migration: Keep s->rp_state.from_dst_file open until migration ends

This will be used in the next patch to allow COLO to reuse the return path.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260302-colo_unit_test_multifd-v11-20-d653fb3b1d80@web.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/migration.c

@@ -97,7 +97,8 @@ static GSList *migration_blockers[MIG_MODE__MAX];
 
 static bool migration_object_check(MigrationState *ms, Error **errp);
 static bool migration_switchover_start(MigrationState *s, Error **errp);
-static bool close_return_path_on_source(MigrationState *s);
+static bool stop_return_path_thread_on_source(MigrationState *s);
+static void migration_release_dst_files(MigrationState *ms);
 static void migration_completion_end(MigrationState *s);
 
 static void migration_downtime_start(MigrationState *s)
@@ -1281,7 +1282,8 @@ static void migration_cleanup(MigrationState *s)
     cpr_state_close();
     cpr_transfer_source_destroy(s);
 
-    close_return_path_on_source(s);
+    stop_return_path_thread_on_source(s);
+    migration_release_dst_files(s);
 
     if (s->migration_thread_running) {
         bql_unlock();
@@ -2217,6 +2219,7 @@ static void migration_release_dst_files(MigrationState *ms)
      * locking needed because this qemufile should only be managed by
      * return path thread.
      */
+    assert(!ms->rp_state.rp_thread_created);
     if (ms->postcopy_qemufile_src) {
         migration_ioc_unregister_yank_from_file(ms->postcopy_qemufile_src);
         qemu_file_shutdown(ms->postcopy_qemufile_src);
@@ -2224,7 +2227,9 @@ static void migration_release_dst_files(MigrationState *ms)
         ms->postcopy_qemufile_src = NULL;
     }
 
-    qemu_fclose(file);
+    if (file) {
+        qemu_fclose(file);
+    }
 }
 
 /*
@@ -2410,7 +2415,7 @@ static void open_return_path_on_source(MigrationState *ms)
 }
 
 /* Return true if error detected, or false otherwise */
-static bool close_return_path_on_source(MigrationState *ms)
+static bool stop_return_path_thread_on_source(MigrationState *ms)
 {
     if (!ms->rp_state.rp_thread_created) {
         return false;
@@ -2432,7 +2437,6 @@ static bool close_return_path_on_source(MigrationState *ms)
 
     qemu_thread_join(&ms->rp_state.rp_thread);
     ms->rp_state.rp_thread_created = false;
-    migration_release_dst_files(ms);
     trace_migration_return_path_end_after();
 
     /* Return path will persist the error in MigrationState when quit */
@@ -2795,7 +2799,7 @@ static void migration_completion(MigrationState *s)
         goto fail;
     }
 
-    if (close_return_path_on_source(s)) {
+    if (stop_return_path_thread_on_source(s)) {
         goto fail;
     }
 
@@ -2949,7 +2953,8 @@ static MigThrError postcopy_pause(MigrationState *s)
          * path and just wait for the thread to finish. It will be
          * re-created when we resume.
          */
-        close_return_path_on_source(s);
+        stop_return_path_thread_on_source(s);
+        migration_release_dst_files(s);
 
         /*
          * Current channel is possibly broken. Release it.  Note that this is