lldb dived into python stack

[j8xixo12]

I have studied cpython for a while, I found python interpreter stored many informations into PyFrameObject, what I care about is line number.

In order to get python script's line number, I used co_lontab which is bytecode offset <----> line number mapping to calculate python source code line number.

The cpython also provides a helper function PyCode_Addr2Line(PyCodeObject *co, int addrq) to convert bytecode offset to line number, and PyFrame_GetLineNumber(PyFrameObject *f) also can get line number directly from PyFrameObject.

Here is my test python script

def foo1():
    print("Hi")
    return 1 + 1

def foo2(input):
    print(input)
    return

def main():
    a = foo1()
    foo2("1234")
    return

if __name__ == '__main__':
    main()

I used br s -n _PyEval_EvalFrame -c '((int)strcmp(PyUnicode_AsUTF8(f->f_code->co_name), "foo1")) == 0' to set breakpoint at first line of main().

Here is the output

(lldb) br s -n _PyEval_EvalFrame -c '((int)strcmp(PyUnicode_AsUTF8(f->f_code->co_name), "foo1")) == 0'
Breakpoint 18: 3 locations.
(lldb) r /tmp/test.py
Process 15283 launched: '/Users/laijunxu/.pyenv/versions/3.9-dev-debug/bin/python3.9' (x86_64)
Process 15283 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 18.1
    frame #0: 0x00000001000b9753 python3.9`_PyEval_EvalFrame(tstate=0x0000000100a06460, f=0x0000000100cf45f0, throwflag=0) at pycore_ceval.h:40:12
   37   static inline PyObject*
   38   _PyEval_EvalFrame(PyThreadState *tstate, PyFrameObject *f, int throwflag)
   39   {
-> 40       return tstate->interp->eval_frame(tstate, f, throwflag);
   41   }
   42
   43   extern PyObject *_PyEval_EvalCode(
Target 0: (python3.9) stopped.
(lldb) py-list
    5    def foo2(input):
    6        print(input)
    7        return
    8
    9    def main():
  >10        a = foo1()
   11        foo2("1234")
   12        return
   13
   14    if __name__ == '__main__':
   15        main()
(lldb) p PyFrame_GetLineNumber(f)
(int) $47704 = 10

I tried to use PyFrame_GetLineNumber to get line number, and I got what I want, but when I tried
using br s -n _PyEval_EvalFrame -c '(int)PyFrame_GetLineNumber(f) == 10' to put breakpoint at the same line
lldb didn't hit the breakpoint.....

I don't know what's going on?

[yungyuc]

If lldb conditional br doesn't work, how about trying to mimic what pdb does for (Python) breakpoint? I assume pdb doesn't use either gdb nor lldb.

[j8xixo12]

I plan to write some gdb or lldb script that user can directly put break point into python script.
I wish to finish it before end of Chinese new year.
Hope I can accomplish it as schedule.

[yungyuc]

I think getting one of the debugger script to work is good enough as the first step. Once you have a prototype, we can start to discuss how to make a PR.