External Command Output Parsing#

Tuist parses JSON and header payloads from external system commands (primarily simctl and devicectl) and HTTP responses. Two code paths are known to surface a raw NSCocoaErrorDomain Code=3840 "Unable to parse empty data." instead of a typed Tuist error when input is empty or malformed. This is an open hardening gap tracked in issue #13747.

simctl JSON — SimulatorController#

SimulatorController.devices() and runtimes() run xcrun simctl list … --json and feed stdout directly to JSONSerialization.jsonObject. If simctl exits 0 with empty stdout, JSONSerialization throws the raw 3840 error; there is no surrounding catch block to convert it to a SimulatorControllerError 1 2.

SimulatorControllerError has three typed cases — simctlError(String), deviceNotFound, simulatorNotFound(udid:) — but none for a JSON parse failure 3. The recommended fix from the issue report is to catch empty or non-JSON stdout and throw a typed simctlError that includes the command, exit status, and stderr.

devicectl JSON — DeviceController#

DeviceController.findAvailableDevices() writes devicectl list devices --json-output to a temporary file, then wraps the JSONDecoder call in a do/catch that converts any failure — including JSON parse errors — to DeviceControllerError.fetchingDevicesFailed 4. This surfaces a clean user-facing message ("Fetching the list of devices failed.") but discards the underlying decode error, making it hard to diagnose root causes (see also issue #12052).

HTTP x-tuist-warnings Header — OutputWarningsMiddleware#

OutputWarningsMiddleware reads the x-tuist-warnings response header (base64-encoded JSON array of warning strings) and logs each warning via AlertController. The bug: Data(base64Encoded: "") returns a non-nil, zero-byte Data value, so the couldntConvertToData guard passes and JSONSerialization.jsonObject throws 3840 for an empty header value 5. The fix is to treat an empty header value as "no warnings" and return early before decoding.

Error Types Reference#

Key Files#

• cli/Sources/TuistCore/Simulator/SimulatorController.swift — simctl invocation and JSON parsing
• cli/Sources/TuistAutomation/Device/DeviceController.swift — devicectl invocation and JSON parsing with error wrapping
• cli/Sources/TuistHTTP/OutputWarningsMiddleware.swift — HTTP warning header parsing
• GitHub Issue #13747 — root-cause analysis and suggested fixes 6