Discard compressed input buffer on backward gzip seek

CHANGELOG.rst

@@ -11,6 +11,8 @@ Changelog
 version 1.8.1-dev
 -----------------
 + Restore PyPy wheel builds.
++ Fix backward seeking in igzip files by discarding buffered compressed data
+  when rewinding the underlying file.
 
 version 1.8.0
 -----------------

src/isal/isal_zlibmodule.c

@@ -1918,6 +1918,10 @@ GzipReader_seek(GzipReader *self, PyObject *args, PyObject *kwargs)
         if (seek_result == NULL) {
             return NULL;
         }
+        Py_DECREF(seek_result);
+        /* Discard compressed data buffered before rewinding the file. */
+        self->current_pos = self->input_buffer;
+        self->buffer_end = self->input_buffer;
         self->stream_phase = GzipReader_HEADER;
         self->_pos = 0;
         self->all_bytes_read = 0;

tests/test_seek_buffer_reset.py

@@ -0,0 +1,30 @@
+"""Force backward seeks past the outer buffer and through the raw reader."""
+import gzip
+import io
+import random
+
+from isal import igzip
+
+import pytest
+
+
+@pytest.mark.parametrize('writer', [gzip.compress, igzip.compress])
+@pytest.mark.parametrize('random_data', [False, True])
+def test_backward_seek_discards_compressed_buffer(writer, random_data):
+    data = (random.Random(42).randbytes(1048576) if random_data
+            else b'hello world' * 100000)
+    with igzip.open(io.BytesIO(writer(data)), 'rb') as reader:
+        for offset in (12345, 0, 750000, 10, 32768, 0):
+            assert reader.seek(offset) == offset
+            assert reader.read(400) == data[offset:offset+400]
+        reader.seek(0)
+        assert reader.read() == data
+
+
+@pytest.mark.parametrize('size', [110, 8192, 32768])
+def test_raw_reader_backward_seek(size):
+    data = random.Random(42).randbytes(size)
+    reader = igzip._GzipReader(io.BytesIO(gzip.compress(data)))
+    assert reader.read(1) == data[:1]
+    assert reader.seek(0) == 0
+    assert reader.readall() == data