[2.7] bpo-30622: Change NPN detection: (GH-2079) (#3316)

* 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:

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

[1] https://github.com/openssl/openssl/commit/68b33cc5c7.
(cherry picked from commit b2d096bd2a)

Modules/_ssl.c

@@ -280,7 +280,7 @@ 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
@@ -1502,7 +1502,7 @@ static PyObject *PySSL_version(PySSLSocket *self)
     return PyUnicode_FromString(version);
 }
 
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
 static PyObject *PySSL_selected_npn_protocol(PySSLSocket *self) {
     const unsigned char *out;
     unsigned int outlen;
@@ -2140,7 +2140,7 @@ context_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         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
@@ -2218,7 +2218,7 @@ 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
@@ -2248,7 +2248,7 @@ set_ciphers(PySSLContext *self, PyObject *args)
     Py_RETURN_NONE;
 }
 
-#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,
@@ -2272,7 +2272,9 @@ 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,
@@ -2307,7 +2309,7 @@ _selectNPN_cb(SSL *s,
 static PyObject *
 _set_npn_protocols(PySSLContext *self, PyObject *args)
 {
-#ifdef OPENSSL_NPN_NEGOTIATED
+#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
     Py_buffer protos;
 
     if (!PyArg_ParseTuple(args, "s*:set_npn_protocols", &protos))
@@ -4305,7 +4307,7 @@ init_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;