gh-91924: Fix __lltrace__ for non-UTF-8 stdout encoding

[vstinner]

Fix lltrace debug feature if the stdout encoding is not UTF-8.

If the stdout encoding is not UTF-8, the first call to
lltrace_resume_frame() indirectly sets lltrace to 0 when calling
unicode_check_encoding_errors() which calls
encodings.search_function().

[vstinner]

Python 3.10 is also affected: lltrace must be set to 0 in prtrace(). In Python 3.10, the feature is enabled by setting __ltrace__ with a single L, whereas Python 3.11 and 3.12 uses __lltrace__ with two L!

[sweeneyde]

Making lltrace a local variable would fix 3.10 as well, right?

[vstinner]

The __lltrace__ feature is only mentioned in the doc of Python debug build: https://docs.python.org/dev/using/configure.html#python-debug-build

[sweeneyde]

Spelling was originally changed by 3c1e481, but changed back by 37965d2

[vstinner]

My first implementation set lltrace to 1 at the end of lltrace_instruction() and lltrace_resume_frame() to workaround side effects of recusive calls to _PyEval_EvalFrame().

diff --git a/Python/ceval.c b/Python/ceval.c
index 230198b41f6a5..4d4681099e0df 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -148,6 +148,9 @@ lltrace_instruction(_PyInterpreterFrame *frame,
         printf("%d: %s\n", offset * 2, opname);
     }
     fflush(stdout);
+
+    // gh-91924: PyObject_Print() can indirectly set lltrace to 0
+    lltrace = 1;
 }
 static void
 lltrace_resume_frame(_PyInterpreterFrame *frame)
@@ -179,6 +182,9 @@ lltrace_resume_frame(_PyInterpreterFrame *frame)
     printf("\n");
     fflush(stdout);
     PyErr_Restore(type, value, traceback);
+
+    // gh-91924: PyObject_Print() can indirectly set lltrace to 0
+    lltrace = 1;
 }
 #endif
 static int call_trace(Py_tracefunc, PyObject *,

But @sweeneyde is right, making the variable local is enough. I updated my PR.

[vstinner]

cc @serhiy-storchaka @markshannon @gvanrossum

[serhiy-storchaka]

👍

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-93212 is a backport of this pull request to the 3.11 branch.

[vstinner]

In the 3.10 branch, making lltrace variable local would require to change call_function(). I prefer to minimize changes and so I wrote a simpler change: PR #93214.