fallocate does not prevent ENOSPC on write

[Jakob Unterwurzacher]

I have a user who is reporting ENOSPC errors when running gocryptfs on
top of btrfs (ticket: https://github.com/rfjakob/gocryptfs/issues/395 ).

What is interesting is that the error gets thrown at write time. This
is not supposed to happen, because gocryptfs does

    fallocate(..., FALLOC_FL_KEEP_SIZE, ...)

before writing.

I wrote a minimal reproducer in C: https://github.com/rfjakob/fallocate_write
This is what it looks like on ext4:

    $ ../fallocate_write/fallocate_write
    reading from /dev/urandom
    writing to ./blob.379Q8P
    writing blocks of 132096 bytes each
    [...]
    fallocate failed: No space left on device

On btrfs, it will instead look like this:

    [...]
    pwrite failed: No space left on device

Is this a bug in btrfs' fallocate implementation or am I reading the
guarantees that fallocate gives me wrong?

Thanks!
Jakob