Tracebacks should include values of local variables

[aidanholm]

• possible privacy concern if URIs/bookmark/history details are included in tracebacks
  • it's difficult to detect which variables this applies to from a generic traceback()
• difficult to display some variables
  • functions (but these won't usually be relevant)
  • tables need to be summarized in some fashion

[taobert]

https://github.com/ignacio/StackTracePlus may be relevant.
I've not used it, but it seems to support local variables, including tables.
It doesn't print function source, (which appears to be a non-trivial exercise).
Implementation seems to be as easy as:

local STP = require "StackTracePlus"
debug.traceback = STP.stacktrace

With respect to privacy concerns,
i think that guessing which fields are sensitive would be impossible to get right,
and would only lead to Luakit devs being blamed when someone (inevitably) posts that data.
I think, rather, that it'd be better to leave the fancier stack trace off by default,
and allow it to be enabled by users when/if they need to collect that data,
and that there should be warnings (in the docs, setting description, console output, etc)
that data may be leaked if a stack trace is generated,
and extra warnings to read, understand and redact any stack traces posted.

Edit (at 20210321):
So, the two-line implementation above doesn't (seem to) work with luajit.
Based on a StackTracePlus pull request, adding the following to the top of rc.lua:

if not package.loaded.StackTracePlus then
    local STP = require 'StackTracePlus'
    debug.traceback = STP.stacktrace
    assert(                          --rethrows error
        xpcall(                      --calls main function
            debug.getinfo( 1 ).func, --retrieves main function from call stack
            STP.stacktrace, ... ))   --passes through all arguments (may not work with 5.1, should work with LuaJIT)
    return  --terminate, so control flow won't move on to running
            --the rest of the main function again (we already ran that)
end

changes this stack trace:

[    1.742987] E [core/common/lualib]: Lua error: assertion failed!
                 Traceback:
                 (1) [C]                                                     in function assert
                 (2) /home/tao/.config/luakit/plugins/tabhistory/init.lua:61 in function _attach_signals_to_notebook
                 (3) /home/tao/.config/luakit/plugins/tabhistory/init.lua:97 in function sigfunc
                 (4) /usr/local/share/luakit/lib/lousy/signal.lua:84 in function emit_signal
                 (5) /usr/local/share/luakit/lib/window.lua:756 in function new
                 (6) /usr/local/share/luakit/lib/session.lua:127 in function restore_file
                 (7) /usr/local/share/luakit/lib/session.lua:161 in function restore
                 (8) /home/tao/.config/luakit/rc.lua:231 in main chunk

to:

[    1.534240] E [core/common/lualib]: Lua error: /home/tao/.config/luakit/plugins/tabhistory/init.lua:61: assertion failed!
                 Stack Traceback
                 ===============
                 (1) global C function 'assert'
                 (2) Lua upvalue '_attach_signals_to_notebook' at file '/home/tao/.config/luakit/plugins/tabhistory/init.lua:61'
                        Local variables:
                 (3) Lua local 'sigfunc' at file '/home/tao/.config/luakit/plugins/tabhistory/init.lua:97'
                        Local variables:
                         w = table: 0x42003c10  {emit_signal:function: 0x4038aca0, ibar:table: 0x410dc6b0, tablist:table: 0x41ac6e70 (more...)}
                 (4) Lua field 'emit_signal' at file '/usr/local/share/luakit/lib/lousy/signal.lua:84'
                        Local variables:
                         object = table: 0x41e69698  {methods:table: 0x4038abe0, emit_signal:function: 0x4037b930, build:function: 0x40390500 (more...)}
                         signame = string: "init"
                         d = table: 0x41e5c540  {signals:table: 0x4037e750, module:true}
                         sigfuncs = table: 0x41455138  {1:function: 0x408dbb30, 2:function: 0x402a34a0, 3:function: 0x41286828 (more...)}
                         (for generator) = C function: 0x41e52ea0
                         (for state) = table: 0x41455138  {1:function: 0x408dbb30, 2:function: 0x402a34a0, 3:function: 0x41286828 (more...)}
                         (for control) = number: 9
                         _ = number: 9
                         sigfunc = Lua function '(anonymous)' (defined at line 92 of chunk /home/tao/.config/luakit/plugins/tabhistory/init.lua)
                         ret = nil
                         (*temporary) = table: 0x401c7060  {}
                 (5) Lua field 'new' at file '/usr/local/share/luakit/lib/window.lua:756'
                        Local variables:
                         args = table: 0x40c08428  {1:luakit://newtab/}
                         w = table: 0x42003c10  {emit_signal:function: 0x4038aca0, ibar:table: 0x410dc6b0, tablist:table: 0x41ac6e70 (more...)}
                 (6) Lua upvalue 'restore_file' at file '/usr/local/share/luakit/lib/session.lua:127'
                        Local variables:
                         file = string: "/home/tao/.config/luakit/session"
                         delete = nil
                         ok = boolean: true
                         wins = table: 0x41ccbd28  {1:table: 0x41c99488}
                         state = table: 0x4088d0f8  {}
                         w = nil
                         (for generator) = C function: 0x41e52ea0
                         (for state) = table: 0x41ccbd28  {1:table: 0x41c99488}
                         (for control) = number: 1
                         _ = number: 1
                         win = table: 0x41c99488  {tab_groups:table: 0x40d09228, closed:table: 0x412819a8, open:table: 0x402ab268 (more...)}
                         (for generator) = C function: 0x41e52ea0
                         (for state) = table: 0x402ab268  {8:table: 0x4012c080, 2:table: 0x40575eb0, 5:table: 0x4079afe0, 3:table: 0x402d4598 (more...)}
                         (for control) = number: 1
                         _ = number: 1
                         item = table: 0x41d36420  {ti:1, current:false, uri:https://github.com/luakit/luakit/issues/626, session_state:
                 Traceback:
                 (1) [C]                                in function assert
                 (2) /home/tao/.config/luakit/rc.lua:10 in main chunk