bdb.clear_all_file_breaks skips breakpoints when multiple breakpoints share a line #149015

Bug report

Bug description:

bdb.Bdb.clear_all_file_breaks(filename) iterates over a list while mutating
it via Breakpoint.deleteMe(), which removes elements from the same list.
When two or more breakpoints exist at the same (filename, lineno), this
classic iterate-and-delete pattern skips alternate elements, leaving orphan
Breakpoint instances in Breakpoint.bplist and Breakpoint.bpbynumber.

The same module already uses the defensive slice-copy pattern in
clear_break() (the sibling function), so this is a clear local
inconsistency, not a contractual choice.

Internal asymmetry (smoking gun):

In Lib/bdb.py:

clear_break() — defensive copy:

for bp in Breakpoint.bplist[filename, lineno][:]:
    bp.deleteMe()

clear_all_file_breaks() — no copy:

for line in self.breaks[filename]:
    blist = Breakpoint.bplist[filename, line]
    for bp in blist:        # iterating raw list
        bp.deleteMe()        # mutates blist

Both functions delete breakpoints via the same deleteMe() mechanism, which
removes from Breakpoint.bplist[(filename, lineno)]. The asymmetric defense
between sibling functions is the strongest evidence this is an oversight
rather than a deliberate design choice.

Reproducer:

import bdb

class MyDbg(bdb.Bdb): pass

dbg = MyDbg()
# Three breakpoints at the same (file, line)
dbg.set_break(__file__, 10)
dbg.set_break(__file__, 10)
dbg.set_break(__file__, 10)

# Snapshot before clearing
key = (bdb.canonic(__file__), 10)
before = list(bdb.Breakpoint.bplist.get(key, []))
print(f"Before: {len(before)} breakpoints in Breakpoint.bplist[{key}]")

# Clear all in this file
dbg.clear_all_file_breaks(bdb.canonic(__file__))

# Snapshot after clearing
after = list(bdb.Breakpoint.bplist.get(key, []))
print(f"After:  {len(after)} breakpoints in Breakpoint.bplist[{key}]")
print(f"Orphans in Breakpoint.bpbynumber: "
      f"{[n for n, b in enumerate(bdb.Breakpoint.bpbynumber) if b is not None]}")

Expected behavior:

Before: 3 breakpoints in Breakpoint.bplist[...]
After:  0 breakpoints in Breakpoint.bplist[...]
Orphans in Breakpoint.bpbynumber: []

Actual behavior (Python 3.14.3):

Before: 3 breakpoints in Breakpoint.bplist[...]
After:  1 breakpoints in Breakpoint.bplist[...]   ← orphan remains
Orphans in Breakpoint.bpbynumber: [<some index>]   ← orphan remains

For N breakpoints at the same (file, line), floor(N/2) orphans remain.

Suggested fix (one-character diff):

--- a/Lib/bdb.py
+++ b/Lib/bdb.py
@@ -755,7 +755,7 @@
         if filename not in self.breaks:
             return 'There are no breakpoints in %s' % filename
         for line in self.breaks[filename]:
             blist = Breakpoint.bplist[filename, line]
-            for bp in blist:
+            for bp in blist[:]:
                 bp.deleteMe()
         del self.breaks[filename]

This matches the existing pattern at clear_break() in the same file.

Suggested test (Lib/test/test_bdb.py):

def test_clear_all_file_breaks_with_multiple_bps_same_line(self):
    """Regression test: clear_all_file_breaks must remove all breakpoints,
    even when multiple breakpoints share the same (file, line)."""
    dbg = bdb.Bdb()
    src = bdb.canonic(__file__)
    dbg.set_break(src, 10)
    dbg.set_break(src, 10)
    dbg.set_break(src, 10)
    self.assertEqual(len(bdb.Breakpoint.bplist[(src, 10)]), 3)
    dbg.clear_all_file_breaks(src)
    self.assertNotIn((src, 10), bdb.Breakpoint.bplist)

Versions:

• Reproduced on Python 3.14.3 (Windows, MINGW64).
• Reading source confirms identical pattern in main and 3.13 branches.
• Behavior identical regardless of whether the breakpoints are set with
  conditions, ignore counts, or via different Bdb subclasses.

Related (not duplicate):

Issue #54770 (bpo-10561, fixed 2010) covers a different scenario: the
clear bpnumber pdb command potentially deleting more than one breakpoint.
That issue addresses clear_bpbynumber / breakpoint identity. The bug
reported here is in clear_all_file_breaks and concerns iterate-and-delete
on the shared Breakpoint.bplist list. No prior issue or PR specifically
targets this asymmetry between clear_break (defensive) and
clear_all_file_breaks (vulnerable).

Linked PRs

• gh-149015: avoid iterate-and-delete issue in Bdb.clear_all_file_breaks #149039
• gh-149015: Fix iterate-and-delete issue in Bdb.clear_all_file_breaks #149843
• gh-149015: Fix clear_all_file_breaks for breakpoints sharing a line in bdb #159748
• [3.15] gh-149015: Fix clear_all_file_breaks for breakpoints sharing a line in bdb (GH-159748) #159801
• [3.14] gh-149015: Fix clear_all_file_breaks for breakpoints sharing a line in bdb (GH-159748) #159802