bpo-34759: Fix error handling in ssl 'unwrap()' (GH-9468)

OpenSSL follows the convention that whenever you call a function, it
returns an error indicator value; and if this value is negative, then
you need to go look at the actual error code to see what happened.

Commit c6fd1c1 introduced a small mistake in
_ssl__SSLSocket_shutdown_impl: instead of checking whether the error
indicator was negative, it started checking whether the actual error
code was negative, and it turns out that the error codes are never
negative. So the effect was that 'unwrap()' lost the ability to raise
SSL errors.


https://bugs.python.org/issue34759

Lib/test/test_ssl.py

@@ -1715,6 +1715,47 @@ def test_private_init(self):
         with self.assertRaisesRegex(TypeError, "public constructor"):
             ssl.SSLObject(bio, bio)
 
+    def test_unwrap(self):
+        client_ctx, server_ctx, hostname = testing_context()
+        c_in = ssl.MemoryBIO()
+        c_out = ssl.MemoryBIO()
+        s_in = ssl.MemoryBIO()
+        s_out = ssl.MemoryBIO()
+        client = client_ctx.wrap_bio(c_in, c_out, server_hostname=hostname)
+        server = server_ctx.wrap_bio(s_in, s_out, server_side=True)
+
+        # Loop on the handshake for a bit to get it settled
+        for _ in range(5):
+            try:
+                client.do_handshake()
+            except ssl.SSLWantReadError:
+                pass
+            if c_out.pending:
+                s_in.write(c_out.read())
+            try:
+                server.do_handshake()
+            except ssl.SSLWantReadError:
+                pass
+            if s_out.pending:
+                c_in.write(s_out.read())
+        # Now the handshakes should be complete (don't raise WantReadError)
+        client.do_handshake()
+        server.do_handshake()
+
+        # Now if we unwrap one side unilaterally, it should send close-notify
+        # and raise WantReadError:
+        with self.assertRaises(ssl.SSLWantReadError):
+            client.unwrap()
+
+        # But server.unwrap() does not raise, because it reads the client's
+        # close-notify:
+        s_in.write(c_out.read())
+        server.unwrap()
+
+        # And now that the client gets the server's close-notify, it doesn't
+        # raise either.
+        c_in.write(s_out.read())
+        client.unwrap()
 
 class SimpleBackgroundTests(unittest.TestCase):
     """Tests that connect to a simple server running in the background"""

Modules/_ssl.c

@@ -2583,9 +2583,9 @@ _ssl__SSLSocket_shutdown_impl(PySSLSocket *self)
             break;
     }
 
-    if (err.ssl < 0) {
+    if (ret < 0) {
         Py_XDECREF(sock);
-        return PySSL_SetError(self, err.ssl, __FILE__, __LINE__);
+        return PySSL_SetError(self, ret, __FILE__, __LINE__);
     }
     if (sock)
         /* It's already INCREF'ed */