pdb: breakpoint() is ignored after quitting a previous session with q #160643

Bug description:

Since gh-138641 (#159918, shipped in 3.15.1 and backported to 3.14), Bdb.set_trace() returns early when self.enterframe is set. Bdb.set_enterframe() clears it after yield without try/finally, so when an exception escapes trace_dispatch(), for example BdbQuit after q in the interactive interpreter, enterframe stays set and every later breakpoint() is ignored.

# printf 'q\nq\nq\n' | python repro.py
import sys, bdb
sys.ps1 = '>>> '   # as in the interactive interpreter
def f(i):
    breakpoint()
    return i
for i in range(3):
    try:
        print('call', i, '->', f(i))
    except bdb.BdbQuit:
        print('BdbQuit caught', i)

On main only the first call stops:

> .../repro.py(5)f()
-> breakpoint()
(Pdb) BdbQuit caught 0
call 1 -> 1
call 2 -> 2

On 3.14.7 the debugger stops on every call. The same happens in a real REPL: call a function that has breakpoint(), type q, call it again, and it no longer stops.

CPython versions tested on:

3.13, 3.14, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-160643: Reset Bdb.enterframe when trace dispatch raises #160644