lightning/doc/HACKING.md

Welcome, fellow coder!

This repository contains a code to run a lightning protocol daemon. It's broken into subdaemons, with the idea being that we can add more layers of separation between different clients and extra barriers to exploits.

It is designed to implement the lightning protocol as specified in various BOLTs.

Getting Started

It's in C, to encourage alternate implementations. Patches are welcome! You should read our Style Guide.

To read the code, you'll probably need to understand ccan/tal: it's a hierarchical memory allocator, where each allocation has a parent, and thus lifetimes are grouped. eg. a struct bitcoin_tx has a pointer to an array of struct bitcoin_tx_input; they are allocated off the struct bitcoind_tx, so freeing the struct bitcoind_tx frees them all. Tal also supports destructors, which are usually used to remove things from lists, etc.

Some routines use take(): take() marks a pointer as to be consumed (e.g. freed automatically before return) by a called function. It can safely accept NULL pointers. Functions whose prototype in headers has the macro TAKES can have the specific argument as a take() call. Use this sparingly, as it can be very confusing.

The more complex daemons use async io (ccan/io): you register callbacks and they happen once I/O is available, then you return what to do next. This does not use threads, so the code flow is generally fairly simple.

The Components

Here's a list of parts, with notes:

• ccan - useful routines from http://ccodearchive.net

  • Use make update-ccan to update it.
  • Use make update-ccan CCAN_NEW="mod1 mod2..." to add modules
  • Do not edit this! If you want a wrapper, add one to common/utils.h.

• bitcoin/ - bitcoin script, signature and transaction routines.

  • Not a complete set, but enough for our purposes.

• external/ - external libraries from other sources

  • libbacktrace - library to provide backtraces when things go wrong.
  • libsodium - encryption library (should be replaced soon with built-in)
  • libwally-core - bitcoin helper library
  • secp256k1 - bitcoin curve encryption library within libwally-core
  • jsmn - tiny JSON parsing helper
  • libbase58 - base58 address encoding/decoding library.

• tools/ - tools for building

  • check-bolt.c: check the source code contains correct BOLT quotes (as used by check-source)
  • generate-wire.py: generate marshal/unmarshal routines from extracts from BOLT specs, and as specified by subdaemons.
  • mockup.sh / update-mocks.sh: tools to generate mock functions for unit tests.

• devtools/ - tools for developers

  • Currently just bolt11-cli for decoding bolt11

• contrib/ - python support and other stuff which doesn't belong :)

• wire/ - basic marshalling/un

• common/ - routines needed by any two or more of the directories below

• cli/ - commandline utility to control lightning daemon.

• lightningd/ - master daemon which controls the subdaemons and passes peer file descriptors between them.

• wallet/ - database code used by master for tracking what's happening.

• hsmd/ - daemon which looks after the cryptographic secret, and performs commitment signing.

• gossipd/ - daemon to chat to peers which don't have any channels, and maintains routing information and broadcasts gossip.

• openingd/ - daemon to open a channel for a single peer.

• channeld/ - daemon to operate a single peer once channel is operating normally.

• closingd/ - daemon to handle mutual closing negotiation with a single peer.

• onchaind/ - daemon to hand a single channel which has had its funding transaction spent.

Debugging

You can debug crashing subdaemons with the argument --dev-debugger=lightning_channeld, where channeld is the subdaemon name. It will print out (to stderr) a command such as:

gdb -ex 'attach 22398' -ex 'p debugger_connected=1' \
  lightningd/lightning_hsmd

Run this command to start debugging. You may need to type return one more time to exit the infinite while loop, otherwise you can type continue to begin.

Database

c-lightning state is persisted in lightning-dir. It is a sqlite database stored in the lightningd.sqlite3 file, typically under ~/.lightning. You can run queries against this file like so:

$ sqlite3 ~/.lightning/lightningd.sqlite3 \
  "SELECT HEX(prev_out_tx), prev_out_index, status FROM outputs"

Or you can launch into the sqlite3 repl and check things out from there:

$ sqlite3 ~/.lightning/lightningd.sqlite3
SQLite version 3.21.0 2017-10-24 18:55:49
Enter ".help" for usage hints.
sqlite> .tables
channel_configs  invoices         peers            vars
channel_htlcs    outputs          shachain_known   version
channels         payments         shachains
sqlite> .schema outputs
...

Some data is stored as raw bytes, use HEX(column) to pretty print these.

Make sure that clightning is not running when you query the database, as some queries may lock the database and cause crashes.

Common variables

Table vars contains global variables used by lightning node.

$ sqlite3 ~/.lightning/lightningd.sqlite3
SQLite version 3.21.0 2017-10-24 18:55:49
Enter ".help" for usage hints.
sqlite> .headers on
sqlite> select * from vars;
name|val
next_pay_index|2
bip32_max_index|4
...

Variables:

• next_pay_index next resolved invoice counter that will get assigned.
• bip32_max_index last wallet derivation counter.

Note: Each time newaddr command is called, bip32_max_index counter is increased to the last derivation index. Each address generated after bip32_max_index is not included as lightning funds.

Testing

There are three kinds of tests. For best results, you should have valgrind installed, and build with DEVELOPER=1.

• source tests - run by make check-source, looks for whitespace, header order, and checks formatted quotes from BOLTs if BOLTDIR exists (currently disabled, since BOLTs are being re-edited).

• unit tests - run by make check, these are run-*.c files in test/ subdirectories which can test routines inside C source files. You should insert /* AUTOGENERATED MOCKS START */ and /* AUTOGENERATED MOCKS END */ lines, and make update-mocks will automatically generate stub functions which will allow you to link (which will conveniently crash if they're called).

• blackbox tests - run by make check or directly as PYTHONPATH=contrib/pylightning python3 tests/test_lightningd.py -f. You can run these much faster by putting VALGRIND=0 after make check, which has the added bonus of doing memory leak detection. You can also append LightningDTests.TESTNAME to run a single test.

Our Travis CI instance (see .travis.yml) runs all these for each pull request.

Subtleties

There are a few subtleties you should be aware of as you modify deeper parts of the code:

• structeq will not work on some structures. For example, it will not work with struct short_channel_id --- use short_channel_id_eq for comparing those.
• command_success, command_fail, and command_fail_detailed will free the cmd you pass in. This also means that if you tal-allocated anything from the cmd, they will also get freed at those points and will no longer be accessible afterwards.
• When making a structure part of a list, you will instance a struct list_node. This has to be the first field of the structure, or else dev-memleak command will think your structure has leaked.

Further Information

Feel free to ask questions on the lightning-dev mailing list, or on #c-lightning on IRC, or email me at rusty@rustcorp.com.au.

Cheers!
Rusty.