The peephole optimizer in Python 2.7 and later (and probably a *lot* earlier) has a bug where if the optimizer entirely optimizes away the last line(s) of a function, the lnotab references invalid bytecode offsets:

>>> def f(cond1, cond2):
...     while 1:
...         return 3
...     while 1:
...         return 5
...     return 6
... 
>>> list(dis.findlinestarts(f.__code__))
[(0, 3), (4, 5), (8, 6)]
>>> len(f.__code__.co_code)
8
>>> f.__code__.co_code[8]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: index out of range

The problem is that the lnotab-readjustment in Python/peephole.c doesn't account for trailing NOPs in a bytecode string. I haven't been able to reproduce this before Python 3.8, probably because the optimizer wasn't capable of optimizing things aggressively enough to end a bytecode string with NOPs.

I have a fix for this bug already.

There's also a bug where the optimizer may bail out on optimizing a code object *after* updating the lnotab (the last 'goto exitUnchanged' in Python/peephole.c). That bug has existed since Python 3.6, but it's not clear to me how much this actually affects.

As mentioned in the PR (GH-15970), I don't think we should fix this bug. We can, but it involves replacing PyCode_Optimize() (which is public but undocumented, with confusing refcount effects) with a stub, and very careful surgery on the code of the peephole optimizer. I tried three different ways and I keep running into unexpected side-effects of my changes, because of how the optimizer is called by the compiler.

It is the case that other changes in 3.8 make this bug more apparent, but it's always been around (at least since lnotab was introduced). At this point I think the best thing to do is to document that lnotab can have invalid bytecode offsets, and then reconsider serious refactoring and redesign of the peephole optimizer if it's going to be kept around in 3.9. (Right now there's talk about replacing it with a more sensible CFG-based optimizer.)

Since we modify the content of the bytes object in any case, we can shrink it in-place by setting its Py_SIZE(). But it would be better to fill the end of it with "no-op" fillers.

Setting Py_SIZE of the bytes object is possible, but gross and not how you're supposed to operate on bytes.

I'm also not entirely convinced lnotab isn't reused in ways it shouldn't. The peephole optimizer already does gross things and is tied very intimately into the compiler and assembler structs, and any change I tried caused weird side-effects. I'm not comfortable making these changes without extensive rewrites of those bits of the code, which Mark Shannon is already working on for different reasons.

The current lnotab format doesn't really have the concept of 'no-op fillers', because zero-increment entries are used to add to previous entries. Adding the concept could mean breaking third-party consumers of lnotab. Of all the uses of lnotab that I could find, dis.findlinestarts() was the only one that didn't ignore the invalid entries. I think just documenting the current behaviour (which, just as a reminder, has been around forever, but is just more obvious in Python 3.8) and fixing dis.findlinestarts() is enough of a fix for the foreseeable future. See #60283.

New changeset c816503 by Gregory P. Smith (T. Wouters) in branch 'master':
bpo-38115: Deal with invalid bytecode offsets in lnotab (GH-16079)
c816503

New changeset 36c6fa9 by Gregory P. Smith in branch '3.8':
bpo-38115: Deal with invalid bytecode offsets in lnotab (GH-16079) (GH-16464)
36c6fa9

thanks Thomas!