Backtracing support in Swift ============================ When things go wrong, it's always useful to be able to get a backtrace showing where the problem occurred in your program. Broadly speaking there are three circumstances where you might want a backtrace, namely: * Program crashes * Runtime errors * Specific user-defined program events Historically, Swift has tended to lean on operating system crash catching support for the first two of these, and hasn't really provided any built-in support for the latter. This is fine for Darwin, where the operating system provides a comprehensive system-wide crash catching facility; it's just about OK on Windows, which also has system-wide crash logging; but it isn't great elsewhere, in particular on Linux where a lot of server-side Swift programs currently rely on a separate package to provide them with some level of backtrace support when errors happen. What does Swift now support? ---------------------------- Swift now supports: * Automatic crash catching and backtrace generation out of the box. * Built-in symbolication. * A choice of unwind algorithms, including "fast", DWARF and SEH. * Interactive(!) crash/runtime error catching. Crash catching is enabled by default, and won't interfere with any system-wide crash reporters you might be using. How do I configure backtracing? ------------------------------- There is an environment variable, ``SWIFT_BACKTRACE``, that can be used to configure Swift's crash catching and backtracing support. The variable should contain a ``,``-separated list of ``key=value`` pairs. Supported keys are as follows: +-----------------+---------+--------------------------------------------------+ | Key | Default | Meaning | +=================+=========+==================================================+ | enable | yes* | Set to ``no`` to disable crash catching, or | | | | ``tty`` to enable only if stdin is a terminal. | +-----------------+---------+--------------------------------------------------+ | demangle | yes | Set to ``no`` to disable demangling. | +-----------------+---------+--------------------------------------------------+ | interactive | tty | Set to ``no`` to disable interaction, or ``yes`` | | | | to enable always. | +-----------------+---------+--------------------------------------------------+ | color | tty | Set to ``yes`` to enable always, or ``no`` to | | | | disable. Uses ANSI escape sequences. | +-----------------+---------+--------------------------------------------------+ | timeout | 30s | Time to wait for interaction when a crash | | | | occurs. Setting this to ``none`` or ``0s`` will | | | | disable interaction. | +-----------------+---------+--------------------------------------------------+ | unwind | auto | Specifies which unwind algorithm to use. | | | | ``auto`` means to choose appropriately for the | | | | platform. Other options are ``fast``, which | | | | does a naïve stack walk; and ``precise``, which | | | | uses exception handling data to perform an | | | | unwind. | +-----------------+---------+--------------------------------------------------+ | preset | auto | Specifies which set of preset formatting options | | | | to use. Options are ``friendly``, ``medium`` or | | | | ``full``. ``auto`` means to use ``friendly`` if | | | | interactive, and ``full`` otherwise. | +-----------------+---------+--------------------------------------------------+ | sanitize | preset | If ``yes``, we will try to process paths to | | | | remove PII. Exact behaviour is platform | | | | dependent. | +-----------------+---------+--------------------------------------------------+ | threads | preset | Options are ``all`` to show backtraces for every | | | | thread, or ``crashed`` to show only the crashing | | | | thread. | +-----------------+---------+--------------------------------------------------+ | registers | preset | Options are ``none``, ``all`` or ``crashed``. | +-----------------+---------+--------------------------------------------------+ | images | preset | Options are ``none``, ``all``, or ``mentioned``, | | | | which only displays images mentioned in a | | | | backtrace. | +-----------------+---------+--------------------------------------------------+ | limit | 64 | Limits the length of the captured backtrace. See | | | | below for a discussion of its behaviour. Can be | | | | set to ``none`` to mean no limit. | +-----------------+---------+--------------------------------------------------+ | top | 16 | Specify a minimum number of frames to capture | | | | from the top of the stack. See below for more. | +-----------------+---------+--------------------------------------------------+ | cache | yes | Set to ``no`` to disable symbol caching. This | | | | only has effect on platforms that have a symbol | | | | cache that can be controlled by the runtime. | +-----------------+---------+--------------------------------------------------+ | output-to | stdout | Set to ``stderr`` to send the backtrace to the | | | | standard error instead of standard output. This | | | | may be useful in some CI