bpo-43794: OpenSSL 3.0.0: set OP_IGNORE_UNEXPECTED_EOF by default (GH-25309)

Signed-off-by: Christian Heimes <christian@python.org>
diff --git a/Doc/library/ssl.rst b/Doc/library/ssl.rst
index 9333168..587d3b3 100644
--- a/Doc/library/ssl.rst
+++ b/Doc/library/ssl.rst
@@ -893,6 +893,14 @@
 
    .. versionadded:: 3.6
 
+.. data:: OP_IGNORE_UNEXPECTED_EOF
+
+   Ignore unexpected shutdown of TLS connections.
+
+   This option is only available with OpenSSL 3.0.0 and later.
+
+   .. versionadded:: 3.10
+
 .. data:: HAS_ALPN
 
    Whether the OpenSSL library has built-in support for the *Application-Layer
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index c0e040d..831f411 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -151,6 +151,7 @@ def data_file(*name):
 OP_SINGLE_ECDH_USE = getattr(ssl, "OP_SINGLE_ECDH_USE", 0)
 OP_CIPHER_SERVER_PREFERENCE = getattr(ssl, "OP_CIPHER_SERVER_PREFERENCE", 0)
 OP_ENABLE_MIDDLEBOX_COMPAT = getattr(ssl, "OP_ENABLE_MIDDLEBOX_COMPAT", 0)
+OP_IGNORE_UNEXPECTED_EOF = getattr(ssl, "OP_IGNORE_UNEXPECTED_EOF", 0)
 
 # Ubuntu has patched OpenSSL and changed behavior of security level 2
 # see https://bugs.python.org/issue41561#msg389003
@@ -1168,7 +1169,8 @@ def test_options(self):
         # SSLContext also enables these by default
         default |= (OP_NO_COMPRESSION | OP_CIPHER_SERVER_PREFERENCE |
                     OP_SINGLE_DH_USE | OP_SINGLE_ECDH_USE |
-                    OP_ENABLE_MIDDLEBOX_COMPAT)
+                    OP_ENABLE_MIDDLEBOX_COMPAT |
+                    OP_IGNORE_UNEXPECTED_EOF)
         self.assertEqual(default, ctx.options)
         ctx.options |= ssl.OP_NO_TLSv1
         self.assertEqual(default | ssl.OP_NO_TLSv1, ctx.options)
diff --git a/Misc/NEWS.d/next/Library/2021-04-09-16-14-22.bpo-43794.-1XPDH.rst b/Misc/NEWS.d/next/Library/2021-04-09-16-14-22.bpo-43794.-1XPDH.rst
new file mode 100644
index 0000000..64894bd
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-04-09-16-14-22.bpo-43794.-1XPDH.rst
@@ -0,0 +1 @@
+Add :data:`ssl.OP_IGNORE_UNEXPECTED_EOF` constants (OpenSSL 3.0.0)
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 3ee61e3..c08665b 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -3203,6 +3203,10 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
 #ifdef SSL_OP_SINGLE_ECDH_USE
     options |= SSL_OP_SINGLE_ECDH_USE;
 #endif
+#ifdef SSL_OP_IGNORE_UNEXPECTED_EOF
+    /* Make OpenSSL 3.0.0 behave like 1.1.1 */
+    options |= SSL_OP_IGNORE_UNEXPECTED_EOF;
+#endif
     SSL_CTX_set_options(self->ctx, options);
 
     /* A bare minimum cipher list without completely broken cipher suites.
@@ -6313,6 +6317,10 @@ sslmodule_init_constants(PyObject *m)
     PyModule_AddIntConstant(m, "OP_NO_RENEGOTIATION",
                             SSL_OP_NO_RENEGOTIATION);
 #endif
+#ifdef SSL_OP_IGNORE_UNEXPECTED_EOF
+    PyModule_AddIntConstant(m, "OP_IGNORE_UNEXPECTED_EOF",
+                            SSL_OP_IGNORE_UNEXPECTED_EOF);
+#endif
 
 #ifdef X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT
     PyModule_AddIntConstant(m, "HOSTFLAG_ALWAYS_CHECK_SUBJECT",