Bug fixed inline GDB pretty printing

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2024-08-13 14:11:03

Re: my previous guide to writing inline GDB pretty printers, it turns 
out it had bugs. Here is how to fix them.

Firstly, my thanks to Braden Ganetsky for reporting the bugs, then pair 
programming over Slack with me late last night to diagnose and fix the 
issues. We have cowritten a conversion script which we now believe is 
actually correct -- it takes a Python GDB pretty printer, and emits a C 
(not C++) header file with all the correct markup. It can be found at:

https://github.com/ned14/quickcpplib/blob/master/scripts/generate_gdb_printer.py

The first bug fixed is correct escaping: we need to double escape the 
original Python in the right kind of way to cause the escaped inline 
assembler block to emit the right escaped text for GDB to load it 
correct as Python. Getting the double escaping correct is tricky, but we 
think we have it now.

The second bug fixed is correct composure of multiple inline GDB pretty 
printers - Braden found only the first was having any effect. It turns 
out this is due to a misreading of the GDB docs from 
https://sourceware.org/gdb/current/onlinedocs/gdb.html/dotdebug_005fgdb_005fscripts-section.html#dotdebug_005fgdb_005fscripts-section 
where this example:

```
asm(
".pushsection \".debug_gdb_scripts\", \"MS\",@progbits,1\n"
".byte " XSTRING (SECTION_SCRIPT_ID_PYTHON_TEXT) "\n"
".ascii \"gdb.inlined-script\\n\"\n"
".ascii \"class test_cmd (gdb.Command):\\n\"\n"
...
```

... does not make it clear that the string "gdb.inlined-script" is 
actually the unique identifier of that script used to eliminate 
duplicates. It needs to be unique per inlined script unless you 
specifically want duplicates, and our Python converter above has 
remedied this. Generally you ought to name it after your library and 
maybe possibly a git SHA or version depending on how unstable your 
library is, and ensure that the pretty printer matches only that version 
of your library.

Thanks once again to Braden for his help.

Note that the Outcome GDB pretty printer which has shipped in the 
pending Boost release is buggy and will not work properly. The next 
Boost release will fix it. You can of course copy the fixed pretty 
printer from the github repo over this Boost release.

Niall