systems. | +-----------------+---------+--------------------------------------------------+ | swift-backtrace | | If specified, gives the full path to the | | | | swift-backtrace binary to use for crashes. | | | | Otherwise, Swift will locate the binary relative | | | | to the runtime library, or using ``SWIFT_ROOT``. | +-----------------+---------+--------------------------------------------------+ (*) On macOS, this defaults to ``tty`` rather than ``yes``. Backtrace limits ---------------- The limit settings are provided both to prevent runaway backtraces and to allow for a sensible backtrace to be produced even when a function has blown the stack through excessive recursion. Typically in the latter case you want to capture some frames at the top of the stack so that you can see how the recursion was entered, and the frames at the bottom of the stack where the actual fault occurred. 1. There are ``limit`` or fewer frames. In this case we will display all the frames in the backtrace. Note that this _includes_ the case where there are exactly ``limit`` frames. 2. There are more than ``limit`` frames. a. ``top`` is ``0``. We will display the first ``limit - 1`` frames followed by ``...`` to indicate that more frames exist. b. ``top`` is less than ``limit - 1``. We will display ``limit - 1 - top`` frames from the bottom of the stack, then a ``...``, then ``top`` frames from the top of the stack. c. ``top`` is greater or equal to ``limit - 1``. We will display ``...``, followed by ``limit - 1`` frames from the top of the stack. For example, let's say we have a stack containing 10 frames numbered here 1 to 10, with 10 being the innermost frame. With ``limit`` set to 5, you would see:: 10 9 8 7 ... With ``limit`` set to 5 and ``top`` to 2, you would instead see:: 10 9 ... 2 1 And with ``limit`` set to 5 and ``top`` to 4 or above, you would see:: ... 4 3 2 1 What is the swift-backtrace binary? ----------------------------------- ``swift-backtrace`` is a program that gets invoked when your program crashes. We do this because when a program crashes, it is potentially in an invalid state and there is very little that is safe for us to do. By executing an external helper program, we ensure that we do not interfere with the way the program was going to crash (so that system-wide crash catchers will still generate the correct information), and we are also able to use any functionality we need to generate a decent backtrace, including symbolication (which might in general require memory allocation, fetching and reading remote files and so on). You shouldn't try to run ``swift-backtrace`` yourself; it has unusual requirements, which vary from platform to platform. Instead, it will be triggered automatically by the runtime. System specifics ---------------- macOS ^^^^^ On macOS, we catch crashes and other events using a signal handler. At time of writing, this is installed for the following signals: +--------------+--------------------------+-------------------------------------+ | Signal | Description | Comment | +====+=========+==========================+=====================================+ | 3 | SIGQUIT | Quit program | | +----+---------+--------------------------+-------------------------------------+ | 4 | SIGILL | Illegal instruction | | +----+---------+--------------------------+-------------------------------------+ | 5 | SIGTRAP | Trace trap | | +----+---------+--------------------------+-------------------------------------+ | 6 | SIGABRT | Abort program | | +----+---------+--------------------------+-------------------------------------+ | 8 | SIGFPE | Floating point exception | On Intel, integer divide by zero | | | | | also triggers this. | +----+---------+--------------------------+-------------------------------------+ | 10 | SIGBUS | Bus error | | +----+---------+--------------------------+-------------------------------------+ | 11 | SIGSEGV | Segmentation violation | | +----+---------+--------------------------+-------------------------------------+ If crash catching is enabled, the signal handler will be installed for any process that links the Swift runtime. If you replace the handlers for any of these signals, your program will no longer produce backtraces for program failures that lead to the handler you have replaced. Additionally, the runtime will configure an alternate signal handling stack, so that stack overflows can be successfully trapped. Note that the runtime will not install its signal handlers for a signal if it finds that there is already a handler for that signal. Similarly if something else has already configured an alternate signal stack, it will leave that stack alone. Once the backtracer has finished handling the crash, it will allow the crashing program to continue and crash normally, which will result in the usual Crash Reporter log file being generated. Crash catching *cannot* be enabled for setuid binaries. This is intentional as doing so might create a security hole. Other Darwin (iOS, tvOS) ^^^^^^^^^^^^^^^^^^^^^^^^ Crash catching is not enabled for non-macOS Darwin. You should continue to look at the system-provided crash logs.