Fix race condition with BIO methods initialization in libpq with threads

The libpq code in charge of creating per-connection SSL objects was
prone to a race condition when loading the custom BIO methods needed by
my_SSL_set_fd().  As BIO methods are stored as a static variable, the
initialization of a connection could fail because it could be possible
to have one thread refer to my_bio_methods while it is being manipulated
by a second concurrent thread.

This error has been introduced by 8bb14cd, that has removed
ssl_config_mutex around the call of my_SSL_set_fd(), that itself sets
the custom BIO methods used in libpq.  Like previously, the BIO method
initialization is now protected by the existing ssl_config_mutex, itself
initialized earlier for WIN32.

While on it, document that my_bio_methods is protected by
ssl_config_mutex, as this can be easy to miss.

Reported-by: Willi Mann
Author: Willi Mann, Michael Paquier
Discussion: https://postgr.es/m/e77abc4c-4d03-4058-a9d7-ef0035657e04@celonis.com
Backpatch-through: 12

Author: michaelpq

src/interfaces/libpq/fe-secure-openssl.c

@@ -1820,6 +1820,7 @@ PQsslAttribute(PGconn *conn, const char *attribute_name)
 #define BIO_set_data(bio, data) (bio->ptr = data)
 #endif
 
+/* protected by ssl_config_mutex */
 static BIO_METHOD *my_bio_methods;
 
 static int
@@ -1885,6 +1886,13 @@ my_sock_write(BIO *h, const char *buf, int size)
 static BIO_METHOD *
 my_BIO_s_socket(void)
 {
+	BIO_METHOD *res;
+
+	if (pthread_mutex_lock(&ssl_config_mutex))
+		return NULL;
+
+	res = my_bio_methods;
+
 	if (!my_bio_methods)
 	{
 		BIO_METHOD *biom = (BIO_METHOD *) BIO_s_socket();
@@ -1893,39 +1901,51 @@ my_BIO_s_socket(void)
 
 		my_bio_index = BIO_get_new_index();
 		if (my_bio_index == -1)
-			return NULL;
+			goto err;
 		my_bio_index |= (BIO_TYPE_DESCRIPTOR | BIO_TYPE_SOURCE_SINK);
-		my_bio_methods = BIO_meth_new(my_bio_index, "libpq socket");
-		if (!my_bio_methods)
-			return NULL;
+		res = BIO_meth_new(my_bio_index, "libpq socket");
+		if (!res)
+			goto err;
 
 		/*
 		 * As of this writing, these functions never fail. But check anyway,
 		 * like OpenSSL's own examples do.
 		 */
-		if (!BIO_meth_set_write(my_bio_methods, my_sock_write) ||
-			!BIO_meth_set_read(my_bio_methods, my_sock_read) ||
-			!BIO_meth_set_gets(my_bio_methods, BIO_meth_get_gets(biom)) ||
-			!BIO_meth_set_puts(my_bio_methods, BIO_meth_get_puts(biom)) ||
-			!BIO_meth_set_ctrl(my_bio_methods, BIO_meth_get_ctrl(biom)) ||
-			!BIO_meth_set_create(my_bio_methods, BIO_meth_get_create(biom)) ||
-			!BIO_meth_set_destroy(my_bio_methods, BIO_meth_get_destroy(biom)) ||
-			!BIO_meth_set_callback_ctrl(my_bio_methods, BIO_meth_get_callback_ctrl(biom)))
+		if (!BIO_meth_set_write(res, my_sock_write) ||
+			!BIO_meth_set_read(res, my_sock_read) ||
+			!BIO_meth_set_gets(res, BIO_meth_get_gets(biom)) ||
+			!BIO_meth_set_puts(res, BIO_meth_get_puts(biom)) ||
+			!BIO_meth_set_ctrl(res, BIO_meth_get_ctrl(biom)) ||
+			!BIO_meth_set_create(res, BIO_meth_get_create(biom)) ||
+			!BIO_meth_set_destroy(res, BIO_meth_get_destroy(biom)) ||
+			!BIO_meth_set_callback_ctrl(res, BIO_meth_get_callback_ctrl(biom)))
 		{
-			BIO_meth_free(my_bio_methods);
-			my_bio_methods = NULL;
-			return NULL;
+			goto err;
 		}
 #else
-		my_bio_methods = malloc(sizeof(BIO_METHOD));
-		if (!my_bio_methods)
-			return NULL;
-		memcpy(my_bio_methods, biom, sizeof(BIO_METHOD));
-		my_bio_methods->bread = my_sock_read;
-		my_bio_methods->bwrite = my_sock_write;
+		res = malloc(sizeof(BIO_METHOD));
+		if (!res)
+			goto err;
+		memcpy(res, biom, sizeof(BIO_METHOD));
+		res->bread = my_sock_read;
+		res->bwrite = my_sock_write;
 #endif
 	}
-	return my_bio_methods;
+
+	my_bio_methods = res;
+	pthread_mutex_unlock(&ssl_config_mutex);
+	return res;
+
+err:
+#ifdef HAVE_BIO_METH_NEW
+	if (res)
+		BIO_meth_free(res);
+#else
+	if (res)
+		free(res);
+#endif
+	pthread_mutex_unlock(&ssl_config_mutex);
+	return NULL;
 }
 
 /* This should exactly match OpenSSL's SSL_set_fd except for using my BIO */