This is useful for deallocating disk blocks as part of an alternate disk
file implementation.
Additionally, add an offset argument to the existing fallocate utility
function; this allows you to grow an existing file.
Sam always had the best descriptions:
utils.fallocate(fd, size) allocates <size> bytes for the file referred
to by <fd>. It allows for keeping a reserve of an additional N bytes
or X% of the filesystem free. If neither fallocate() or
posix_fallocate() C functions are avaialble, utils.fallocate() will
log a warning (once only) and not actually allocate space.
utils.punch_hole(fd, offset, length) deallocates <length> bytes
starting at <offset> from the file referred to by <fd>. It uses the C
function fallocate(). If fallocate() is not available, calls to
utils.punch_hole() will raise an exception.
Since these both use the fallocate syscall, refactor that a bit and get
rid of FallocateWrapper. We add a new _LibcWrapper to do some
lazy-loading of a C function and expose whether the function is actually
available in Python, though. This allows utils.fallocate and
utils.punch_hole to keep their fancy logic pretty well-contained.
Modernized the tests for utils.fallocate() and utils.punch_hole().
Co-Authored-By: Samuel Merritt <sam@swiftstack.com>
Change-Id: Ieac30a477d784905c94742ee3d0898d7e0194b39
dbacdcf01c