gh-160121: Correct repeated calls to decompress() with max_length=0

Lib/test/test_bz2.py

@@ -967,6 +967,29 @@ def testDecompressorChunksMaxsize(self):
         self.assertEqual(out, self.BIG_TEXT)
         self.assertEqual(bzd.unused_data, b"")
 
+    def test_decompressor_max_length_zero(self):
+        # max_length=0 buffers the input and never produces output,
+        # however many times it is repeated.
+        bzd = BZ2Decompressor()
+        for _ in range(20):
+            self.assertEqual(bzd.decompress(b'', max_length=0), b'')
+        self.assertEqual(bzd.decompress(self.DATA, max_length=0), b'')
+        for _ in range(20):
+            self.assertEqual(bzd.decompress(b'', max_length=0), b'')
+        self.assertFalse(bzd.needs_input)
+        self.assertFalse(bzd.eof)
+        self.assertEqual(bzd.decompress(b''), self.TEXT)
+        self.assertTrue(bzd.eof)
+
+        # Output pending from an earlier call is kept for later.
+        bzd = BZ2Decompressor()
+        out = bzd.decompress(self.DATA, max_length=1)
+        for _ in range(20):
+            self.assertEqual(bzd.decompress(b'', max_length=0), b'')
+        self.assertFalse(bzd.needs_input)
+        self.assertEqual(out + bzd.decompress(b''), self.TEXT)
+        self.assertTrue(bzd.eof)
+
     def test_decompressor_inputbuf_1(self):
         # Test reusing input buffer after moving existing
         # contents to beginning

Lib/test/test_zlib.py

@@ -1231,6 +1231,29 @@ def testDecompressorChunksMaxsize(self):
         self.assertEqual(out, self.BIG_TEXT)
         self.assertEqual(zlibd.unused_data, b"")
 
+    def test_decompressor_max_length_zero(self):
+        # max_length=0 buffers the input and never produces output,
+        # however many times it is repeated.
+        zlibd = zlib._ZlibDecompressor()
+        for _ in range(20):
+            self.assertEqual(zlibd.decompress(b'', max_length=0), b'')
+        self.assertEqual(zlibd.decompress(self.DATA, max_length=0), b'')
+        for _ in range(20):
+            self.assertEqual(zlibd.decompress(b'', max_length=0), b'')
+        self.assertFalse(zlibd.needs_input)
+        self.assertFalse(zlibd.eof)
+        self.assertEqual(zlibd.decompress(b''), self.TEXT)
+        self.assertTrue(zlibd.eof)
+
+        # Output pending from an earlier call is kept for later.
+        zlibd = zlib._ZlibDecompressor()
+        out = zlibd.decompress(self.DATA, max_length=1)
+        for _ in range(20):
+            self.assertEqual(zlibd.decompress(b'', max_length=0), b'')
+        self.assertFalse(zlibd.needs_input)
+        self.assertEqual(out + zlibd.decompress(b''), self.TEXT)
+        self.assertTrue(zlibd.eof)
+
     def test_decompressor_inputbuf_1(self):
         # Test reusing input buffer after moving existing
         # contents to beginning

Lib/test/test_zstd.py

@@ -853,6 +853,29 @@ def test_decompress_empty_content_frame(self):
         self.assertEqual(d.unused_data, b'')
         self.assertEqual(d.unused_data, b'') # twice
 
