[Libguestfs] [nbdkit PATCH v3 14/15] todo: Mention possibility of caching .can_FOO callbacks

[Eric Blake]

Recent patches clarified documentation to point out that within
the life of a single connection, the .can_FOO helpers should
return consistent results, and that callers may cache those
results.  But at least in the case of .can_fua, we aren't really
caching things; depending on the overhead involved, calling out
to the plugin's .can_fua on every .pwrite with FUA requested
may be noticeable overhead compared to caching it.  Any cache
must not be a static variable, as it can differ between
connections.

Signed-off-by: Eric Blake <eblake at redhat.com>
---
 TODO | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/TODO b/TODO
index b6fb0b1..1e0f483 100644
--- a/TODO
+++ b/TODO
@@ -40,6 +40,10 @@ General ideas for improvements
   ones like offset) can fail to initialize if they can't guarantee
   strict alignment and don't want to deal with bounce buffers.

+* Add per-connection caching of .can_FOO callbacks (we already have
+  some: .can_write is only called once, but .can_fua is called on
+  every request with the FUA flag set).
+
 Suggestions for plugins
 -----------------------

-- 
2.14.3