tests: add test for calling decompress multiple times

This test was previously failing in the C backend due to the
`assert(!self->unused_data)`. The previous commit refactored the
code so `self->unused_data` is only assigned after zstd frame
completion (not exhaustion of input). So the assertion should be
upheld.

The bug fixes are also documented in the changelog.

Closes #180.

Author: TheNeuralBit

docs/news.rst

@@ -82,6 +82,14 @@ Other Actions Not Blocking Release
 0.19.0 (not yet released)
 =========================
 
+Bug Fixes
+---------
+
+* The C backend implementation of ``ZstdDecompressionObj.decompress()`` could
+  have raised an assertion in cases where the function was called multiple
+  times on an instance. In non-debug builds, calls to this method could have
+  leaked memory.
+
 Changes
 -------
 

tests/test_decompressor_decompressobj.py

@@ -73,6 +73,27 @@ def test_reuse(self):
             dobj.decompress(data)
             self.assertEqual(dobj.flush(), b"")
 
+    def test_multiple_decompress_calls(self):
+        expected = b"foobar" * 10
+        data = zstd.ZstdCompressor(level=1).compress(expected)
+
+        N = 3
+        partitioned_data = [
+            data[len(data) * i // N : len(data) * (i + 1) // N]
+            for i in range(N)
+        ]
+
+        dctx = zstd.ZstdDecompressor()
+        dobj = dctx.decompressobj()
+
+        for partition in partitioned_data[:-1]:
+            decompressed = dobj.decompress(partition)
+            self.assertEqual(decompressed, b"")
+            self.assertEqual(dobj.unused_data, b"")
+
+        decompressed = dobj.decompress(partitioned_data[-1])
+        self.assertEqual(decompressed, expected)
+
     def test_bad_write_size(self):
         dctx = zstd.ZstdDecompressor()