Files
addr2line
adler
ahash
aho_corasick
ansi_term
anyhow
arc_swap
arrayref
arrayvec
ascii
assert_matches
async_stream
async_stream_impl
async_trait
atty
auto_enums
auto_enums_core
auto_enums_derive
backoff
backtrace
base32
base64
bincode
bip39
bitflags
bitvec
blake3
block_buffer
block_padding
borsh
borsh_derive
borsh_derive_internal
borsh_schema_derive_internal
bs58
bstr
bv
byte_slice_cast
byte_unit
bytecount
byteorder
bytes
bzip2
bzip2_sys
cargo_build_bpf
cargo_metadata
cargo_platform
cargo_test_bpf
cast
cc
cfg_if
chrono
chrono_humanize
clap
colored
combine
console
const_fn
constant_time_eq
core_affinity
cpufeatures
crc32fast
criterion_stats
crossbeam_channel
crossbeam_deque
crossbeam_epoch
crossbeam_queue
crossbeam_utils
crunchy
crypto_mac
csv
csv_core
ctrlc
curve25519_dalek
dashmap
derivative
derive_more
derive_utils
dialoguer
digest
dir_diff
dirs_next
dirs_sys_next
dlopen
dlopen_derive
doc_comment
dtoa
ed25519
ed25519_dalek
either
encoding_rs
enum_iterator
enum_iterator_derive
env_logger
ethabi
ethbloom
ethereum
ethereum_types
evm
evm_bridge
evm_core
evm_gasometer
evm_rpc
evm_runtime
evm_state
evm_utils
failure
failure_derive
fake_simd
fast_math
fd_lock
filetime
fixed_hash
flate2
fnv
foreign_types
foreign_types_shared
form_urlencoded
fs_extra
futures
futures_channel
futures_core
futures_executor
futures_io
futures_macro
futures_sink
futures_task
futures_util
async_await
future
io
lock
sink
stream
task
gag
generic_array
gethostname
getrandom
gimli
globset
goauth
goblin
h2
half
hash256_std_hasher
hash32
hash_db
hashbrown
heck
hex
hidapi
histogram
hmac
hmac_drbg
http
http_body
httparse
httpdate
humantime
hyper
hyper_rustls
hyper_tls
idna
ieee754
impl_codec
impl_rlp
impl_serde
indexed
indexmap
indicatif
inflector
cases
camelcase
case
classcase
kebabcase
pascalcase
screamingsnakecase
sentencecase
snakecase
tablecase
titlecase
traincase
numbers
deordinalize
ordinalize
string
constants
deconstantize
demodulize
pluralize
singularize
suffix
foreignkey
input_buffer
instant
iovec
ipnet
itertools
itoa
jemalloc_ctl
jemalloc_sys
jemallocator
jobserver
jsonrpc_client_transports
jsonrpc_core
jsonrpc_core_client
jsonrpc_derive
jsonrpc_http_server
jsonrpc_pubsub
jsonrpc_server_utils
jsonrpc_ws_server
keccak
keccak_hash
keccak_hasher
kernel32
lazy_static
lazycell
libc
libloading
librocksdb_sys
linked_hash_map
lock_api
log
lru
matches
maybe_uninit
memchr
memmap2
memoffset
mime
mime_guess
miniz_oxide
mio
mio_extras
miow
native_tls
net2
nix
num_cpus
num_derive
num_enum
num_enum_derive
num_integer
num_traits
number_prefix
object
once_cell
opaque_debug
openssl
openssl_probe
openssl_sys
ouroboros
ouroboros_macro
parity_scale_codec
parity_scale_codec_derive
parity_ws
parking_lot
parking_lot_core
paste
paste_impl
paw
paw_attributes
paw_raw
pbkdf2
percent_encoding
pest
pickledb
pin_project
pin_project_lite
pin_utils
plain
ppv_lite86
pretty_hex
primitive_types
proc_macro2
proc_macro_crate
proc_macro_error
proc_macro_error_attr
proc_macro_hack
proc_macro_nested
prost
prost_derive
prost_types
quote
radium
rand
rand_chacha
rand_core
rand_isaac
raptorq
rayon
rayon_core
reed_solomon_erasure
regex
regex_automata
regex_syntax
remove_dir_all
reqwest
retain_mut
ring
ripemd160
rlp
rlp_derive
rocksdb
rpassword
rustc_demangle
rustc_hash
rustc_hex
rustls
rustversion
ryu
same_file
scopeguard
scroll
scroll_derive
