Libguestfs - Feb 2021 - [nbdkit PATCH] ext2: Disable .can_multi_conn

Although passing through the plugin's multi-conn status happens to be
safe due to our current .thread_model forbidding parallel connections,
explicitly overriding it now will help us remember to think about the
issue if we later improve the filter to allow parallelism.
---
 filters/ext2/ext2.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/filters/ext2/ext2.c b/filters/ext2/ext2.c
index 549ef5ed..05ff634e 100644
--- a/filters/ext2/ext2.c
+++ b/filters/ext2/ext2.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2020 Red Hat Inc.
+ * Copyright (C) 2017-2021 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -305,6 +305,20 @@ ext2_can_cache (struct nbdkit_next_ops *next_ops, void
*nxdata, void *handle)
   return NBDKIT_CACHE_EMULATE;
 }

+static int
+ext2_can_multi_conn (struct nbdkit_next_ops *next_ops, void *nxdata,
+                     void *handle)
+{
+  /* Since we do not permit parallel connections, it does not matter
+   * what we advertise here, and we could just as easily inherit the
+   * plugin's .can_multi_conn for now.  But realistically, if we
+   * adjust .thread_model, we cannot advertise support unless .flush
+   * is consistent, and that would require inspecting the ext2 source
+   * code, so for now, we hard-code a safe answer.
+   */
+  return 0;
+}
+
 /* It might be possible to relax this, but it's complicated.
  *
  * It's desirable for ?nbdkit -r? to behave the same way as
@@ -470,6 +484,7 @@ static struct nbdkit_filter filter = {
   .close              = ext2_close,
   .can_fua            = ext2_can_fua,
   .can_cache          = ext2_can_cache,
+  .can_multi_conn     = ext2_can_multi_conn,
   .export_description = ext2_export_description,
   .get_size           = ext2_get_size,
   .pread              = ext2_pread,
-- 
2.30.1