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
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
use {sys, Token};
use event_imp::{self as event, Ready, Event, Evented, PollOpt};
use std::{fmt, io, ptr, usize};
use std::cell::UnsafeCell;
use std::{mem, ops, isize};
#[cfg(all(unix, not(target_os = "fuchsia")))]
use std::os::unix::io::AsRawFd;
#[cfg(all(unix, not(target_os = "fuchsia")))]
use std::os::unix::io::RawFd;
use std::sync::{Arc, Mutex, Condvar};
use std::sync::atomic::{AtomicUsize, AtomicPtr, AtomicBool};
use std::sync::atomic::Ordering::{self, Acquire, Release, AcqRel, Relaxed, SeqCst};
use std::time::{Duration, Instant};

// Poll is backed by two readiness queues. The first is a system readiness queue
// represented by `sys::Selector`. The system readiness queue handles events
// provided by the system, such as TCP and UDP. The second readiness queue is
// implemented in user space by `ReadinessQueue`. It provides a way to implement
// purely user space `Evented` types.
//
// `ReadinessQueue` is is backed by a MPSC queue that supports reuse of linked
// list nodes. This significantly reduces the number of required allocations.
// Each `Registration` / `SetReadiness` pair allocates a single readiness node
// that is used for the lifetime of the registration.
//
// The readiness node also includes a single atomic variable, `state` that
// tracks most of the state associated with the registration. This includes the
// current readiness, interest, poll options, and internal state. When the node
// state is mutated, it is queued in the MPSC channel. A call to
// `ReadinessQueue::poll` will dequeue and process nodes. The node state can
// still be mutated while it is queued in the channel for processing.
// Intermediate state values do not matter as long as the final state is
// included in the call to `poll`. This is the eventually consistent nature of
// the readiness queue.
//
// The readiness node is ref counted using the `ref_count` field. On creation,
// the ref_count is initialized to 3: one `Registration` handle, one
// `SetReadiness` handle, and one for the readiness queue. Since the readiness queue
// doesn't *always* hold a handle to the node, we don't use the Arc type for
// managing ref counts (this is to avoid constantly incrementing and
// decrementing the ref count when pushing & popping from the queue). When the
// `Registration` handle is dropped, the `dropped` flag is set on the node, then
// the node is pushed into the registration queue. When Poll::poll pops the
// node, it sees the drop flag is set, and decrements it's ref count.
//
// The MPSC queue is a modified version of the intrusive MPSC node based queue
// described by 1024cores [1].
//
// The first modification is that two markers are used instead of a single
// `stub`. The second marker is a `sleep_marker` which is used to signal to
// producers that the consumer is going to sleep. This sleep_marker is only used
// when the queue is empty, implying that the only node in the queue is
// `end_marker`.
//
// The second modification is an `until` argument passed to the dequeue
// function. When `poll` encounters a level-triggered node, the node will be
// immediately pushed back into the queue. In order to avoid an infinite loop,
// `poll` before pushing the node, the pointer is saved off and then passed
// again as the `until` argument. If the next node to pop is `until`, then
// `Dequeue::Empty` is returned.
//
// [1] http://www.1024cores.net/home/lock-free-algorithms/queues/intrusive-mpsc-node-based-queue


/// Polls for readiness events on all registered values.
///
/// `Poll` allows a program to monitor a large number of `Evented` types,
/// waiting until one or more become "ready" for some class of operations; e.g.
/// reading and writing. An `Evented` type is considered ready if it is possible
/// to immediately perform a corresponding operation; e.g. [`read`] or
/// [`write`].
///
/// To use `Poll`, an `Evented` type must first be registered with the `Poll`
/// instance using the [`register`] method, supplying readiness interest. The
/// readiness interest tells `Poll` which specific operations on the handle to
/// monitor for readiness. A `Token` is also passed to the [`register`]
/// function. When `Poll` returns a readiness event, it will include this token.
/// This associates the event with the `Evented` handle that generated the
/// event.
///
/// [`read`]: tcp/struct.TcpStream.html#method.read
/// [`write`]: tcp/struct.TcpStream.html#method.write
/// [`register`]: #method.register
///
/// # Examples
///
/// A basic example -- establishing a `TcpStream` connection.
