Swift FileHandle: readabilityHandler + availableData can crash with NSFileHandleOperationException (EAGAIN) if another path makes the fd non-blocking

Symptom: the process aborts with *** -[NSConcreteFileHandle availableData]: Resource temporarily unavailable (an uncaught NSException) in a closure registered as readabilityHandler on a Pipe. Cause: process exit code (for example Process.terminationHandler) drained the pipe by setting readabilityHandler = nil, switching the same fd to O_NONBLOCK with fcntl, and reading it dry. Setting the handler to nil does not cancel an invocation the dispatch source has already queued or started. That invocation then calls availableData on an empty non-blocking fd: read() returns EAGAIN, and availableData raises an Objective-C exception instead of returning empty Data, which Swift cannot catch. It is intermittent and becomes much more likely when the child writes to stderr exactly as it exits (for example shell job lines like Terminated/Killed). Fix: in the handler use the throwing API try? handle.read(upToCount:) (macOS 10.15.4+) and treat an error as no data. Or drain without changing the fd flags (use poll before read), on the same serial queue as the handler, or on a dup of the fd.