bpo-30622: Change NPN detection: (#2079)

* Change NPN detection:

Version breakdown, support disabled (pre-patch/post-patch):
- pre-1.0.1: OPENSSL_NPN_NEGOTIATED will not be defined -> False/False
- 1.0.1 and 1.0.2: OPENSSL_NPN_NEGOTIATED will not be defined ->
False/False
- 1.1.0+: OPENSSL_NPN_NEGOTIATED will be defined and
OPENSSL_NO_NEXTPROTONEG will be defined -> True/False

Version breakdown support enabled (pre-patch/post-patch):
- pre-1.0.1: OPENSSL_NPN_NEGOTIATED will not be defined -> False/False
- 1.0.1 and 1.0.2: OPENSSL_NPN_NEGOTIATED will be defined and
OPENSSL_NO_NEXTPROTONEG will not be defined -> True/True
- 1.1.0+: OPENSSL_NPN_NEGOTIATED will be defined and
OPENSSL_NO_NEXTPROTONEG will not be defined -> True/True

* Refine NPN guard:

- If NPN is disabled, but ALPN is available we need our callback
- Make clinic's ssl behave the same way

This created a working ssl module for me, with NPN disabled and ALPN
enabled for OpenSSL 1.1.0f.

Concerns to address:
The initial commit for NPN support into OpenSSL [1], had the
OPENSSL_NPN_* variables defined inside the OPENSSL_NO_NEXTPROTONEG
guard. The question is if that ever made it into a release.
This would need an ugly hack, something like:

	#if defined(OPENSSL_NO_NEXTPROTONEG) && \
		!defined(OPENSSL_NPN_NEGOTIATED)
	#	define OPENSSL_NPN_UNSUPPORTED 0
	#	define OPENSSL_NPN_NEGOTIATED 1
	#	define OPENSSL_NPN_NO_OVERLAP 2
	#endif

[1] https://github.com/openssl/openssl/commit/68b33cc5c7

Modules/_ssl.c

@@ -315,7 +315,7 @@ static unsigned int _ssl_locks_count = 0;
 typedef struct {
     PyObject_HEAD
     SSL_CTX *ctx;
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
     unsigned char *npn_protocols;
     int npn_protocols_len;
 #endif
@@ -1697,7 +1697,7 @@ _ssl__SSLSocket_version_impl(PySSLSocket *self)
     return PyUnicode_FromString(version);
 }
 
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
 /*[clinic input]
 _ssl._SSLSocket.selected_npn_protocol
 [clinic start generated code]*/
@@ -2647,7 +2647,7 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
         return NULL;
     }
     self->ctx = ctx;
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
     self->npn_protocols = NULL;
 #endif
 #ifdef HAVE_ALPN
@@ -2782,7 +2782,7 @@ context_dealloc(PySSLContext *self)
     PyObject_GC_UnTrack(self);
     context_clear(self);
     SSL_CTX_free(self->ctx);
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
     PyMem_FREE(self->npn_protocols);
 #endif
 #ifdef HAVE_ALPN
@@ -2860,7 +2860,7 @@ _ssl__SSLContext_get_ciphers_impl(PySSLContext *self)
 #endif
 
 
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG) || defined(HAVE_ALPN)
 static int
 do_protocol_selection(int alpn, unsigned char **out, unsigned char *outlen,
                       const unsigned char *server_protocols, unsigned int server_protocols_len,
@@ -2884,7 +2884,9 @@ do_protocol_selection(int alpn, unsigned char **out, unsigned char *outlen,
 
     return SSL_TLSEXT_ERR_OK;
 }
+#endif
 
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
 /* this callback gets passed to SSL_CTX_set_next_protos_advertise_cb */
 static int
 _advertiseNPN_cb(SSL *s,
@@ -2927,7 +2929,7 @@ _ssl__SSLContext__set_npn_protocols_impl(PySSLContext *self,
                                          Py_buffer *protos)
 /*[clinic end generated code: output=72b002c3324390c6 input=319fcb66abf95bd7]*/
 {
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
     PyMem_Free(self->npn_protocols);
     self->npn_protocols = PyMem_Malloc(protos->len);
     if (self->npn_protocols == NULL)
@@ -5397,7 +5399,7 @@ PyInit__ssl(void)
     Py_INCREF(r);
     PyModule_AddObject(m, "HAS_ECDH", r);
 
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
     r = Py_True;
 #else
     r = Py_False;

Modules/clinic/_ssl.c.h

@@ -132,7 +132,7 @@ _ssl__SSLSocket_version(PySSLSocket *self, PyObject *Py_UNUSED(ignored))
     return _ssl__SSLSocket_version_impl(self);
 }
 
-#if defined(OPENSSL_NPN_NEGOTIATED)
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
 
 PyDoc_STRVAR(_ssl__SSLSocket_selected_npn_protocol__doc__,
 "selected_npn_protocol($self, /)\n"
@@ -151,7 +151,7 @@ _ssl__SSLSocket_selected_npn_protocol(PySSLSocket *self, PyObject *Py_UNUSED(ign
     return _ssl__SSLSocket_selected_npn_protocol_impl(self);
 }
 
-#endif /* defined(OPENSSL_NPN_NEGOTIATED) */
+#endif /* defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG) */
 
 #if defined(HAVE_ALPN)