Add def multiprocessing_breakpoint function

Using the builtin `breakpoint` function does not work with
multiprocessing, as the Process forks has different STDIN so the pdb can
not attach to it. Use this `multiprocessing_breakpoint` function
instead, when you need a breakpoint for debugging.

pyproject.toml

@@ -46,6 +46,7 @@ addopts = "--cov=unblob --cov=tests --cov-branch --cov-fail-under=90"
 [tool.vulture]
 paths = ["unblob/"]
 exclude = ["unblob/_py/"]
+ignore_names = ["breakpointhook"]
 
 [build-system]
 requires = ["poetry-core>=1.0.0"]

unblob/logging.py

@@ -1,4 +1,6 @@
 import logging
+import pdb
+import sys
 from os import getpid
 from pathlib import Path
 from typing import Any
@@ -79,3 +81,18 @@ def configure_logger(verbose: bool, extract_root: Path):
         wrapper_class=structlog.make_filtering_bound_logger(log_level),
         processors=processors,
     )
+
+
+class _MultiprocessingPdb(pdb.Pdb):
+    def interaction(self, *args, **kwargs):
+        _stdin = sys.stdin
+        try:
+            sys.stdin = open("/dev/stdin")
+            pdb.Pdb.interaction(self, *args, **kwargs)
+        finally:
+            sys.stdin = _stdin
+
+
+def multiprocessing_breakpoint():
+    """Call this in Process forks instead of the builtin `breakpoint` function for debugging with PDB."""
+    return _MultiprocessingPdb().set_trace(frame=sys._getframe(1))

unblob/processing.py

@@ -1,6 +1,7 @@
 import multiprocessing
 import stat
 import statistics
+import sys
 from operator import attrgetter
 from pathlib import Path
 from typing import List, Optional
@@ -12,7 +13,7 @@
 from .file_utils import iterate_file
 from .finder import search_chunks_by_priority
 from .iter_utils import pairwise
-from .logging import noformat
+from .logging import multiprocessing_breakpoint, noformat
 from .math import shannon_entropy
 from .models import ProcessingConfig, Task, UnknownChunk, ValidChunk
 
@@ -83,6 +84,9 @@ def process_file(  # noqa: C901
 def _process_task_queue(
     task_queue: multiprocessing.JoinableQueue, config: ProcessingConfig
 ):
+    # Set custom function to breakpoint() call in the sub-processes for easier debugging
+    sys.breakpointhook = multiprocessing_breakpoint
+
     while True:
         logger.debug("Waiting for Task")
         task = task_queue.get()