sct
secp256k1
secp256k1_sys
semver
semver_parser
serde
serde_bytes
serde_cbor
serde_derive
serde_json
serde_urlencoded
serde_yaml
sha1
sha2
sha3
signal_hook
signal_hook_registry
signature
simpl
simple_logger
slab
smallvec
smpl_jwt
snafu
snafu_derive
socket2
solana_account_decoder
solana_accounts_bench
solana_banking_bench
solana_banks_client
solana_banks_interface
solana_banks_server
solana_bench_exchange
solana_bench_streamer
solana_bench_tps
solana_bench_tps_evm
solana_bpf_loader_program
solana_budget_program
solana_clap_utils
solana_cli
solana_cli_config
solana_cli_output
solana_client
solana_config_program
solana_core
solana_crate_features
solana_csv_to_validator_infos
solana_dos
solana_download_utils
solana_evm_loader_program
solana_exchange_program
solana_failure_program
solana_faucet
solana_frozen_abi
solana_frozen_abi_macro
solana_genesis
solana_ip_address
solana_ip_address_server
solana_ledger
solana_ledger_tool
solana_ledger_udev
solana_local_cluster
solana_log_analyzer
solana_logger
solana_measure
solana_merkle_root_bench
solana_merkle_tree
solana_metrics
solana_net_shaper
solana_net_utils
solana_noop_program
solana_notifier
solana_ownable
solana_perf
solana_poh_bench
solana_program
solana_program_test
solana_ramp_tps
solana_rayon_threadlimit
solana_rbpf
solana_remote_wallet
solana_runtime
solana_sdk
solana_sdk_macro
solana_secp256k1_program
solana_sleep_program
solana_stake_accounts
solana_stake_monitor
solana_stake_o_matic
solana_stake_program
solana_storage_bigtable
solana_storage_proto
solana_store_tool
solana_streamer
solana_sys_tuner
solana_tokens
solana_transaction_status
solana_upload_perf
solana_version
solana_vest_program
solana_vote_program
solana_watchtower
spin
spl_associated_token_account
spl_memo
spl_token
stable_deref_trait
standback
static_assertions
strsim
structopt
structopt_derive
subtle
symlink
syn
synstructure
sysctl
tar
tarpc
tarpc_plugins
tempfile
termcolor
terminal_size
textwrap
thiserror
thiserror_impl
thread_scoped
time
time_macros
time_macros_impl
tiny_keccak
tinyvec
tinyvec_macros
tokio
fs
future
io
loom
macros
net
park
process
runtime
signal
stream
sync
task
time
util
tokio_codec
tokio_executor
tokio_fs
tokio_io
tokio_reactor
tokio_rustls
tokio_serde
tokio_sync
tokio_tcp
tokio_threadpool
tokio_tls
tokio_util
toml
tonic
tower
tower_balance
tower_buffer
tower_discover
tower_layer
tower_limit
tower_load
tower_load_shed
tower_make
tower_ready_cache
tower_retry
tower_service
tower_timeout
tower_util
tracing
tracing_attributes
tracing_core
tracing_futures
trees
triedb
triehash
try_lock
tungstenite
typenum
ucd_trie
uint
unicase
unicode_bidi
unicode_normalization
unicode_segmentation
unicode_width
unicode_xid
unix_socket
unreachable
untrusted
url
users
utf8
utf8_width
vec_map
velas
velas_account_program
velas_faucet
velas_genesis
velas_gossip
velas_install
velas_install_init
velas_keygen
velas_test_validator
velas_validator
void
walkdir
want
webpki
webpki_roots
websocket
websocket_base
winapi
ws2_32
xattr
yaml_rust
zeroize
zeroize_derive
zstd
zstd_safe
zstd_sys
  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
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
//! An unbounded set of futures.
//!
//! This module is only available when the `std` or `alloc` feature of this
//! library is activated, and it is activated by default.

