[Bug] CustomThread silently discards exceptions from background solver threads — callers receive None on failure

[tejassinghbhati]

Summary

Summary

CustomThread extends threading.Thread to capture and return the result of a background thread's target function via a custom join(). However, if the target raises an exception, the exception is silently swallowed — self._return stays None and the exception is lost forever. The calling code receives None as if the thread completed successfully, making it impossible to distinguish a clean None return from a thread crash.

This is especially dangerous for model solver threads, where a silent failure gives users false confidence that a run completed — while results are never actually produced.

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Affected File

API/Classes/Base/CustomThreadClass.py

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Current Behaviour

class CustomThread(Thread):
    def run(self):
        if self._target is not None:
            # ❌ If _target raises, exception is silently swallowed here
            self._return = self._target(*self._args, **self._kwargs)

    def join(self):
        Thread.join(self)
        # ❌ Returns None whether thread succeeded or crashed
        return self._return

If the solver subprocess raises a FileNotFoundError, RuntimeError, or any other exception:

• self._return is never assigned → stays None
• The exception is lost — it does not propagate to the main thread
• join() returns None to the caller, which looks identical to a valid empty result
• No error is logged, no user feedback is shown

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Expected behavior

Expected Behaviour

• If a background thread raises an exception, that exception must propagate to the caller when join() is called
• join() should re-raise the exception so the route handler can catch it and return a proper error response to the frontend
• No silent failures — the exception must never be discarded

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Proposed Fix

import sys

class CustomThread(Thread):
    def __init__(self, group=None, target=None, name=None, args=(), kwargs={}, Verbose=None):
        Thread.__init__(self, group, target, name, args, kwargs)
        self._return = None
        self._exc_info = None  # stores exception if thread crashes

    def run(self):
        if self._target is not None:
            try:
                self._return = self._target(*self._args, **self._kwargs)
            except Exception:
                self._exc_info = sys.exc_info()  # capture full traceback

    def join(self):
        Thread.join(self)
        if self._exc_info is not None:
            # re-raise the original exception with its original traceback
            raise self._exc_info[1].with_traceback(self._exc_info[2])
        return self._return

Reproduction steps

Steps to Reproduce

from API.Classes.Base.CustomThreadClass import CustomThread

def failing_task():
    raise RuntimeError("Solver failed: file not found")

t = CustomThread(target=failing_task)
t.start()
result = t.join()

# Bug: result is None, no exception is raised
# Expected: RuntimeError should propagate here

print(result)  # prints None — silent failure

Environment

Environment

Field	Value
Affected file	API/Classes/Base/CustomThreadClass.py
Python	3.10+
Impact	All background solver runs using CustomThread

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Acceptance Criteria

• Exceptions raised inside a CustomThread target propagate to the caller on join()
• The original traceback is preserved (not replaced with a new one)
• join() still returns the target's return value on success
• A unit test is added: one for successful return, one for exception propagation

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Labels

bug · correctness · threading · GSoC-2026

Logs or screenshots

[SeaCelo]

Thanks. Closing as resolved by #46