wolfram-library-link 0.5.0

//! Utilities for catching panics, capturing a backtrace, and extracting the panic
//! message.

use std::collections::HashMap;
use std::panic::{self, UnwindSafe};
use std::process;
use std::sync::{self, Mutex};
use std::thread::{self, ThreadId};
use std::time::Instant;

#[cfg(feature = "panic-failure-backtraces")]
use backtrace::Backtrace;

use once_cell::sync::Lazy;

use crate::expr::{Expr, Symbol};

static CAUGHT_PANICS: Lazy<Mutex<HashMap<ThreadId, (Instant, CaughtPanic)>>> =
    Lazy::new(|| Default::default());

/// Information from a caught panic.
///
/// Returned by [`call_and_catch_panic()`].
#[derive(Clone)]
pub struct CaughtPanic {
    /// Note: In certain circumstances, this message will NOT match the message used
    /// in panic!(). This can happen when user code changes the panic hook, or when
    /// the panic occurs in a different thread from the one `call_and_catch_panic()`
    /// was called in.
    ///
    /// An inaccurate instance of `CaughtPanic` can also be reported when panic's
    /// occur in multiple threads at once.
    message: Option<String>,
    location: Option<String>,

    #[cfg(feature = "panic-failure-backtraces")]
    backtrace: Option<Backtrace>,
}

impl CaughtPanic {
    pub(crate) fn to_pretty_expr(&self) -> Expr {
        let CaughtPanic {
            message,
            location,

            #[cfg(feature = "panic-failure-backtraces")]
            backtrace,
        } = self.clone();

        let message = Expr::string(message.unwrap_or("Rust panic (no message)".into()));
        let location = Expr::string(location.unwrap_or("Unknown".into()));

        let backtrace = {
            // Avoid showing the backtrace if it hasn't been explicitly requested by the user.
            // This avoids calling `.resolve()` below, which can sometimes be very slow (100s of
            // millisends).
            if !cfg!(feature = "panic-failure-backtraces") || !should_show_backtrace() {
                Expr::normal(Symbol::new("System`Missing"), vec![Expr::string(
                    "NotEnabled",
                )])
            } else {
                #[cfg(feature = "panic-failure-backtraces")]
                {
                    display_backtrace(backtrace)
                }

                #[cfg(not(feature = "panic-failure-backtraces"))]
                unreachable!()
            }
        };

        // Failure["RustPanic", <|
        //     "MessageTemplate" -> "Rust LibraryLink function panic: `message`",
        //     "MessageParameters" -> <| "message" -> "..." |>,
        //     "SourceLocation" -> "...",
        //     "Backtrace" -> "..."
        // |>]
        Expr::normal(Symbol::new("System`Failure"), vec![
            Expr::string("RustPanic"),
            Expr::normal(Symbol::new("System`Association"), vec![
                Expr::normal(Symbol::new("System`Rule"), vec![
                    Expr::string("MessageTemplate"),
                    Expr::string("Rust LibraryLink function panic: `message`"),
                ]),
                Expr::normal(Symbol::new("System`Rule"), vec![
                    Expr::string("MessageParameters"),
                    Expr::normal(Symbol::new("System`Association"), vec![Expr::normal(
                        Symbol::new("System`Rule"),
                        vec![Expr::string("message"), message],
                    )]),
                ]),
                Expr::normal(Symbol::new("System`Rule"), vec![
                    Expr::string("SourceLocation"),
                    location,
                ]),
                Expr::normal(Symbol::new("System`Rule"), vec![
                    Expr::string("Backtrace"),
                    backtrace,
                ]),
            ]),
        ])
    }
}

fn should_show_backtrace() -> bool {
    std::env::var(crate::BACKTRACE_ENV_VAR).is_ok()
}

#[cfg(feature = "panic-failure-backtraces")]
fn display_backtrace(bt: Option<Backtrace>) -> Expr {
    let bt: Expr = if let Some(mut bt) = bt {
        // Resolve the symbols in the frames of the backtrace.
        bt.resolve();

        // Expr::string(format!("{:?}", bt))

        let mut frames = Vec::new();
        for (index, frame) in bt.frames().into_iter().enumerate() {
            use backtrace::{BacktraceSymbol, SymbolName};

