shelve.Shelf.sync() leaves writeback disabled if writing a cached entry fails

[christianaurichzm]

Bug report

Bug description:

Shelf.sync() sets self.writeback to False while it writes the cached entries back, and only sets it back to True after the loop. If writing one of the entries raises (for example, a cached object that can no longer be pickled), the exception propagates as expected, but writeback stays False. The shelf keeps working, and later mutations of accessed objects are silently lost:

import shelve

class Unpicklable:
    def __reduce__(self):
        raise RuntimeError("cannot pickle")

d = {}
s = shelve.Shelf(d, writeback=True)
s["a"] = []
s["a"].append(Unpicklable())
try:
    s.sync()
except RuntimeError:
    pass
print(s.writeback)  # False, expected True

s["a"].clear()
s["b"] = [1]
s["b"].append(2)
s.close()
print(shelve.Shelf(d)["b"])  # [1], expected [1, 2]

The cached entries are still in s.cache after the failure, so the caller could fix the offending object and call sync() again, but by then the shelf is no longer in writeback mode.

Restoring writeback in a finally block would fix this, leaving the cache intact so sync() can be retried.

CPython versions tested on:

3.13, 3.14, 3.15, 3.16, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-157976: Fix shelve.Shelf.sync() leaving writeback disabled on error #157977