Make the statbuf field public. This means you no longer have to use
req->ptr - though that still works and will continue to work for the
foreseeable future.
Fixes#704.
`#if FOO` (where FOO is undefined) is a legal construct in C89 and C99
but gcc, clang and sparse complain loudly about it at higher warning
levels.
Squelch those warnings. Makes the code more consistent as well.
uv_fs_poll_t has an embedded uv_timer_t handle that got closed at a time when
the memory of the encapsulating handle might already have been deallocated.
Solve that by moving the poller's state into a structure that is allocated on
the heap and can be freed independently.
* the callback gets called only once on error, not repeatedly...
* ...unless the error reason changes from e.g. UV_ENOENT to UV_EACCES
* the callback receives pointers to uv_statbuf_t objects so it can inspect what
changed
