brick_server new client API - copy/6

[tatsuya6502]

To help a client application to implement "copy on write" snapshot feature, add the following lightweight server-side operations. These operations should be very efficient because they are metadata-only operations and do not have to read/copy the value on a disk.

• copy/6 - copy the value and user metadata to other key(s).
  • copy(table(), From::key(), To::key(), exp_time(), [do_op_flag()], timeout())
  • copy(table(), From::key(), To::[key()], exp_time(), [do_op_flag()], timeout())

Limitation:

• From::key() and To:key() must have the same key prefix (they must be stored in the same chain.)

Design Notes:

• To be a lightweight operation, use in-memory operation ?KEY_SWITCHAROO to copy only the pointer to the value.
• When an hlog file containing From key is frozen, do not to use ?KEY_SWITCHAROO. Instead, do a disk-based operation; load the value of From key from an hlog on a disk and put the value to To key(s). More details on https://github.com/hibari/hibari/issues/33#issuecomment-31649930
• Maybe we want remove rename/6 operation from brick_server? The same thing can be done by a micro-transaction: do([txn, copy(OldKey, NewKey), delete(OldKey)]) Note that the current implementation of rename/6 is not an equivalent of the micro-transaction but a simple do operation: do([copy(OldKey, NewKey), delete(OldKey)]) One good thing to keep rename/6 is that it has its own operation counter for the brick operation statistics, but it does not seem to be a must-have feature.

[tatsuya6502]

Set the target milestone to v0.3.0.