self.assertEqual(fp._left, fp._compress_left)
+ arr = fp.read(read_length)
+ current_pos += read_length
+ self.assertEqual(fp.tell(), current_pos)
+ self.assertEqual(arr, txt[current_pos - read_length:current_pos])
+ self.assertEqual(fp._left, fp._compress_left)
+ read_count = sio.bytes_read - old_count
+ self.assertLessEqual(read_count, read_buffer_size)
+
+ # backward seek
+ old_count = sio.bytes_read
+ backward_seek_len = 5003
+ fp.seek(-backward_seek_len, os.SEEK_CUR)
+ current_pos -= backward_seek_len
+ self.assertEqual(fp.tell(), current_pos)
+ self.assertEqual(fp._left, fp._compress_left)
+ arr = fp.read(read_length)
+ current_pos += read_length
+ self.assertEqual(fp.tell(), current_pos)
+ self.assertEqual(arr, txt[current_pos - read_length:current_pos])
+ self.assertEqual(fp._left, fp._compress_left)
+ read_count = sio.bytes_read - old_count
+ self.assertLessEqual(read_count, read_buffer_size)
+
+ # eof flags test
+ fp.seek(0, os.SEEK_END)
+ fp.seek(12345, os.SEEK_SET)
+ current_pos = 12345
+ arr = fp.read(read_length)
+ current_pos += read_length
+ self.assertEqual(arr, txt[current_pos - read_length:current_pos])
+
+
if __name__ == "__main__":
unittest.main()
diff --git a/Lib/zipfile/__init__.py b/Lib/zipfile/__init__.py
index 052ef47..24531c1 100644
--- a/Lib/zipfile/__init__.py
+++ b/Lib/zipfile/__init__.py
@@ -1184,13 +1184,15 @@ def seek(self, offset, whence=os.SEEK_SET):
self._offset = buff_offset
read_offset = 0
# Fast seek uncompressed unencrypted file
- elif self._compress_type == ZIP_STORED and self._decrypter is None and read_offset > 0:
+ elif self._compress_type == ZIP_STORED and self._decrypter is None and read_offset != 0:
# disable CRC checking after first seeking - it would be invalid
self._expected_crc = None
# seek actual file taking already buffered data into account
read_offset -= len(self._readbuffer) - self._offset
self._fileobj.seek(read_offset, os.SEEK_CUR)
self._left -= read_offset
+ self._compress_left -= read_offset
+ self._eof = self._left <= 0
read_offset = 0
# flush read buffer
self._readbuffer = b''
diff --git a/Misc/NEWS.d/next/Library/2024-12-21-03-20-12.gh-issue-128131.QpPmNt.rst b/Misc/NEWS.d/next/Library/2024-12-21-03-20-12.gh-issue-128131.QpPmNt.rst
new file mode 100644
index 0000000..f4c4ebc
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-12-21-03-20-12.gh-issue-128131.QpPmNt.rst
@@ -0,0 +1,2 @@
+Completely support random access of uncompressed unencrypted read-only
+zip files obtained by :meth:`ZipFile.open <zipfile.ZipFile.open>`.