+    def test_decompressor_max_length_zero(self):
+        # max_length=0 buffers the input and never produces output,
+        # however many times it is repeated.
+        d = ZstdDecompressor()
+        for _ in range(20):
+            self.assertEqual(d.decompress(b'', max_length=0), b'')
+        self.assertEqual(d.decompress(COMPRESSED_100_PLUS_32KB, max_length=0), b'')
+        for _ in range(20):
+            self.assertEqual(d.decompress(b'', max_length=0), b'')
+        self.assertFalse(d.needs_input)
+        self.assertFalse(d.eof)
+        self.assertEqual(d.decompress(b''), DECOMPRESSED_100_PLUS_32KB)
+        self.assertTrue(d.eof)
+
+        # Output pending from an earlier call is kept for later.
+        d = ZstdDecompressor()
+        out = d.decompress(COMPRESSED_100_PLUS_32KB, max_length=1)
+        for _ in range(20):
+            self.assertEqual(d.decompress(b'', max_length=0), b'')
+        self.assertFalse(d.needs_input)
+        self.assertEqual(out + d.decompress(b''), DECOMPRESSED_100_PLUS_32KB)
+        self.assertTrue(d.eof)
+
 class DecompressorFlagsTestCase(unittest.TestCase):
 
     @classmethod
@@ -1167,6 +1190,18 @@ def test_decompressor_skippable(self):
         d = ZstdDecompressor()
         dat = d.decompress(SKIPPABLE_FRAME, 0)
 
+        self.assertEqual(dat, b'')
+        self.assertFalse(d.eof)
+        self.assertFalse(d.needs_input)
+
+        dat = d.decompress(b'', 0)
+
+        self.assertEqual(dat, b'')
+        self.assertFalse(d.eof)
+        self.assertFalse(d.needs_input)
+
+        dat = d.decompress(b'')
+
         self.assertEqual(dat, b'')
         self.assertTrue(d.eof)
         self.assertFalse(d.needs_input)

Misc/NEWS.d/next/Library/2026-10-22-23-01-17.gh-issue-160000.mL0zRo.rst

@@ -0,0 +1,5 @@
+Fix :meth:`compression.zstd.ZstdDecompressor.decompress` raising an error when
+called repeatedly with ``max_length=0``. Calling ``decompress()`` with
+``max_length=0`` on the Zstandard decompressor, :class:`bz2.BZ2Decompressor`
+and :class:`!zlib._ZlibDecompressor` now always buffers the input and returns
+``b''`` without decompressing anything.

Modules/_bz2module.c

@@ -421,6 +421,12 @@ decompress_buf(BZ2Decompressor *d, Py_ssize_t max_length)
         goto error;
     }
 
+    /* With max_length == 0 there is no room for output.  Don't call into
+       libbz2: any input is kept in the input buffer for the next call. */
+    if (max_length == 0) {
+        goto finish;
+    }
+
     for (;;) {
         int bzret;
         /* On a 64-bit system, buffer length might not fit in avail_out, so we
@@ -456,6 +462,7 @@ decompress_buf(BZ2Decompressor *d, Py_ssize_t max_length)
         }
     }
 
+finish:
     result = OutputBuffer_Finish(&buffer, bzs->avail_out);
     if (result != NULL) {
         return result;

Modules/_zstd/decompressor.c

@@ -226,6 +226,12 @@ decompress_lock_held(ZstdDecompressor *self, ZSTD_inBuffer *in,
     }
     assert(out.pos == 0);
 
+    /* With max_length == 0 there is no room for output.  Don't call into
+       libzstd: any input is kept in the input buffer for the next call. */
+    if (max_length == 0) {
+        goto finish;
+    }
+
     while (1) {
         /* Decompress */
         Py_BEGIN_ALLOW_THREADS
@@ -269,6 +275,7 @@ decompress_lock_held(ZstdDecompressor *self, ZSTD_inBuffer *in,
         }
     }
 
+finish:
     /* Return a bytes object */
     ret = _OutputBuffer_Finish(&buffer, &out);
     if (ret != NULL) {

Modules/zlibmodule.c

@@ -1488,6 +1488,12 @@ decompress_buf(ZlibDecompressor *self, Py_ssize_t max_length)
 
     int err = Z_OK;
 
+    /* With max_length == 0 there is no room for output.  Don't call into
+       zlib: any input is kept in the input buffer for the next call. */
+    if (max_length == 0) {
+        return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
+    }
+
     /* When sys.maxsize is passed as default use DEF_BUF_SIZE as start buffer.
        In this particular case the data may not necessarily be very big, so
        it is better to grow dynamically.*/