ErasureCode: abstract interface

The erasure coded pool relies on this abstract interface to encode and
decode the chunks stored in the OSD. It has been designed to be
generic enough to accomodate the libraries and algorithms that are
most likely to be used. It does not claim to be universal.

http://tracker.ceph.com/issues/5878 refs #5878

Signed-off-by: Loic Dachary <loic@dachary.org>
ErasureCode: example implementation : K=2 M=1

An erasure code implementation designed for tests. Although it is fully
functional and could be used on actual data, it is mainly provided for
testing purposes. It splits data in two, computes an XOR parity and
can sustain the loss of one chunk.

The constructor will usleep(3) for parameters["usleep"] microseconds
so that the caller can create race conditions.

http://tracker.ceph.com/issues/5878 refs #5878

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

When dynamically loaded, a plugin is expected to define

    int __erasure_code_init(char *plugin_name);

When called, it is responsible for registering an ErasureCodePlugin
derived object that provides a factory method from which the concrete
implementation of the ErasureCodeInterface object can be generated:

  virtual int factory(const map<std::string,std::string> &parameters,
                      ErasureCodeInterfaceRef *erasure_code) {
    *erasure_code = ErasureCodeInterfaceRef(new ErasureCodeExample(parameters));
    return 0;
  }

The plugin instance contains the library data member which is used to
store the handle of the shared library. It is opaque to the plugin.

http://tracker.ceph.com/issues/5878 refs #5878

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

A ErasureCodePluginRegistry singleton holds all erasure plugin objects
derived from ErasureCodePlugin and dlopen(2) handles for the lifetime
of the OSD and is cleaned up by the destructor.

The registry has a single entry point ( method factory ) and should
be used as follows:

  map<std::string,std::string> parameters;
  parameters["directory"] = "/usr/lib/ceph/erasure-code";
  ErasureCodeInterfaceRef erasure_code;
  ErasureCodePluginRegistry &instance = ErasureCodePluginRegistry::instance();
  instance.factory("jerasure", parameters, &erasure_code));

If the plugin requested ( "jerasure" in the example above ) is not
found in the *plugins* data member, the load method is called and will:

  * dlopen(parameters["erasure-code-directory"] + "jerasure")
  * f = dlsym("__erasure_code_init")
  * f("jerasure")
  * check that it registered "jerasure"

The plugin is expected to do something like

  instance.add(plugin_name, new ErasureCodePluginJerasure());

to register itself.

The factory method is protected with a Mutex to avoid race
conditions when using the same plugin from two threads.

The erasure_codelib_LTLIBRARIES variable is added to the Makefile
and the plugins are expected to add themselves and be installed
in the $(libdir)/erasure-code

http://tracker.ceph.com/issues/5878 refs #5878

Signed-off-by: Loic Dachary <loic@dachary.org>
ErasureCodePlugin: plugin registry tests and example

libec_example.la is a fully functional plugin based on
ErasureCodeExample to test the ErasureCodePlugin abstract
interface. It is dynamically loaded to test the
ErasureCodePluginRegistry implementation.

Although the plugin is built in the test directory, it will be
installed. noinst_LTLIBRARIES won't build the shared library, only the
static version which is not suitable for testing.

http://tracker.ceph.com/issues/5878 refs #5878

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #518 from dachary/wip-5878

erasure plugin mechanism and abstract API

Reviewed-by: Samuel Just <sam.just@inktank.com>