///
/// ```
/// # use std::error::Error;
/// # fn try_main() -> Result<(), Box<Error>> {
/// use mio::{Events, Poll, Ready, PollOpt, Token};
/// use mio::net::TcpStream;
///
/// use std::net::{TcpListener, SocketAddr};
///
/// // Bind a server socket to connect to.
/// let addr: SocketAddr = "127.0.0.1:0".parse()?;
/// let server = TcpListener::bind(&addr)?;
///
/// // Construct a new `Poll` handle as well as the `Events` we'll store into
/// let poll = Poll::new()?;
/// let mut events = Events::with_capacity(1024);
///
/// // Connect the stream
/// let stream = TcpStream::connect(&server.local_addr()?)?;
///
/// // Register the stream with `Poll`
/// poll.register(&stream, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
///
/// // Wait for the socket to become ready. This has to happens in a loop to
/// // handle spurious wakeups.
/// loop {
///     poll.poll(&mut events, None)?;
///
///     for event in &events {
///         if event.token() == Token(0) && event.readiness().is_writable() {
///             // The socket connected (probably, it could still be a spurious
///             // wakeup)
///             return Ok(());
///         }
///     }
/// }
/// #     Ok(())
/// # }
/// #
/// # fn main() {
/// #     try_main().unwrap();
/// # }
/// ```
///
/// # Edge-triggered and level-triggered
///
/// An [`Evented`] registration may request edge-triggered events or
/// level-triggered events. This is done by setting `register`'s
/// [`PollOpt`] argument to either [`edge`] or [`level`].
///
/// The difference between the two can be described as follows. Supposed that
/// this scenario happens:
///
/// 1. A [`TcpStream`] is registered with `Poll`.
/// 2. The socket receives 2kb of data.
/// 3. A call to [`Poll::poll`] returns the token associated with the socket
///    indicating readable readiness.
/// 4. 1kb is read from the socket.
/// 5. Another call to [`Poll::poll`] is made.
///
/// If when the socket was registered with `Poll`, edge triggered events were
/// requested, then the call to [`Poll::poll`] done in step **5** will
/// (probably) hang despite there being another 1kb still present in the socket
/// read buffer. The reason for this is that edge-triggered mode delivers events
/// only when changes occur on the monitored [`Evented`]. So, in step *5* the
/// caller might end up waiting for some data that is already present inside the
/// socket buffer.
///
/// With edge-triggered events, operations **must** be performed on the
/// `Evented` type until [`WouldBlock`] is returned. In other words, after
/// receiving an event indicating readiness for a certain operation, one should
/// assume that [`Poll::poll`] may never return another event for the same token
/// and readiness until the operation returns [`WouldBlock`].
///
/// By contrast, when level-triggered notifications was requested, each call to
/// [`Poll::poll`] will return an event for the socket as long as data remains
/// in the socket buffer. Generally, level-triggered events should be avoided if
/// high performance is a concern.
///
/// Since even with edge-triggered events, multiple events can be generated upon
/// receipt of multiple chunks of data, the caller has the option to set the
/// [`oneshot`] flag. This tells `Poll` to disable the associated [`Evented`]
/// after the event is returned from [`Poll::poll`]. The subsequent calls to
/// [`Poll::poll`] will no longer include events for [`Evented`] handles that
/// are disabled even if the readiness state changes. The handle can be
/// re-enabled by calling [`reregister`]. When handles are disabled, internal
/// resources used to monitor the handle are maintained until the handle is
/// dropped or deregistered. This makes re-registering the handle a fast
/// operation.
///
/// For example, in the following scenario:
///
/// 1. A [`TcpStream`] is registered with `Poll`.
/// 2. The socket receives 2kb of data.
/// 3. A call to [`Poll::poll`] returns the token associated with the socket
///    indicating readable readiness.
/// 4. 2kb is read from the socket.
/// 5. Another call to read is issued and [`WouldBlock`] is returned
/// 6. The socket receives another 2kb of data.
/// 7. Another call to [`Poll::poll`] is made.
