nbd/server: do not poll within a coroutine context

Coroutines are not supposed to block. Instead, they should yield.

The client performs TLS upgrade outside of an AIOContext, during
synchronous handshake; this still requires g_main_loop.  But the
server responds to TLS upgrade inside a coroutine, so a nested
g_main_loop is wrong.  Since the two callbacks no longer share more
than the setting of data.complete and data.error, it's just as easy to
use static helpers instead of trying to share a common code path.  It
is also possible to add assertions that no other code is interfering
with the eventual path to qio reaching the callback, whether or not it
required a yield or main loop.

Fixes: f95910f ("nbd: implement TLS support in the protocol negotiation")
Signed-off-by: Zhu Yangyang <zhuyangyang14@huawei.com>
[eblake: move callbacks to their use point, add assertions]
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20240408160214.1200629-5-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

nbd/client.c

@@ -596,13 +596,31 @@ static int nbd_request_simple_option(QIOChannel *ioc, int opt, bool strict,
     return 1;
 }
 
+/* Callback to learn when QIO TLS upgrade is complete */
+struct NBDTLSClientHandshakeData {
+    bool complete;
+    Error *error;
+    GMainLoop *loop;
+};
+
+static void nbd_client_tls_handshake(QIOTask *task, void *opaque)
+{
+    struct NBDTLSClientHandshakeData *data = opaque;
+
+    qio_task_propagate_error(task, &data->error);
+    data->complete = true;
+    if (data->loop) {
+        g_main_loop_quit(data->loop);
+    }
+}
+
 static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
                                         QCryptoTLSCreds *tlscreds,
                                         const char *hostname, Error **errp)
 {
     int ret;
     QIOChannelTLS *tioc;
-    struct NBDTLSHandshakeData data = { 0 };
+    struct NBDTLSClientHandshakeData data = { 0 };
 
     ret = nbd_request_simple_option(ioc, NBD_OPT_STARTTLS, true, errp);
     if (ret <= 0) {
@@ -619,18 +637,20 @@ static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
         return NULL;
     }
     qio_channel_set_name(QIO_CHANNEL(tioc), "nbd-client-tls");
-    data.loop = g_main_loop_new(g_main_context_default(), FALSE);
     trace_nbd_receive_starttls_tls_handshake();
     qio_channel_tls_handshake(tioc,
-                              nbd_tls_handshake,
+                              nbd_client_tls_handshake,
                               &data,
                               NULL,
                               NULL);
 
     if (!data.complete) {
+        data.loop = g_main_loop_new(g_main_context_default(), FALSE);
         g_main_loop_run(data.loop);
+        assert(data.complete);
+        g_main_loop_unref(data.loop);
     }
-    g_main_loop_unref(data.loop);
+
     if (data.error) {
         error_propagate(errp, data.error);
         object_unref(OBJECT(tioc));

nbd/common.c

@@ -47,17 +47,6 @@ int nbd_drop(QIOChannel *ioc, size_t size, Error **errp)
 }
 
 
-void nbd_tls_handshake(QIOTask *task,
-                       void *opaque)
-{
-    struct NBDTLSHandshakeData *data = opaque;
-
-    qio_task_propagate_error(task, &data->error);
-    data->complete = true;
-    g_main_loop_quit(data->loop);
-}
-
-
 const char *nbd_opt_lookup(uint32_t opt)
 {
     switch (opt) {

nbd/nbd-internal.h

@@ -72,16 +72,6 @@ static inline int nbd_write(QIOChannel *ioc, const void *buffer, size_t size,
     return qio_channel_write_all(ioc, buffer, size, errp) < 0 ? -EIO : 0;
 }
 
-struct NBDTLSHandshakeData {
-    GMainLoop *loop;
-    bool complete;
-    Error *error;
-};
-
-
-void nbd_tls_handshake(QIOTask *task,
-                       void *opaque);
-
 int nbd_drop(QIOChannel *ioc, size_t size, Error **errp);
 
 #endif

nbd/server.c

@@ -748,6 +748,23 @@ static int nbd_negotiate_handle_info(NBDClient *client, Error **errp)
     return rc;
 }
 
+/* Callback to learn when QIO TLS upgrade is complete */
+struct NBDTLSServerHandshakeData {
+    bool complete;
+    Error *error;
+    Coroutine *co;
+};
+
+static void nbd_server_tls_handshake(QIOTask *task, void *opaque)
+{
+    struct NBDTLSServerHandshakeData *data = opaque;
+
+    qio_task_propagate_error(task, &data->error);
+    data->complete = true;
+    if (!qemu_coroutine_entered(data->co)) {
+        aio_co_wake(data->co);
+    }
+}
 
 /* Handle NBD_OPT_STARTTLS. Return NULL to drop connection, or else the
  * new channel for all further (now-encrypted) communication. */
@@ -756,7 +773,7 @@ static QIOChannel *nbd_negotiate_handle_starttls(NBDClient *client,
 {
     QIOChannel *ioc;
     QIOChannelTLS *tioc;
-    struct NBDTLSHandshakeData data = { 0 };
+    struct NBDTLSServerHandshakeData data = { 0 };
 
     assert(client->opt == NBD_OPT_STARTTLS);
 
@@ -777,17 +794,18 @@ static QIOChannel *nbd_negotiate_handle_starttls(NBDClient *client,
 
     qio_channel_set_name(QIO_CHANNEL(tioc), "nbd-server-tls");
     trace_nbd_negotiate_handle_starttls_handshake();
-    data.loop = g_main_loop_new(g_main_context_default(), FALSE);
+    data.co = qemu_coroutine_self();
     qio_channel_tls_handshake(tioc,
-                              nbd_tls_handshake,
+                              nbd_server_tls_handshake,
                               &data,
                               NULL,
                               NULL);
 
     if (!data.complete) {
-        g_main_loop_run(data.loop);
+        qemu_coroutine_yield();
+        assert(data.complete);
     }
-    g_main_loop_unref(data.loop);
+
     if (data.error) {
         object_unref(OBJECT(tioc));
         error_propagate(errp, data.error);