      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205

    use std::cell::{Cell, RefCell};
    use std::fmt;
    use std::marker::PhantomData;

    #[derive(Debug, Clone, Copy)]
    pub(crate) enum EnterContext {
        #[cfg_attr(not(feature = "rt"), allow(dead_code))]
        Entered {
            allow_blocking: bool,
        },
        NotEntered,
    }

    impl EnterContext {
        pub(crate) fn is_entered(self) -> bool {
            matches!(self, EnterContext::Entered { .. })
        }
    }

    thread_local!(static ENTERED: Cell<EnterContext> = Cell::new(EnterContext::NotEntered));

    /// Represents an executor context.
    pub(crate) struct Enter {
        _p: PhantomData<RefCell<()>>,
    }

    cfg_rt! {
        use crate::park::thread::ParkError;

        use std::time::Duration;

        /// Marks the current thread as being within the dynamic extent of an
        /// executor.
        pub(crate) fn enter(allow_blocking: bool) -> Enter {
            if let Some(enter) = try_enter(allow_blocking) {
                return enter;
            }

            panic!(
                "Cannot start a runtime from within a runtime. This happens \
                because a function (like `block_on`) attempted to block the \
                current thread while the thread is being used to drive \
                asynchronous tasks."
            );
        }

        /// Tries to enter a runtime context, returns `None` if already in a runtime
        /// context.
        pub(crate) fn try_enter(allow_blocking: bool) -> Option<Enter> {
            ENTERED.with(|c| {
                if c.get().is_entered() {
                    None
                } else {
                    c.set(EnterContext::Entered { allow_blocking });
                    Some(Enter { _p: PhantomData })
                }
            })
        }
    }

    // Forces the current "entered" state to be cleared while the closure
    // is executed.
    //
    // # Warning
    //
    // This is hidden for a reason. Do not use without fully understanding
    // executors. Misuing can easily cause your program to deadlock.
    cfg_rt_multi_thread! {
        pub(crate) fn exit<F: FnOnce() -> R, R>(f: F) -> R {
            // Reset in case the closure panics
            struct Reset(EnterContext);
            impl Drop for Reset {
                fn drop(&mut self) {
                    ENTERED.with(|c| {
                        assert!(!c.get().is_entered(), "closure claimed permanent executor");
                        c.set(self.0);
                    });
                }
            }

            let was = ENTERED.with(|c| {
                let e = c.get();
                assert!(e.is_entered(), "asked to exit when not entered");
                c.set(EnterContext::NotEntered);
                e
            });

            let _reset = Reset(was);
            // dropping _reset after f() will reset ENTERED
            f()
        }
    }

    cfg_rt! {
        /// Disallow blocking in the current runtime context until the guard is dropped.
        pub(crate) fn disallow_blocking() -> DisallowBlockingGuard {
            let reset = ENTERED.with(|c| {
                if let EnterContext::Entered {
                    allow_blocking: true,
                } = c.get()
                {
                    c.set(EnterContext::Entered {
                        allow_blocking: false,
                    });
                    true
                } else {
                    false
                }
            });
            DisallowBlockingGuard(reset)
        }

        pub(crate) struct DisallowBlockingGuard(bool);
        impl Drop for DisallowBlockingGuard {
            fn drop(&mut self) {
                if self.0 {
                    // XXX: Do we want some kind of assertion here, or is "best effort" okay?
                    ENTERED.with(|c| {
                        if let EnterContext::Entered {
                            allow_blocking: false,
                        } = c.get()
                        {
                            c.set(EnterContext::Entered {
                                allow_blocking: true,
                            });
                        }
                    })
                }
            }
        }
    }

    cfg_rt_multi_thread! {
        /// Returns true if in a runtime context.
        pub(crate) fn context() -> EnterContext {
            ENTERED.with(|c| c.get())
        }
    }

    cfg_rt! {
        impl Enter {
            /// Blocks the thread on the specified future, returning the value with
            /// which that future completes.
            pub(crate) fn block_on<F>(&mut self, f: F) -> Result<F::Output, ParkError>
            where
                F: std::future::Future,
            {
                use crate::park::thread::CachedParkThread;

                let mut park = CachedParkThread::new();
                park.block_on(f)
            }

            /// Blocks the thread on the specified future for **at most** `timeout`
            ///
            /// If the future completes before `timeout`, the result is returned. If
            /// `timeout` elapses, then `Err` is returned.
            pub(crate) fn block_on_timeout<F>(&mut self, f: F, timeout: Duration) -> Result<F::Output, ParkError>
            where
                F: std::future::Future,
            {
                use crate::park::Park;
                use crate::park::thread::CachedParkThread;
                use std::task::Context;
                use std::task::Poll::Ready;
                use std::time::Instant;

                let mut park = CachedParkThread::new();
                let waker = park.get_unpark()?.into_waker();
                let mut cx = Context::from_waker(&waker);

                pin!(f);
                let when = Instant::now() + timeout;

                loop {
                    if let Ready(v) = crate::coop::budget(|| f.as_mut().poll(&mut cx)) {
                        return Ok(v);
                    }

                    let now = Instant::now();

                    if now >= when {
                        return Err(());
                    }

                    park.park_timeout(when - now)?;
                }
            }
        }
    }

    impl fmt::Debug for Enter {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Enter").finish()
        }
    }

    impl Drop for Enter {
        fn drop(&mut self) {
            ENTERED.with(|c| {
                assert!(c.get().is_entered());
                c.set(EnterContext::NotEntered);
            });
        }
    }