fix: enable tracebacks for "user"/custom sqlite functions

A bit niche but I tried setting my bareasc prefix to an empty string,
and was getting an obtuse error. This should help make clearer what is
happening when queries fail.

The exception is not properly raised up the stack in the first place
because it happens across 2 FFI boundaries: the DB query
(Python -> SQLite), and the custom DB function (SQLite -> Python).
Thus Python cannot forwarded it back to itself through SQLite, and it's
treated as an "unraisable" exception.

We could override `sys.unraisablehook` to not print anything for the
original exception, and store it in a global for the outer Python
interpreter to fetch and raise properly, but that's pretty hacky,
limited to a single DB instance and query at once, and risks swallowing
other "unraisable" exceptions.
Instead we just tell the user to look above for what Python prints.

Sample output:
```
Exception ignored in: <function unidecode_expect_ascii at
0x7f7fa20bb060>
Traceback (most recent call last):
  File "site-packages/unidecode/__init__.py", line 60, in
unidecode_expect_ascii
    bytestring = string.encode('ASCII')
                 ^^^^^^^^^^^^^
AttributeError: 'bytes' object has no attribute 'encode'
Traceback (most recent call last):
  File "site-packages/beets/dbcore/db.py", line 988, in query
    cursor = self.db._connection().execute(statement, subvals)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
sqlite3.OperationalError: user-defined function raised exception

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "site-packages/beets/__main__.py", line 9, in <module>
    sys.exit(main())
             ^^^^^^
  File "site-packages/beets/ui/__init__.py", line 1865, in main
    _raw_main(args)
  File "site-packages/beets/ui/__init__.py", line 1852, in _raw_main
    subcommand.func(lib, suboptions, subargs)
  File "site-packages/beets/ui/commands.py", line 1599, in list_func
    list_items(lib, decargs(args), opts.album)
  File "site-packages/beets/ui/commands.py", line 1594, in list_items
    for item in lib.items(query):
                ^^^^^^^^^^^^^^^^
  File "site-packages/beets/library.py", line 1695, in items
    return self._fetch(Item, query, sort or
self.get_default_item_sort())

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "site-packages/beets/library.py", line 1673, in _fetch
    return super()._fetch(model_cls, query, sort)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "site-packages/beets/dbcore/db.py", line 1301, in _fetch
    rows = tx.query(sql, subvals)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "site-packages/beets/dbcore/db.py", line 991, in query
    raise DBCustomFunctionError()
beets.dbcore.db.DBCustomFunctionError: beets defined SQLite function
failed; see the other errors above for details
```

beets/dbcore/db.py

@@ -64,6 +64,16 @@ class DBAccessError(Exception):
     """
 
 
+class DBCustomFunctionError(Exception):
+    """A sqlite function registered by beets failed."""
+
+    def __init__(self):
+        super().__init__(
+            "beets defined SQLite function failed; "
+            "see the other errors above for details"
+        )
+
+
 class FormattedMapping(Mapping[str, str]):
     """A `dict`-like formatted view of a model.
 
@@ -947,6 +957,12 @@ class Transaction:
             self._mutated = False
             self.db._db_lock.release()
 
+        if (
+            isinstance(exc_value, sqlite3.OperationalError)
+            and exc_value.args[0] == "user-defined function raised exception"
+        ):
+            raise DBCustomFunctionError()
+
     def query(
         self, statement: str, subvals: Sequence[SQLiteType] = ()
     ) -> list[sqlite3.Row]:
@@ -1007,6 +1023,13 @@ class Database:
                 "sqlite3 must be compiled with multi-threading support"
             )
 
+        # Print tracebacks for exceptions in user defined functions
+        # See also `self.add_functions` and `DBCustomFunctionError`.
+        #
+        # `if`: use feature detection because PyPy doesn't support this.
+        if hasattr(sqlite3, "enable_callback_tracebacks"):
+            sqlite3.enable_callback_tracebacks(True)
+
         self.path = path
         self.timeout = timeout
 

test/test_dbcore.py

@@ -23,6 +23,7 @@ from tempfile import mkstemp
 import pytest
 
 from beets import dbcore
+from beets.dbcore.db import DBCustomFunctionError
 from beets.library import LibModel
 from beets.test import _common
 from beets.util import cached_classproperty
@@ -31,6 +32,13 @@ from beets.util import cached_classproperty
 # have multiple models with different numbers of fields.
 
 
+@pytest.fixture
+def db(model):
+    db = model(":memory:")
+    yield db
+    db._connection().close()
+
+
 class SortFixture(dbcore.query.FieldSort):
     pass
 
@@ -784,3 +792,25 @@ class ResultsIteratorTest(unittest.TestCase):
             self.db._fetch(ModelFixture1, dbcore.query.FalseQuery()).get()
             is None
         )
+
+
+class TestException:
+    @pytest.mark.parametrize("model", [DatabaseFixture1])
+    @pytest.mark.filterwarnings(
+        "ignore: .*plz_raise.*: pytest.PytestUnraisableExceptionWarning"
+    )
+    @pytest.mark.filterwarnings(
+        "error: .*: pytest.PytestUnraisableExceptionWarning"
+    )
+    def test_custom_function_error(self, db: DatabaseFixture1):
+        def plz_raise():
+            raise Exception("i haz raized")
+
+        db._connection().create_function("plz_raise", 0, plz_raise)
+
+        with db.transaction() as tx:
+            tx.mutate("insert into test (field_one) values (1)")
+
+        with pytest.raises(DBCustomFunctionError):
+            with db.transaction() as tx:
+                tx.query("select * from test where plz_raise()")