            // TODO: Show all the symbols, not just the last one. A frame will be
            //       associated with more than one symbol if any inlining occured, so this
            //       would help show better backtraces in optimized builds.
            let bt_symbol: Option<&BacktraceSymbol> = frame.symbols().last();

            let name: String = bt_symbol
                .and_then(BacktraceSymbol::name)
                .as_ref()
                .map(|sym: &SymbolName| format!("{}", sym))
                .unwrap_or("<unknown>".into());

            // Skip frames from within the `backtrace` crate itself.
            if name.starts_with("backtrace::") {
                continue;
            }

            let filename = bt_symbol.and_then(BacktraceSymbol::filename);
            let lineno = bt_symbol.and_then(BacktraceSymbol::lineno);
            let file_and_line: String = match (filename, lineno) {
                (Some(path), Some(lineno)) => format!("{}:{}", path.display(), lineno),
                (Some(path), None) => format!("{}", path.display()),
                _ => "".into(),
            };

            // Row[{
            //     %[index.to_string()],
            //     ": ",
            //     'file_and_line,
            //     'name
            // }]
            frames.push(Expr::normal(Symbol::new("System`Row"), vec![Expr::normal(
                Symbol::new("System`List"),
                vec![
                    Expr::string(index.to_string()),
                    Expr::string(": "),
                    Expr::string(file_and_line),
                    Expr::string(name),
                ],
            )]));
        }

        let frames = Expr::normal(Symbol::new("System`List"), frames);
        // Set ImageSize so that the lines don't wordwrap for very long function names,
        // which makes the backtrace hard to read.

        // Column['frames, ImageSize -> {1200, Automatic}]
        Expr::normal(Symbol::new("System`Column"), vec![
            frames,
            Expr::normal(Symbol::new("System`Rule"), vec![
                Expr::symbol(Symbol::new("System`ImageSize")),
                Expr::normal(Symbol::new("System`List"), vec![
                    Expr::from(1200i64),
                    Expr::symbol(Symbol::new("System`Automatic")),
                ]),
            ]),
        ])
    } else {
        Expr::string("<unable to capture backtrace>")
    };

    // Row[{
    //     Style["Backtrace", Bold],
    //     ": ",
    //     Style['bt, FontSize -> 13, FontFamily -> "Source Code Pro"]
    // }]
    Expr::normal(Symbol::new("System`Row"), vec![Expr::normal(
        Symbol::new("System`List"),
        vec![
            Expr::normal(Symbol::new("System`Style"), vec![
                Expr::string("Backtrace"),
                Expr::symbol(Symbol::new("System`Bold")),
            ]),
            Expr::string(": "),
            Expr::normal(Symbol::new("System`Style"), vec![
                bt,
                Expr::normal(Symbol::new("System`Rule"), vec![
                    Expr::symbol(Symbol::new("System`FontSize")),
                    Expr::from(13i16),
                ]),
                Expr::normal(Symbol::new("System`Rule"), vec![
                    Expr::symbol(Symbol::new("System`FontFamily")),
                    Expr::string("Source Code Pro"),
                ]),
            ]),
        ],
    )])
}

/// Call `func` and catch any unwinding panic which occurs during that call, returning
/// information from the caught panic in the form of a `CaughtPanic`.
///
/// NOTE: `func` should not set it's own panic hook, or unset the panic hook set upon
///       calling it. Doing so would likely interfere with the operation of this function.
pub fn call_and_catch_panic<T, F>(func: F) -> Result<T, CaughtPanic>
where
    F: FnOnce() -> T + UnwindSafe,
{
    // Set up the panic hook. If calling `func` triggers a panic, the panic message string
    // and location will be saved into CAUGHT_PANICS.
    //
    // The panic hook is reset to the default handler before we return.
    let prev_hook = panic::take_hook();
    let _: () = panic::set_hook(Box::new(custom_hook));

    // Call `func`, catching any panic's which occur. The `Err` produced by `catch_unwind`
    // is an opaque object we can't get any information from; this is why it's necessary
    // to set the panic hook, which *does* get an inspectable object.
    let result: Result<T, ()> = panic::catch_unwind(|| func()).map_err(|_| ());

    // Return to the previously set hook (will be the default hook if no previous hook was
    // set).
    panic::set_hook(prev_hook);