use futures_core::future::Future;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
use futures_task::{FutureObj, LocalFutureObj, Spawn, LocalSpawn, SpawnError};
use crate::task::AtomicWaker;
use core::cell::UnsafeCell;
use core::fmt::{self, Debug};
use core::iter::FromIterator;
use core::marker::PhantomData;
use core::mem;
use core::pin::Pin;
use core::ptr;
use core::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release, SeqCst};
use core::sync::atomic::{AtomicPtr, AtomicBool};
use alloc::sync::{Arc, Weak};

mod abort;

mod iter;
pub use self::iter::{Iter, IterMut, IterPinMut, IterPinRef};

mod task;
use self::task::Task;

mod ready_to_run_queue;
use self::ready_to_run_queue::{ReadyToRunQueue, Dequeue};


/// A set of futures which may complete in any order.
///
/// This structure is optimized to manage a large number of futures.
/// Futures managed by [`FuturesUnordered`] will only be polled when they
/// generate wake-up notifications. This reduces the required amount of work
/// needed to poll large numbers of futures.
///
/// [`FuturesUnordered`] can be filled by [`collect`](Iterator::collect)ing an
/// iterator of futures into a [`FuturesUnordered`], or by
/// [`push`](FuturesUnordered::push)ing futures onto an existing
/// [`FuturesUnordered`]. When new futures are added,
/// [`poll_next`](Stream::poll_next) must be called in order to begin receiving
/// wake-ups for new futures.
///
/// Note that you can create a ready-made [`FuturesUnordered`] via the
/// [`collect`](Iterator::collect) method, or you can start with an empty set
/// with the [`FuturesUnordered::new`] constructor.
///
/// This type is only available when the `std` or `alloc` feature of this
/// library is activated, and it is activated by default.
#[must_use = "streams do nothing unless polled"]
pub struct FuturesUnordered<Fut> {
    ready_to_run_queue: Arc<ReadyToRunQueue<Fut>>,
    head_all: AtomicPtr<Task<Fut>>,
    is_terminated: AtomicBool,
}

unsafe impl<Fut: Send> Send for FuturesUnordered<Fut> {}
unsafe impl<Fut: Sync> Sync for FuturesUnordered<Fut> {}
impl<Fut> Unpin for FuturesUnordered<Fut> {}

impl Spawn for FuturesUnordered<FutureObj<'_, ()>> {
    fn spawn_obj(&self, future_obj: FutureObj<'static, ()>)
        -> Result<(), SpawnError>
    {
        self.push(future_obj);
        Ok(())
    }
}

impl LocalSpawn for FuturesUnordered<LocalFutureObj<'_, ()>> {
    fn spawn_local_obj(&self, future_obj: LocalFutureObj<'static, ()>)
        -> Result<(), SpawnError>
    {
        self.push(future_obj);
        Ok(())
    }
}

// FuturesUnordered is implemented using two linked lists. One which links all
// futures managed by a `FuturesUnordered` and one that tracks futures that have
// been scheduled for polling. The first linked list allows for thread safe
// insertion of nodes at the head as well as forward iteration, but is otherwise
// not thread safe and is only accessed by the thread that owns the
// `FuturesUnordered` value for any other operations. The second linked list is
// an implementation of the intrusive MPSC queue algorithm described by
// 1024cores.net.
//
// When a future is submitted to the set, a task is allocated and inserted in
// both linked lists. The next call to `poll_next` will (eventually) see this
// task and call `poll` on the future.
//
// Before a managed future is polled, the current context's waker is replaced
// with one that is aware of the specific future being run. This ensures that
// wake-up notifications generated by that specific future are visible to
// `FuturesUnordered`. When a wake-up notification is received, the task is
// inserted into the ready to run queue, so that its future can be polled later.
//
// Each task is wrapped in an `Arc` and thereby atomically reference counted.
// Also, each task contains an `AtomicBool` which acts as a flag that indicates
// whether the task is currently inserted in the atomic queue. When a wake-up
// notifiaction is received, the task will only be inserted into the ready to
// run queue if it isn't inserted already.

impl<Fut> Default for FuturesUnordered<Fut> {
    fn default() -> Self {
        Self::new()
    }
}

impl<Fut> FuturesUnordered<Fut> {
    /// Constructs a new, empty [`FuturesUnordered`].
    ///
    /// The returned [`FuturesUnordered`] does not contain any futures.
    /// In this state, [`FuturesUnordered::poll_next`](Stream::poll_next) will
    /// return [`Poll::Ready(None)`](Poll::Ready).
    pub fn new() -> Self {
        let stub = Arc::new(Task {
            future: UnsafeCell::new(None),
            next_all: AtomicPtr::new(ptr::null_mut()),
            prev_all: UnsafeCell::new(ptr::null()),
            len_all: UnsafeCell::new(0),
            next_ready_to_run: AtomicPtr::new(ptr::null_mut()),
            queued: AtomicBool::new(true),
            ready_to_run_queue: Weak::new(),
        });
        let stub_ptr = &*stub as *const Task<Fut>;
        let ready_to_run_queue = Arc::new(ReadyToRunQueue {
            waker: AtomicWaker::new(),
            head: AtomicPtr::new(stub_ptr as *mut _),
            tail: UnsafeCell::new(stub_ptr),
            stub,
        });

