[Libguestfs] libnbd completion callback question

[Eric Blake]

I noticed while reading the code that we have a documentation hole that 
may cause memory leaks for clients that are unaware, in relation to 
completion callbacks.

The situation arises as follows: for all commands with a completion 
callback, I checked that the code has clean semantics: either 
nbd_aio_FOO() returns -1 and we never call the callback cleanup, or 
nbd_aio_FOO() returns a cookie and we call the callback cleanup when the 
command is retired.  And since completion callbacks can only ever be 
passed to nbd_aio_FOO() functions, it is simple enough to document that 
when nbd_aio_FOO() fails (possible when calling them while the state 
machine is in the wrong state, or with invalid flags, or a command that 
the server is unwilling to accept - all things we can check client-side 
without traffic to the server), then the callback function was NOT 
registered, and the user must clean any resources then and there to 
avoid a leak.  Only when nbd_aio_FOO() succeeds will the responsibility 
for cleanup be handled by the callback.

But for nbd_block_status and nbd_pread_structured, we have a second 
callback.  And _these_ callbacks have a problem: we can return -1 for 
two different reasons, either because the command was never attempted 
(nbd_aio_FOO failed) and so the callback will never be cleaned, or 
because the command got sent to the server and the server failed it (the 
callback WILL be cleaned).  As this is ambiguous, it risks being a 
memory leak.  So I think we have a bug in our code, and need to 
strengthen our promise of whether the cleanup callback will be called, 
regardless of success or failure, while still minimizing any incompat 
changes that might cause a double-free for existing clients.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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