Re: [Libguestfs] [RFC nbdkit PATCH] multi-conn: New filter

Implement a TODO item of emulating multi-connection consistency via multiple plugin flush calls to allow a client to assume that a flush on a single connection is good enough. This also gives us some fine-tuning over whether to advertise the bit, including some setups that are unsafe but may be useful in timing tests. Testing is interesting: I used the sh plugin to implement a server that intentionally keeps a per-connection cache. Note that this filter assumes that multiple connections will still share the same data (other than caching effects); effects are not guaranteed when trying to mix it with more exotic plugins like info that violate that premise. ---

+static int +multi_conn_flush (struct nbdkit_next_ops *next_ops, void *nxdata, + void *handle, uint32_t flags, int *err) +{ + struct handle *h = handle, *h2; + size_t i; + + if (h->mode == EMULATE) { + /* Optimize for the common case of a single connection: flush all + * writes on other connections, then flush both read and write on + * the current connection, then finally flush all other + * connections to avoid reads seeing stale data, skipping the + * flushes that make no difference according to dirty tracking. + */ + bool updated = h->dirty & WRITE; + + ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); + for (i = 0; i < conns.size; i++) { + h2 = conns.ptr[i]; + if (h == h2) + continue; + if (dirty || h2->dirty & WRITE) { + if (h2->next_ops->flush (h2->nxdata, flags, err) == -1)