Prevent a possible assert in the debugger when a thread detaches (#34955)

When a new thread is attached to the VM, the debugger will add it to the
`thread_to_tls` hash table. When that thread detaches, it will be
removed. Once the thread is attached, if a client is debugging, the
client can use the `CMD_THREAD_GET_FRAME_INFO` command to ask for
details about that thread.

This is a possibility that the thread detaches before the
`CMD_THREAD_GET_FRAME_INFO` command is processed, so the thread may not
exist the the `thread_to_tls` hash table any more.

This is a race condition, but is a valid state, so instead of asserting,
the debugger agent should indicate to the client that the thread no
longer exists, using `ERR_UNLOADED`.

diff --git a/src/mono/mono/mini/debugger-agent.c b/src/mono/mono/mini/debugger-agent.c
index 3742cc0..a46f622 100644 (file)
--- a/src/mono/mono/mini/debugger-agent.c
+++ b/src/mono/mono/mini/debugger-agent.c
@@ -9135,7 +9135,8 @@ thread_commands (int command, guint8 *p, guint8 *end, Buffer *buf)
                mono_loader_lock ();
                tls = (DebuggerTlsData *)mono_g_hash_table_lookup (thread_to_tls, thread);
                mono_loader_unlock ();
-               g_assert (tls);
+               if (tls == NULL)
+                       return ERR_UNLOADED;
 
                compute_frame_info (thread, tls, TRUE); //the last parameter is TRUE to force that the frame info that will be send is synchronised with the debugged thread