        Self {
            head_all: AtomicPtr::new(ptr::null_mut()),
            ready_to_run_queue,
            is_terminated: AtomicBool::new(false),
        }
    }

    /// Returns the number of futures contained in the set.
    ///
    /// This represents the total number of in-flight futures.
    pub fn len(&self) -> usize {
        let (_, len) = self.atomic_load_head_and_len_all();
        len
    }

    /// Returns `true` if the set contains no futures.
    pub fn is_empty(&self) -> bool {
        // Relaxed ordering can be used here since we don't need to read from
        // the head pointer, only check whether it is null.
        self.head_all.load(Relaxed).is_null()
    }

    /// Push a future into the set.
    ///
    /// This method adds the given future to the set. This method will not
    /// call [`poll`](core::future::Future::poll) on the submitted future. The caller must
    /// ensure that [`FuturesUnordered::poll_next`](Stream::poll_next) is called
    /// in order to receive wake-up notifications for the given future.
    pub fn push(&self, future: Fut) {
        let task = Arc::new(Task {
            future: UnsafeCell::new(Some(future)),
            next_all: AtomicPtr::new(self.pending_next_all()),
            prev_all: UnsafeCell::new(ptr::null_mut()),
            len_all: UnsafeCell::new(0),
            next_ready_to_run: AtomicPtr::new(ptr::null_mut()),
            queued: AtomicBool::new(true),
            ready_to_run_queue: Arc::downgrade(&self.ready_to_run_queue),
        });

        // Reset the `is_terminated` flag if we've previously marked ourselves
        // as terminated.
        self.is_terminated.store(false, Relaxed);

        // Right now our task has a strong reference count of 1. We transfer
        // ownership of this reference count to our internal linked list
        // and we'll reclaim ownership through the `unlink` method below.
        let ptr = self.link(task);

        // We'll need to get the future "into the system" to start tracking it,
        // e.g. getting its wake-up notifications going to us tracking which
        // futures are ready. To do that we unconditionally enqueue it for
        // polling here.
        self.ready_to_run_queue.enqueue(ptr);
    }

    /// Returns an iterator that allows inspecting each future in the set.
    pub fn iter(&self) -> Iter<'_, Fut> where Fut: Unpin {
        Iter(Pin::new(self).iter_pin_ref())
    }

    /// Returns an iterator that allows inspecting each future in the set.
    fn iter_pin_ref(self: Pin<&Self>) -> IterPinRef<'_, Fut> {
        let (task, len) = self.atomic_load_head_and_len_all();

        IterPinRef {
            task,
            len,
            pending_next_all: self.pending_next_all(),
            _marker: PhantomData,
        }
    }

    /// Returns an iterator that allows modifying each future in the set.
    pub fn iter_mut(&mut self) -> IterMut<'_, Fut> where Fut: Unpin {
        IterMut(Pin::new(self).iter_pin_mut())
    }

    /// Returns an iterator that allows modifying each future in the set.
    pub fn iter_pin_mut(mut self: Pin<&mut Self>) -> IterPinMut<'_, Fut> {
        // `head_all` can be accessed directly and we don't need to spin on
        // `Task::next_all` since we have exclusive access to the set.
        let task = *self.head_all.get_mut();
        let len = if task.is_null() {
            0
        } else {
            unsafe {
                *(*task).len_all.get()
            }
        };

        IterPinMut {
            task,
            len,
            _marker: PhantomData
        }
    }

    /// Returns the current head node and number of futures in the list of all
    /// futures within a context where access is shared with other threads
    /// (mostly for use with the `len` and `iter_pin_ref` methods).
    fn atomic_load_head_and_len_all(&self) -> (*const Task<Fut>, usize) {
        let task = self.head_all.load(Acquire);
        let len = if task.is_null() {
            0
        } else {
            unsafe {
                (*task).spin_next_all(self.pending_next_all(), Acquire);
                *(*task).len_all.get()
            }
        };