    // If `result` is an `Err`, meaning a panic occured, read information out of
    // CAUGHT_PANICS.
    let result: Result<T, CaughtPanic> = result.map_err(|()| get_caught_panic());

    result
}

fn get_caught_panic() -> CaughtPanic {
    let id = thread::current().id();
    let mut map = acquire_lock();
    // Remove the `CaughtPanic` which should be associated with `id` now.
    let caught_panic = match map.remove(&id) {
        Some((_time, caught_panic)) => caught_panic.clone(),
        None => {
            match map.len() {
                0 => {
                    // This can occur when the user code sets their own panic hook, but
                    // fails to restore the previous panic hook (i.e., the `custom_hook`
                    // we set above).
                    let message = format!(
                        "could not get panic info for current thread. \
                         Operation of custom panic hook was interrupted"
                    );
                    CaughtPanic {
                        message: Some(message),
                        location: None,

                        #[cfg(feature = "panic-failure-backtraces")]
                        backtrace: None,
                    }
                },
                // This case can occur when a panic occurs in a thread spawned by the
                // current thread: the ThreadId stored in CAUGHT_PANICS's is not
                // the ThreadId of the current thread, but the panic still
                // "bubbled up" accross thread boundries to the catch_unwind() call
                // above.
                //
                // We simply guess that the only panic in the HashMap is the right one --
                // it's rare that multiple panic's will occur in multiple threads at the
                // same time (meaning there's more than one entry in the map).
                1 => map.values().next().unwrap().1.clone(),
                // Pick the most recent panic, and hope it's the right one.
                _ => map
                    .values()
                    .max_by(|a, b| a.0.cmp(&b.0))
                    .map(|(_time, info)| info)
                    .cloned()
                    .unwrap(),
            }
        },
    };
    caught_panic
}

fn custom_hook(info: &panic::PanicInfo) {
    let caught_panic = {
        let message: Option<String> = get_panic_message(info);
        let location: Option<String> = info.location().map(ToString::to_string);

        // Don't resolve the backtrace inside the panic hook. This seems to hang for a
        // long time (maybe forever?). Resolving it later, in the ToPrettyExpr impl, seems
        // to work (though it is noticeably slower, takes maybe ~0.5s-1s).
        #[cfg(feature = "panic-failure-backtraces")]
        let backtrace = Some(Backtrace::new_unresolved());

        CaughtPanic {
            message,
            location,

            #[cfg(feature = "panic-failure-backtraces")]
            backtrace,
        }
    };

    // The `ThreadId` of the thread which is currently panic'ing.
    let thread = thread::current();
    let data = (Instant::now(), caught_panic);

    let mut lock = acquire_lock();

    if let Some(_previous) = lock.insert(thread.id(), data) {
        // This situation is unlikely, but it can happen.
        //
        // This panic hook is used for every panic which occurs while it is set. This
        // includes panic's which are caught before reaching the `panic::catch_unwind()`,
        // above in `call_and_catch_panic()`, which happens when the user code also uses
        // `panic::catch_unwind()`. When that occurs, this hook (assuming the user hasn't
        // also set their own panic hook) will create an entry in CAUGHT_PANICS's. That
        // entry is never cleared, because the panic is caught before reaching the call to
        // `remove()` in `call_and_catch_panic()`.
    }
}

fn get_panic_message(info: &panic::PanicInfo) -> Option<String> {
    // Extract the message from `panic!("...")` statements.
    // In this case, the payload is always the static formatting string.
    if let Some(string) = info.payload().downcast_ref::<&str>() {
        return Some(string.to_string());
    }

    // Extract the message from `panic!("... {} ...", arg...)` statements.
    // In this case, the payload has to be a dynamically allocated String to contain
    // the arbitrary formatted arguments.
    if let Some(string) = info.payload().downcast_ref::<String>() {
        return Some(string.to_owned());
    }

    #[cfg(feature = "nightly")]
    if let Some(fmt_arguments) = info.message() {
        return Some(format!("{}", fmt_arguments));
    }

    None
}

/// Attempt to acquire a lock on CAUGHT_PANIC. Exit the current process if we can not,
/// without panic'ing.
fn acquire_lock() -> sync::MutexGuard<'static, HashMap<ThreadId, (Instant, CaughtPanic)>>
{
    let lock = match CAUGHT_PANICS.lock() {
        Ok(lock) => lock,
        Err(_err) => {
            println!(
                "catch_panic: acquire_lock: failed to acquire lock. Exiting process."
            );
            process::exit(-1);
        },
    };
    lock
}