Issue #6042:
lnotab-based tracing is very complicated and isn't documented very well. There
were at least 3 comment blocks purporting to document co_lnotab, and none did a
very good job. This patch unifies them into Objects/lnotab_notes.txt which
tries to completely capture the current state of affairs.
I also discovered that we've attached 2 layers of patches to the basic tracing
scheme. The first layer avoids jumping to instructions that don't start a line,
to avoid problems in if statements and while loops. The second layer
discovered that jumps backward do need to trace at instructions that don't
start a line, so it added extra lnotab entries for 'while' and 'for' loops, and
added a special case for backward jumps within the same line. I replaced these
patches by just treating forward and backward jumps differently.
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 6d02109..8616d5d 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -798,9 +798,11 @@
specifies the local trace function.
``'line'``
- The interpreter is about to execute a new line of code (sometimes multiple
- line events on one line exist). The local trace function is called; *arg*
- is ``None``; the return value specifies the new local trace function.
+ The interpreter is about to execute a new line of code or re-execute the
+ condition of a loop. The local trace function is called; *arg* is
+ ``None``; the return value specifies the new local trace function. See
+ :file:`Objects/lnotab_notes.txt` for a detailed explanation of how this
+ works.
``'return'``
A function (or other code block) is about to return. The local trace