migration: Fix missing join() of rp_thread

It's possible that the migration thread skip the join() of the rp_thread in
below race and crash on src right at finishing migration:

       migration_thread                     rp_thread
       ----------------                     ---------
    migration_completion()
                                        (before rp_thread quits)
                                        from_dst_file=NULL
                                        [thread got scheduled out]
      s->rp_state.from_dst_file==NULL
        (skip join() of rp_thread)
    migrate_fd_cleanup()
      qemu_fclose(s->to_dst_file)
      yank_unregister_instance()
        assert(yank_find_entry())  <------- crash

It could mostly happen with postcopy, but that shouldn't be required, e.g., I
think it could also trigger with MIGRATION_CAPABILITY_RETURN_PATH set.

It's suspected that above race could be the root cause of a recent (but rare)
migration-test break reported by either Dave or PMM:

https://lore.kernel.org/qemu-devel/YPamXAHwan%2FPPXLf@work-vm/

The issue is: from_dst_file is reset in the rp_thread, so if the thread reset
it to NULL fast enough then the migration thread will assume there's no
rp_thread at all.

This could potentially cause more severe issue (e.g. crash) after the yank code.

Fix it by using a boolean to keep "whether we've created rp_thread".

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210722175841.938739-2-peterx@redhat.com>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -2867,6 +2867,7 @@ static int open_return_path_on_source(MigrationState *ms,
 
     qemu_thread_create(&ms->rp_state.rp_thread, "return path",
                        source_return_path_thread, ms, QEMU_THREAD_JOINABLE);
+    ms->rp_state.rp_thread_created = true;
 
     trace_open_return_path_on_source_continue();
 
@@ -2891,6 +2892,7 @@ static int await_return_path_close_on_source(MigrationState *ms)
     }
     trace_await_return_path_close_on_source_joining();
     qemu_thread_join(&ms->rp_state.rp_thread);
+    ms->rp_state.rp_thread_created = false;
     trace_await_return_path_close_on_source_close();
     return ms->rp_state.error;
 }
@@ -3170,7 +3172,7 @@ static void migration_completion(MigrationState *s)
      * it will wait for the destination to send it's status in
      * a SHUT command).
      */
-    if (s->rp_state.from_dst_file) {
+    if (s->rp_state.rp_thread_created) {
         int rp_error;
         trace_migration_return_path_end_before();
         rp_error = await_return_path_close_on_source(s);

migration/migration.h

@@ -195,6 +195,13 @@ struct MigrationState {
         QEMUFile     *from_dst_file;
         QemuThread    rp_thread;
         bool          error;
+        /*
+         * We can also check non-zero of rp_thread, but there's no "official"
+         * way to do this, so this bool makes it slightly more elegant.
+         * Checking from_dst_file for this is racy because from_dst_file will
+         * be cleared in the rp_thread!
+         */
+        bool          rp_thread_created;
         QemuSemaphore rp_sem;
     } rp_state;