common/Preforker: shut up warning

common/Preforker.h: In member function 'void Preforker::daemonize()':
common/Preforker.h:97:40: warning: ignoring return value of 'ssize_t write(int, const void*, size_t)', declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Sage Weil <sage@inktank.com>
osd: do not return data payload for successful writes

We were somewhat inadvertantly returning a data payload for write
operations.  This was a side-effect of the OpContext::ops field being a
reference to MOSDOp::ops: the return data would end up there, and then
the MOSDOpReply ctor would copy it.

Fix this by breaking the ref, and making the do_op() logic also claim
return result data for error values (so that errors can return data to the
caller).

Signed-off-by: Sage Weil <sage@inktank.com>
librbd: remove mostly-useless assign_bid helper

Do it inline.

Signed-off-by: Sage Weil <sage@inktank.com>
cls_rbd: remove old assign_bid method

This method is problematic because it both writes/mutates and returns data,
which means that an untimely client disconnect or peering event will result
in a success to the client with no payload.

It has not been used since v0.52 (18054ba46fe2779d8df8b1a0d69ec93ca6a66c34)
which is pre-bobtail; so this change breaks compatibility with pre-bobtail
librbd clients (at least for image creation).

Signed-off-by: Sage Weil <sage@inktank.com>
osd: enforce RD, WR flags for class methods

Class methods are marked with RD and WR to help the OSD decide when we need
to flush objects or require certain permissions.  Ensure that methods do
not step outside their advertised capabilities by keeping a counter of rd
and wr ops we perform in do_osd_ops() and making sure that class methods,
and any ops the indirectly call, do not break the rules.

Signed-off-by: Sage Weil <sage@inktank.com>
cls/hello: hello, world rados class

Simple example of a rados class doing read, write, and read/modify/write
methods.

Signed-off-by: Sage Weil <sage@inktank.com>