Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
Introduce a syntax helper tagged_id::create_random_id(),
used to create a new Raft server or group id.
Provide a default ordering for tagged ids, for use
in Raft leader discovery, which selects the smallest
id for leader.
Changes to the `configuration` and `tagged_uint64` classes are needed
to overcome limitations of the IDL compiler tool, i.e. we need to
supply a constructor to the struct initializing all the
members (raft::configuration) and also need to make an accessor
function for private members (in case of raft::tagged_uint64).
All other structs mirror raft definitions in exactly the same way
they are declared in `raft.hh`.
`tagged_id` and `tagged_uint64` are used directly instead of their
typedef-ed companions defined in `raft.hh` since we don't want
to introduce indirect dependencies. In such case it can be guaranteed
that no accidental changes made outside of the idl file will affect idl
definitions.
This patch also fixes a minor typo in `snapshot_id_tag` struct used
in `snapshot_id` typedef.
This commit introduce public raft interfaces. raft::server represents
single raft server instance. raft::state_machine represents a user
defined state machine. raft::rpc, raft::rpc_client and raft::storage are
used to allow implementing custom networking and storage layers.
A shared failure detector interface defines keep-alive semantics,
required for efficient implementation of thousands of raft groups.
