The other story begins with adding a C library extension to the monolith. This was some Python code that wrapped a C library which gave access to an internal data store. Unfortunately, the bindings had a bug in their string handling code. To get a string object that could be sent to the Python C APIs, it used a family of functions called
The developer writing these C bindings didn’t realize that strings are immutable in Python, and the C API documentation for strings doesn’t include that bit of information. To copy a string from the C world to Python-land, the bindings would first use this function to initialize a Python string to use as a buffer. It would fill said buffer with the string from the C side by poking at the object’s internal data structures. This did actually work, as it was never seen BEFORE it was filled nor was it modified AFTER it was sent across the wall to Python-land.
Then disaster struck. A configuration change caused the it-has-always-worked function to begin t