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Python 3.11 introduced detailed error locations with caret indicators pointing to the exact expression that caused an exception. When tracebacks are serialized and reconstructed through tblib, this column position information was lost, making debugging reconstructed exceptions harder. This fixes issue ionelmc#76. The solution captures column offsets from traceback objects and embeds them in custom co_linetable bytes when reconstructing code objects. CPython 3.11+ and PyPy 3.11+ use different linetable formats, so separate encoding functions handle each implementation. The fix also ensures __traceback_maker is defined in globals during code execution to prevent NameError at the wrong instruction index, which would capture incorrect column positions from tb_lasti. Add test for __traceback_maker globals bug Without __traceback_maker in the globals dict during exec, the LOAD_NAME instruction raises NameError at tb_lasti=2 instead of RAISE_VARARGS executing at tb_lasti=4. This captures column positions from the wrong bytecode instruction, showing positions for just the name '__traceback_maker' (columns 6-23) instead of the full 'raise __traceback_maker' expression (columns 0-23). This test verifies the fix catches this by asserting the column start is 0, not 6, proving the exception originates from the correct instruction with accurate position information.
hoodmane
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This changes testlib to propagate error context correctly from the tests inside workerd back out to the host pytest session. Previously, any error traceback would always show the host code that looked up the test results which is test support code and not related to the actual cause of the problem. Now it shows correct tracebacks. The techniques here are based on similar techniques in pytest-pyodide. Using tblib + pickle we serialize exceptions. For robustness we use a custom pickler that pickles most things into strings, other than builtin types and types from tblib which we need to handle correctly. On the unpickling side, we synthesize dummy exception classes on the fly for any exceptions that are missing. This is a simpler way to deal with the problems we've had in pytest-pyodide with tracebacks being messed up for exception classes that are only defined in the Pyodide session. I used a fork of tblib called pyodide-tblib for the traceback serializing. It includes ionelmc/python-tblib#91 which fixes column positioning, a fix that makes multiline positions work correctly, and a fix that gets the first line of functions correct. This makes the context exactly match what pytest would show natively.
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Python 3.11 introduced detailed error locations with caret indicators
pointing to the exact expression that caused an exception. When
tracebacks are serialized and reconstructed through tblib, this
column position information was lost, making debugging reconstructed
exceptions harder. This fixes issue #76.
The solution captures column offsets from traceback objects and
embeds them in custom co_linetable bytes when reconstructing code
objects. CPython 3.11+ and PyPy 3.11+ use different linetable
formats, so separate encoding functions handle each implementation.
The fix also ensures __traceback_maker is defined in globals during
code execution to prevent NameError at the wrong instruction index,
which would capture incorrect column positions from tb_lasti.