unit tests for src/common/buffer.{cc,h}

The following static constructors are implemented by opaque classes
defined in buffer.cc ( buffer::raw_char, buffer::raw_posix_aligned
etc. ). Testing the implementation of these classes is done by
variations of the calls to the static constructors.

    copy(const char *c, unsigned len);
    create(unsigned len);
    claim_char(unsigned len, char *buf);
    create_malloc(unsigned len);
    claim_malloc(unsigned len, char *buf);
    create_static(unsigned len, char *buf);
    create_page_aligned(unsigned len);

The raw_mmap_pages class cannot be tested because it is commented out in
raw_posix_aligned. The raw_hack_aligned class is only tested under Cygwin.
The raw_posix_aligned class is not tested under Cygwin.

The unittest_bufferlist.sh script calls unittest_bufferlist with the
CEPH_BUFFER_TRACK=true environment variable to enable the code
tracking the memory usage. It cannot be done within the bufferlist.cc
file itself because it relies on the initialization of a global
variable  ( buffer_track_alloc ).

When raw_posix_aligned is called on DARWIN, the data is not aligned
on CEPH_PAGE_SIZE because it calls valloc(size) which is the equivalent of
memalign(sysconf(_SC_PAGESIZE),size) and not memalign(CEPH_PAGE_SIZE,size).
For this reason the alignment test is de-activated on DARWIN.

The tests are grouped in

TEST(BufferPtr, ... ) for buffer::ptr
TEST(BufferListIterator, ...) for buffer::list::iterator
TEST(BufferList, ...) for buffer::list
TEST(BufferHash, ...) for buffer::hash

and each method ( and all variations of the prototype ) are
included into a single TEST() function.

Although most aspects of the methods are tested, including exceptions
and border cases, inconsistencies are not highlighted . For
instance

    buffer::list::iterator i;
    i.advance(1);

would dereference a buffer::raw NULL pointer although

    buffer::ptr p;
    p.wasted()

asserts instead of dereferencing the buffer::raw NULL pointer. It
would be better to always assert in case a NULL pointer is about to be
used. But this is a minor inconsistency that is probably not worth a
test.

The following buffer::list methods

    ssize_t read_fd(int fd, size_t len);
    int write_fd(int fd) const;

are not fully tested because the border cases cannot be reliably
reproduced. Going thru a pointer indirection when calling the ::writev
or safe_read functions would allow the test to create mockups to synthetize
the conditions for border cases.

tracker.ceph.com/issues/4066 refs #4066

Signed-off-by: Loic Dachary <loic@dachary.org>