        (task, len)
    }

    /// Releases the task. It destorys the future inside and either drops
    /// the `Arc<Task>` or transfers ownership to the ready to run queue.
    /// The task this method is called on must have been unlinked before.
    fn release_task(&mut self, task: Arc<Task<Fut>>) {
        // `release_task` must only be called on unlinked tasks
        debug_assert_eq!(task.next_all.load(Relaxed), self.pending_next_all());
        unsafe {
            debug_assert!((*task.prev_all.get()).is_null());
        }

        // The future is done, try to reset the queued flag. This will prevent
        // `wake` from doing any work in the future
        let prev = task.queued.swap(true, SeqCst);

        // Drop the future, even if it hasn't finished yet. This is safe
        // because we're dropping the future on the thread that owns
        // `FuturesUnordered`, which correctly tracks `Fut`'s lifetimes and
        // such.
        unsafe {
            // Set to `None` rather than `take()`ing to prevent moving the
            // future.
            *task.future.get() = None;
        }

        // If the queued flag was previously set, then it means that this task
        // is still in our internal ready to run queue. We then transfer
        // ownership of our reference count to the ready to run queue, and it'll
        // come along and free it later, noticing that the future is `None`.
        //
        // If, however, the queued flag was *not* set then we're safe to
        // release our reference count on the task. The queued flag was set
        // above so all future `enqueue` operations will not actually
        // enqueue the task, so our task will never see the ready to run queue
        // again. The task itself will be deallocated once all reference counts
        // have been dropped elsewhere by the various wakers that contain it.
        if prev {
            mem::forget(task);
        }
    }

    /// Insert a new task into the internal linked list.
    fn link(&self, task: Arc<Task<Fut>>) -> *const Task<Fut> {
        // `next_all` should already be reset to the pending state before this
        // function is called.
        debug_assert_eq!(task.next_all.load(Relaxed), self.pending_next_all());
        let ptr = Arc::into_raw(task);

        // Atomically swap out the old head node to get the node that should be
        // assigned to `next_all`.
        let next = self.head_all.swap(ptr as *mut _, AcqRel);

        unsafe {
            // Store the new list length in the new node.
            let new_len = if next.is_null() {
                1
            } else {
                // Make sure `next_all` has been written to signal that it is
                // safe to read `len_all`.
                (*next).spin_next_all(self.pending_next_all(), Acquire);
                *(*next).len_all.get() + 1
            };
            *(*ptr).len_all.get() = new_len;

            // Write the old head as the next node pointer, signaling to other
            // threads that `len_all` and `next_all` are ready to read.
            (*ptr).next_all.store(next, Release);

            // `prev_all` updates don't need to be synchronized, as the field is
            // only ever used after exclusive access has been acquired.
            if !next.is_null() {
                *(*next).prev_all.get() = ptr;
            }
        }

        ptr
    }

    /// Remove the task from the linked list tracking all tasks currently
    /// managed by `FuturesUnordered`.
    /// This method is unsafe because it has be guaranteed that `task` is a
    /// valid pointer.
    unsafe fn unlink(&mut self, task: *const Task<Fut>) -> Arc<Task<Fut>> {
        // Compute the new list length now in case we're removing the head node
        // and won't be able to retrieve the correct length later.
        let head = *self.head_all.get_mut();
        debug_assert!(!head.is_null());
        let new_len = *(*head).len_all.get() - 1;

        let task = Arc::from_raw(task);
        let next = task.next_all.load(Relaxed);
        let prev = *task.prev_all.get();
        task.next_all.store(self.pending_next_all(), Relaxed);
        *task.prev_all.get() = ptr::null_mut();

        if !next.is_null() {
            *(*next).prev_all.get() = prev;
        }

        if !prev.is_null() {
            (*prev).next_all.store(next, Relaxed);
        } else {
            *self.head_all.get_mut() = next;
        }

