bpo-38256: Fix binascii.crc32() when inputs are 4+GiB (GH-32000)

When compiled with `USE_ZLIB_CRC32` defined (`configure` sets this on POSIX systems), `binascii.crc32(...)` failed to compute the correct value when the input data was >= 4GiB. Because the zlib crc32 API is limited to a 32-bit length.

This lines it up with the `zlib.crc32(...)` implementation that doesn't have that flaw.

**Performance:** This also adopts the same GIL releasing for larger inputs logic that `zlib.crc32` has, and causes the Windows build to always use zlib's crc32 instead of our slow C code as zlib is a required build dependency on Windows.

1 files changed, 4 insertions, 4 deletions

diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index 4cf1b6e..2fc39a3 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -1420,7 +1420,7 @@ zlib_adler32_impl(PyObject *module, Py_buffer *data, unsigned int value)
 }
 
 /*[clinic input]
-zlib.crc32
+zlib.crc32 -> unsigned_int
 
     data: Py_buffer
     value: unsigned_int(bitwise=True) = 0
@@ -1432,9 +1432,9 @@ Compute a CRC-32 checksum of data.
 The returned checksum is an integer.
 [clinic start generated code]*/
 
-static PyObject *
+static unsigned int
 zlib_crc32_impl(PyObject *module, Py_buffer *data, unsigned int value)
-/*[clinic end generated code: output=63499fa20af7ea25 input=26c3ed430fa00b4c]*/
+/*[clinic end generated code: output=b217562e4fe6d6a6 input=1229cb2fb5ea948a]*/
 {
     /* Releasing the GIL for very small buffers is inefficient
        and may lower performance */
@@ -1455,7 +1455,7 @@ zlib_crc32_impl(PyObject *module, Py_buffer *data, unsigned int value)
     } else {
         value = crc32(value, data->buf, (unsigned int)data->len);
     }
-    return PyLong_FromUnsignedLong(value & 0xffffffffU);
+    return value;
 }