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Debug (dbg)

Debugging on steroids for Minetest mods

Motivation

Lua offers very powerful introspective facilities through its debug library, which unfortunately almost always go unused due to their clunky APIs.

Current state-of-the-art in Minetest mod debugging appears to be print/log/chat "debugging" using serialization or dump, all of which should be rendered obsolete by dbg.

API

Optionally depend on dbg in mod.conf to ensure that it is available for load-time debugging.

dbg()

Shorter alias for dbg.dd().

debug.debug() on steroids: Starts a REPL (read-eval-print-loop); equivalent to a breakpoint. Features include:

  • Full access to locals & upvalues of the calling function
  • Own local table _L where local debug vars get stored
  • Ability to enter expressions (rather than statements)
  • Continuation, which works the same as in the Lua REPL (+ empty lines working)
  • Pretty-printing using dbg.pp, including syntax highlighting

Enter cont to exit without an error. Use err to throw after error debugging sessions (dbg.error, dbg.assert).

Use EOF (Ctrl + D on Unix) to exit & shut down Minetest.

dbg.error(message, [level])

Starts a debugging session at the given (optional) level, printing the error message.

dbg.assert(value, [message])

Returns value. Starts an error debugging session if value is falsey. Error message is optional.

dbg.pp(...)

Pretty-prints the given vararg using the default parameters.

If the argument list of functions is unreliable (see dbg.getargs_reliable below), a question mark (?) will be appended to the argument list to indicate this.

dbg.ppp(params, ...)

Parameterized pretty-print. Requires a custom pretty-printer parameter table params:

  • write = function(str, token_type), where token_type is optional and may be one of nil, boolean, number, string, reference, function or type
  • upvalues = true, whether upvalues should be written

dbg.vars(level)

Returns a virtual variable table of locals & upvalues vars for the given stacklevel that supports the following operations:

  • Getting: vars.varname
  • Setting: vars.varname = value
  • Looping: for varname, value in vars() do ... end

dbg.locals(level)

Returns a virtual variable table of local values at the given stack level.

Locals include upvalues.

dbg.upvals(func)

func may be either a function or a stack level (including nil, which defaults to the stack level of the calling function).

Returns a virtual variable table of upvalues at the given stack level.

dbg.traceback(level)

Formats a stack trace starting at level. Similar to Lua's builtin debug.stacktrace, but shortens paths and accepts no message to prepend.

dbg.stackinfo(level)

Returns a list of info by repeatedly calling debug.getinfo starting with level and working down the stack.

dbg.getvararg(level)

Only available on LuaJIT; on PUC Lua 5.1, dbg.getvararg will be nil.

Returns the vararg at the given stack level.

dbg.getargs(func)

Function parameter list detection doesn't work properly on PUC Lua 5.1; unused params are lost and varargs are turned into arg. Use dbg.getargs_reliable (boolean) to check for reliability.

Returns a table containing the argument names of func in string form (example: {"x", "y", "z", "..."} for function(x, y, z, ...) end).

dbg.shorten_path(path)

Shortens path: If path is a subpath of a mod, it will be shortened to "<modname>:<subpath>".

Security

Debug deliberately exposes the unrestricted debug API globally, as well as the dbg wrapper API, both of which can be abused to exit the mod security sandbox.

Only use dbg in environments where you trust all enabled mods. Adding dbg to secure.trusted_mods (recommended) or disabling mod security (not recommended) is required.

The /lua chatcommand must explicitly be enabled on servers by setting secure.dbg.lua to true; if enabled, server owners risk unprivileged users gaining access through MITM attacks.

Usage

Prerequisites: LuaJIT and a terminal with decent ANSI support are highly recommended.

/dbg

Calls dbg() to start debugging in the console.

/lua <code>

Executes the code and pretty-prints the result(s) to chat. Only available in singleplayer for security reasons (risk of MITM attacks).

Links: GitHub, ContentDB, Minetest Forums

License: Written by Lars Müller and licensed under the MIT license (see License.txt).