Description of problem:
When calling `next_ops->pread(nxdata, ...)` from the .prepare method of an nbdkit-filter without calling `next_ops->get_size(nxdata)` first, the assertion
assert (h->exportsize <= INT64_MAX); /* Guaranteed by negotiation phase */
in nbdkit/server/backend.c/backend_valid_range() will fail, resulting in an error message which is hard to understand. The "Guaranteed by negotiation phase" comment wasn't really helpful either, what did lead to the solution was reading more of the source code and git blame. In fact, the documentation for .prepare in `man nbdkit-filter` suggests that calling `next_ops->pread` should simply work.
Maybe this expected, in which case I think documentation in `man nbdkit-filter` should mention the need to call `get_size()` manually.
The check exists because pread wants to ensure the request is in-bounds, with a sane error reported if not, but get_size failure does not guarantee a sane error return. So yes, I'm leaning towards this being a documentation flaw.
Since all of our filters are in-tree, it's not hard to update them to whatever semantics we find easiest - if we think .pread should just work (by calling .get_size as needed), we could do that. But merely documenting that all filters must ensure .get_size has been called first (which is automatic in most cases, but tricky in .prepare), is reasonable.
Comment 2Richard W.M. Jones
2020-07-09 15:20:24 UTC
See also:
https://www.redhat.com/archives/libguestfs/2020-July/msg00041.html
I don't think we can remove the restriction, since get_size
really must be called (either by .prepare or .get_size of the filter),
but we can probably make the error easier to understand and/or
document the problem.
Description of problem: When calling `next_ops->pread(nxdata, ...)` from the .prepare method of an nbdkit-filter without calling `next_ops->get_size(nxdata)` first, the assertion assert (h->exportsize <= INT64_MAX); /* Guaranteed by negotiation phase */ in nbdkit/server/backend.c/backend_valid_range() will fail, resulting in an error message which is hard to understand. The "Guaranteed by negotiation phase" comment wasn't really helpful either, what did lead to the solution was reading more of the source code and git blame. In fact, the documentation for .prepare in `man nbdkit-filter` suggests that calling `next_ops->pread` should simply work. Maybe this expected, in which case I think documentation in `man nbdkit-filter` should mention the need to call `get_size()` manually.