        // Store the new list length in the head node.
        let head = *self.head_all.get_mut();
        if !head.is_null() {
            *(*head).len_all.get() = new_len;
        }

        task
    }

    /// Returns the reserved value for `Task::next_all` to indicate a pending
    /// assignment from the thread that inserted the task.
    ///
    /// `FuturesUnordered::link` needs to update `Task` pointers in an order
    /// that ensures any iterators created on other threads can correctly
    /// traverse the entire `Task` list using the chain of `next_all` pointers.
    /// This could be solved with a compare-exchange loop that stores the
    /// current `head_all` in `next_all` and swaps out `head_all` with the new
    /// `Task` pointer if the head hasn't already changed. Under heavy thread
    /// contention, this compare-exchange loop could become costly.
    ///
    /// An alternative is to initialize `next_all` to a reserved pending state
    /// first, perform an atomic swap on `head_all`, and finally update
    /// `next_all` with the old head node. Iterators will then either see the
    /// pending state value or the correct next node pointer, and can reload
    /// `next_all` as needed until the correct value is loaded. The number of
    /// retries needed (if any) would be small and will always be finite, so
    /// this should generally perform better than the compare-exchange loop.
    ///
    /// A valid `Task` pointer in the `head_all` list is guaranteed to never be
    /// this value, so it is safe to use as a reserved value until the correct
    /// value can be written.
    fn pending_next_all(&self) -> *mut Task<Fut> {
        // The `ReadyToRunQueue` stub is never inserted into the `head_all`
        // list, and its pointer value will remain valid for the lifetime of
        // this `FuturesUnordered`, so we can make use of its value here.
        &*self.ready_to_run_queue.stub as *const _ as *mut _
    }
}

impl<Fut: Future> Stream for FuturesUnordered<Fut> {
    type Item = Fut::Output;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>)
        -> Poll<Option<Self::Item>>
    {
        // Variable to determine how many times it is allowed to poll underlying
        // futures without yielding.
        //
        // A single call to `poll_next` may potentially do a lot of work before
        // yielding. This happens in particular if the underlying futures are awoken
        // frequently but continue to return `Pending`. This is problematic if other
        // tasks are waiting on the executor, since they do not get to run. This value
        // caps the number of calls to `poll` on underlying futures a single call to
        // `poll_next` is allowed to make.
        //
        // The value is the length of FuturesUnordered. This ensures that each
        // future is polled only once at most per iteration.
        //
        // See also https://github.com/rust-lang/futures-rs/issues/2047.
        let yield_every = self.len();

        // Keep track of how many child futures we have polled,
        // in case we want to forcibly yield.
        let mut polled = 0;

        // Ensure `parent` is correctly set.
        self.ready_to_run_queue.waker.register(cx.waker());

        loop {
            // Safety: &mut self guarantees the mutual exclusion `dequeue`
            // expects
            let task = match unsafe { self.ready_to_run_queue.dequeue() } {
                Dequeue::Empty => {
                    if self.is_empty() {
                        // We can only consider ourselves terminated once we
                        // have yielded a `None`
                        *self.is_terminated.get_mut() = true;
                        return Poll::Ready(None);
                    } else {
                        return Poll::Pending;
                    }
                }
                Dequeue::Inconsistent => {
                    // At this point, it may be worth yielding the thread &
                    // spinning a few times... but for now, just yield using the
                    // task system.
                    cx.waker().wake_by_ref();
                    return Poll::Pending;
                }
                Dequeue::Data(task) => task,
            };

            debug_assert!(task != self.ready_to_run_queue.stub());

            // Safety:
            // - `task` is a valid pointer.
            // - We are the only thread that accesses the `UnsafeCell` that
            //   contains the future
            let future = match unsafe { &mut *(*task).future.get() } {
                Some(future) => future,

                // If the future has already gone away then we're just
                // cleaning out this task. See the comment in
                // `release_task` for more information, but we're basically
                // just taking ownership of our reference count here.
                None => {
                    // This case only happens when `release_task` was called
                    // for this task before and couldn't drop the task
                    // because it was already enqueued in the ready to run
                    // queue.

                    // Safety: `task` is a valid pointer
                    let task = unsafe { Arc::from_raw(task) };

                    // Double check that the call to `release_task` really
                    // happened. Calling it required the task to be unlinked.
                    debug_assert_eq!(
                        task.next_all.load(Relaxed),
                        self.pending_next_all()
                    );
                    unsafe {
                        debug_assert!((*task.prev_all.get()).is_null());
                    }
                    continue
                }
            };

            // Safety: `task` is a valid pointer
            let task = unsafe { self.unlink(task) };

            // Unset queued flag: This must be done before polling to ensure
            // that the future's task gets rescheduled if it sends a wake-up
            // notification **during** the call to `poll`.
            let prev = task.queued.swap(false, SeqCst);
            assert!(prev);

