# `bitcoin-mainsignals`

The `bitcoin-mainsignals` crate is a Rust
translation of the `mainsignals` module of the
Bitcoin codebase, responsible for signaling events
and calling registered callbacks when specific
actions occur within the Bitcoin system.

This crate provides the `MainSignalsInstance`
struct, which represents an instance of the
`MainSignals` class in the Bitcoin system. The
`MainSignalsInstance` struct is used to register
and unregister callbacks for various events, such
as the addition or removal of transactions from
the memory pool, or the connection or
disconnection of a new block to the blockchain.

The `MainSignalsInstance` struct also provides
functions for iterating over all currently
registered callbacks, and for clearing all
currently registered callbacks.

Additionally, the `MainSignalsInstance` struct
provides functions for registering and
unregistering background signal schedulers, which
are responsible for periodically calling certain
registered callbacks.

Note that this crate is part of a direct
translation of the Bitcoin codebase from C++ to
Rust, and some function bodies may still be in the
process of being translated.

In the Bitcoin system, the `MainSignals` module is
an important part of the overall event signaling
and callback system. Its proper use and
integration is critical to the correct functioning
of the system.

98> which callbacks do we register in the bitcoin
system?

In the context of the `bitcoin-mainsignals` crate,
callbacks can be registered for various events
that occur in the Bitcoin system. Some of these
include:

- `block_checked`: Called when a block has been
  checked for validity.

- `block_connected`: Called when a block has been
  connected to the blockchain.

- `block_disconnected`: Called when a block has
  been disconnected from the blockchain.

- `chain_state_flushed`: Called when the chain
  state has been flushed to disk.

- `transaction_added_to_mempool`: Called when
  a transaction has been added to the mempool.

- `transaction_removed_from_mempool`: Called when
  a transaction has been removed from the mempool.

- `updated_block_tip`: Called when the block tip
  of the blockchain has been updated.

These callbacks can be used to perform additional
actions or trigger other events based on these
system events.

99> what is a signal scheduler and how does it
work?

In the context of the Bitcoin system, a signal
scheduler is responsible for executing callbacks
in response to specific signals. It is essentially
a mechanism for asynchronous event handling.

When a signal is triggered, the signal scheduler
identifies any callbacks that have been registered
for that particular signal and adds them to a list
of pending callbacks. The scheduler then executes
each callback in turn, passing along any relevant
data as arguments.

The signal scheduler in Bitcoin uses a background
thread to execute callbacks, which allows the main
thread to continue processing other tasks without
being blocked. The background thread waits for
pending callbacks to be added to the list and then
executes them in the order they were added.

In addition to registering callbacks for specific
signals, it is also possible to register
a background signal scheduler. This scheduler will
execute callbacks on a separate thread, allowing
the main thread to continue processing other tasks
without being blocked.

100> thanks -- are there other event callbacks we
are interested in?

Yes, there are other event callbacks that we might
be interested in. Some examples include:

- `transaction_added_to_mempool`: This event is
  triggered when a new transaction is added to the
  mempool.

- `transaction_removed_from_mempool`: This event
  is triggered when a transaction is removed from
  the mempool.

- `block_connected`: This event is triggered when
  a new block is connected to the main chain.

- `block_disconnected`: This event is triggered
  when a block is disconnected from the main
  chain.

- `block_checked`: This event is triggered when
  a block is checked for validity.

- `updated_block_tip`: This event is triggered
  when the current block tip is updated.

- `chain_state_flushed`: This event is triggered
  when the chain state is flushed to disk.

These events allow us to keep track of important
changes in the bitcoin system and to perform
certain actions in response to those changes.

101> is there a difference between a signal and an
event?

In the context of the Bitcoin system, the terms
"signal" and "event" are often used
interchangeably. They both refer to a notification
that something has happened or is about to happen
within the Bitcoin software, and provide a way for
external code to respond to those changes.

However, in general programming terminology, there
can be some differences between signals and
events. In some systems, a signal refers to a type
of interrupt or notification that is generated by
the operating system, while an event is
a notification that is generated by a specific
application or program. However, this distinction
is not always consistent across different systems
or programming languages.