Re: [PATCH v2] Make GDB compile with Python 3 on MinGW

Simon Marchi
2019-08-15T02:58:24Z

On 2019-08-14 12:06 p.m., Christian Biesinger via gdb-patches wrote:
> [Now with the comment updated too]
>
> PyFile_FromString and PyFile_AsFile have been removed in Python 3.
> There is no obvious replacement that works here, and we can't just
> pass our FILE* to a DLL in Windows because it may use a different
> C runtime.
>
> So we just call a Python function which reads and executes file
> contents. Care must be taken to execute it in the context of
> __main__.
>
> Tested by inverting the ifdef and running the testsuite on Debian
> Linux (even without the patch, I failed at running the testsuite
> on Windows).

I haven't deciphered the Python part yet, but here are some early comments.

Did you test with both Python 2 and 3?

> gdb/ChangeLog:
>
> 2019-08-13 Christian Biesinger <cbiesinger@google.com>
>
> * python/lib/gdb/__init__.py: Add an execute_file function.
> * python/python.c (python_run_simple_file): Call gdb.execute_file
> on Windows.
> ---
> gdb/python/lib/gdb/__init__.py | 23 +++++++++++++++++++++++
> gdb/python/python.c | 23 +++++++++++++++--------
> 2 files changed, 38 insertions(+), 8 deletions(-)
>
> diff --git a/gdb/python/lib/gdb/__init__.py b/gdb/python/lib/gdb/__init__.py
> index af74df80c8..f1adc5ffbe 100644
> --- a/gdb/python/lib/gdb/__init__.py
> +++ b/gdb/python/lib/gdb/__init__.py
> @@ -106,6 +106,29 @@ def execute_unwinders(pending_frame):
>
> return None
>
> +def execute_file(filepath):

I would suggest naming this "_execute_file", to emphasize that's internal,
not to be used by users.

> + """This function is used to replace Python 2's PyRun_SimpleFile.
> +
> + Loads and executes the given file.
> +
> + We could use the runpy module, but its documentation says:
> + "Furthermore, any functions and classes defined by the executed code are
> + not guaranteed to work correctly after a runpy function has returned."
> + """
> + globals = sys.modules['__main__'].__dict__
> + set_file = False
> + if not hasattr(globals, '__file__'):
> + globals['__file__'] = filepath
> + set_file = True

What does setting __file__ help with? A comment explaining why this is needed
would be nice.

> + try:
> + with open(filepath, 'rb') as file:
> + # We pass globals also as locals to match what Python does
> + # in PyRun_SimpleFile.
> + exec(compile(file.read(), filepath, 'exec'), globals, globals)

Could you split this line (the exec and compile) in two? If there is an error
coming out of either of them, and we get a backtrace, it will be easier to know
which one it is if they are on separate lines.

> + finally:
> + if set_file:
> + del globals['__file__']
> +
>
> # Convenience variable to GDB's python directory
> PYTHONDIR = os.path.dirname(os.path.dirname(__file__))
> diff --git a/gdb/python/python.c b/gdb/python/python.c
> index 162470dcc0..617bc0b84c 100644
> --- a/gdb/python/python.c
> +++ b/gdb/python/python.c
> @@ -323,9 +323,8 @@ python_interactive_command (const char *arg, int from_tty)
> A FILE * from one runtime does not necessarily operate correctly in
> the other runtime.
>
> - To work around this potential issue, we create on Windows hosts the
> - FILE object using Python routines, thus making sure that it is
> - compatible with the Python library. */
> + To work around this potential issue, we run code in Python to load
> + the script. */
>
> static void
> python_run_simple_file (FILE *file, const char *filename)
> @@ -339,14 +338,22 @@ python_run_simple_file (FILE *file, const char *filename)
> /* Because we have a string for a filename, and are using Python to
> open the file, we need to expand any tilde in the path first. */
> gdb::unique_xmalloc_ptr<char> full_path (tilde_expand (filename));
> - gdbpy_ref<> python_file (PyFile_FromString (full_path.get (), (char *) "r"));
> - if (python_file == NULL)
> +
> + if (gdb_python_module == nullptr
> + || ! PyObject_HasAttrString (gdb_python_module, "execute_file"))
> {
> - gdbpy_print_stack ();
> - error (_("Error while opening file: %s"), full_path.get ());
> + error (_("Installation error: gdb.execute_file function is missing"));
> + return;
> }
>
> - PyRun_SimpleFile (PyFile_AsFile (python_file.get ()), filename);
> + gdbpy_ref<> return_value
> + (PyObject_CallMethod (gdb_python_module, "execute_file", "s", full_path.get ()));

Wrap the line above to fit in 80 columns:

gdbpy_ref<> return_value
(PyObject_CallMethod (gdb_python_module, "execute_file", "s",
full_path.get ()));

> + if (return_value == nullptr)
> + {
> + /* Use PyErr_PrintEx instead of gdbpy_print_stack to better match the
> + behavior of the non-Windows codepath. */
> + PyErr_PrintEx(0);

I don't think PyErr_PrintEx clears the Python error indicator, and I think we want
it cleared when we exit this function, so should we call PyErr_Clear here?

Simon