            // We're going to need to be very careful if the `poll`
            // method below panics. We need to (a) not leak memory and
            // (b) ensure that we still don't have any use-after-frees. To
            // manage this we do a few things:
            //
            // * A "bomb" is created which if dropped abnormally will call
            //   `release_task`. That way we'll be sure the memory management
            //   of the `task` is managed correctly. In particular
            //   `release_task` will drop the future. This ensures that it is
            //   dropped on this thread and not accidentally on a different
            //   thread (bad).
            // * We unlink the task from our internal queue to preemptively
            //   assume it'll panic, in which case we'll want to discard it
            //   regardless.
            struct Bomb<'a, Fut> {
                queue: &'a mut FuturesUnordered<Fut>,
                task: Option<Arc<Task<Fut>>>,
            }

            impl<Fut> Drop for Bomb<'_, Fut> {
                fn drop(&mut self) {
                    if let Some(task) = self.task.take() {
                        self.queue.release_task(task);
                    }
                }
            }

            let mut bomb = Bomb {
                task: Some(task),
                queue: &mut *self,
            };

            // Poll the underlying future with the appropriate waker
            // implementation. This is where a large bit of the unsafety
            // starts to stem from internally. The waker is basically just
            // our `Arc<Task<Fut>>` and can schedule the future for polling by
            // enqueuing itself in the ready to run queue.
            //
            // Critically though `Task<Fut>` won't actually access `Fut`, the
            // future, while it's floating around inside of wakers.
            // These structs will basically just use `Fut` to size
            // the internal allocation, appropriately accessing fields and
            // deallocating the task if need be.
            let res = {
                let waker = Task::waker_ref(bomb.task.as_ref().unwrap());
                let mut cx = Context::from_waker(&waker);

                // Safety: We won't move the future ever again
                let future = unsafe { Pin::new_unchecked(future) };

                future.poll(&mut cx)
            };
            polled += 1;

            match res {
                Poll::Pending => {
                    let task = bomb.task.take().unwrap();
                    bomb.queue.link(task);

                    if polled == yield_every {
                        // We have polled a large number of futures in a row without yielding.
                        // To ensure we do not starve other tasks waiting on the executor,
                        // we yield here, but immediately wake ourselves up to continue.
                        cx.waker().wake_by_ref();
                        return Poll::Pending;
                    }
                    continue
                }
                Poll::Ready(output) => {
                    return Poll::Ready(Some(output))
                }
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.len();
        (len, Some(len))
    }
}

impl<Fut> Debug for FuturesUnordered<Fut> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "FuturesUnordered {{ ... }}")
    }
}

impl<Fut> Drop for FuturesUnordered<Fut> {
    fn drop(&mut self) {
        // When a `FuturesUnordered` is dropped we want to drop all futures
        // associated with it. At the same time though there may be tons of
        // wakers flying around which contain `Task<Fut>` references
        // inside them. We'll let those naturally get deallocated.
        unsafe {
            while !self.head_all.get_mut().is_null() {
                let head = *self.head_all.get_mut();
                let task = self.unlink(head);
                self.release_task(task);
            }
        }

        // Note that at this point we could still have a bunch of tasks in the
        // ready to run queue. None of those tasks, however, have futures
        // associated with them so they're safe to destroy on any thread. At
        // this point the `FuturesUnordered` struct, the owner of the one strong
        // reference to the ready to run queue will drop the strong reference.
        // At that point whichever thread releases the strong refcount last (be
        // it this thread or some other thread as part of an `upgrade`) will
        // clear out the ready to run queue and free all remaining tasks.
        //
        // While that freeing operation isn't guaranteed to happen here, it's
        // guaranteed to happen "promptly" as no more "blocking work" will
        // happen while there's a strong refcount held.
    }
}

impl<Fut> FromIterator<Fut> for FuturesUnordered<Fut> {
    fn from_iter<I>(iter: I) -> Self
    where
        I: IntoIterator<Item = Fut>,
    {
        let acc = Self::new();
        iter.into_iter().fold(acc, |acc, item| { acc.push(item); acc })
    }
}

impl<Fut: Future> FusedStream for FuturesUnordered<Fut> {
    fn is_terminated(&self) -> bool {
        self.is_terminated.load(Relaxed)
    }
}

impl<Fut> Extend<Fut> for FuturesUnordered<Fut> {
    fn extend<I>(&mut self, iter: I)
    where
        I: IntoIterator<Item = Fut>,
    {
        for item in iter {
            self.push(item);
        }
    }
}