///
/// Assuming the socket was registered with `Poll` with the [`edge`] and
/// [`oneshot`] options, then the call to [`Poll::poll`] in step 7 would block. This
/// is because, [`oneshot`] tells `Poll` to disable events for the socket after
/// returning an event.
///
/// In order to receive the event for the data received in step 6, the socket
/// would need to be reregistered using [`reregister`].
///
/// [`PollOpt`]: struct.PollOpt.html
/// [`edge`]: struct.PollOpt.html#method.edge
/// [`level`]: struct.PollOpt.html#method.level
/// [`Poll::poll`]: struct.Poll.html#method.poll
/// [`WouldBlock`]: https://doc.rust-lang.org/std/io/enum.ErrorKind.html#variant.WouldBlock
/// [`Evented`]: event/trait.Evented.html
/// [`TcpStream`]: tcp/struct.TcpStream.html
/// [`reregister`]: #method.reregister
/// [`oneshot`]: struct.PollOpt.html#method.oneshot
///
/// # Portability
///
/// Using `Poll` provides a portable interface across supported platforms as
/// long as the caller takes the following into consideration:
///
/// ### Spurious events
///
/// [`Poll::poll`] may return readiness events even if the associated
/// [`Evented`] handle is not actually ready. Given the same code, this may
/// happen more on some platforms than others. It is important to never assume
/// that, just because a readiness notification was received, that the
/// associated operation will as well.
///
/// If operation fails with [`WouldBlock`], then the caller should not treat
/// this as an error and wait until another readiness event is received.
///
/// ### Draining readiness
///
/// When using edge-triggered mode, once a readiness event is received, the
/// corresponding operation must be performed repeatedly until it returns
/// [`WouldBlock`]. Unless this is done, there is no guarantee that another
/// readiness event will be delivered, even if further data is received for the
/// [`Evented`] handle.
///
/// For example, in the first scenario described above, after step 5, even if
/// the socket receives more data there is no guarantee that another readiness
/// event will be delivered.
///
/// ### Readiness operations
///
/// The only readiness operations that are guaranteed to be present on all
/// supported platforms are [`readable`] and [`writable`]. All other readiness
/// operations may have false negatives and as such should be considered
/// **hints**. This means that if a socket is registered with [`readable`],
/// [`error`], and [`hup`] interest, and either an error or hup is received, a
/// readiness event will be generated for the socket, but it **may** only
/// include `readable` readiness. Also note that, given the potential for
/// spurious events, receiving a readiness event with `hup` or `error` doesn't
/// actually mean that a `read` on the socket will return a result matching the
/// readiness event.
///
/// In other words, portable programs that explicitly check for [`hup`] or
/// [`error`] readiness should be doing so as an **optimization** and always be
/// able to handle an error or HUP situation when performing the actual read
/// operation.
///
/// [`readable`]: struct.Ready.html#method.readable
/// [`writable`]: struct.Ready.html#method.writable
/// [`error`]: struct.Ready.html#method.error
/// [`hup`]: struct.Ready.html#method.hup
///
/// ### Registering handles
///
/// Unless otherwise noted, it should be assumed that types implementing
/// [`Evented`] will never become ready unless they are registered with `Poll`.
///
/// For example:
///
/// ```
/// # use std::error::Error;
/// # fn try_main() -> Result<(), Box<Error>> {
/// use mio::{Poll, Ready, PollOpt, Token};
/// use mio::net::TcpStream;
/// use std::time::Duration;
/// use std::thread;
///
/// let sock = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
///
/// thread::sleep(Duration::from_secs(1));
///
/// let poll = Poll::new()?;
///
/// // The connect is not guaranteed to have started until it is registered at
/// // this point
/// poll.register(&sock, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
/// #     Ok(())
/// # }
/// #
/// # fn main() {
/// #     try_main().unwrap();
/// # }
/// ```
///
/// # Implementation notes
///
/// `Poll` is backed by the selector provided by the operating system.
///
/// |      OS    |  Selector |
/// |------------|-----------|
/// | Linux      | [epoll]   |
/// | OS X, iOS  | [kqueue]  |
/// | Windows    | [IOCP]    |
/// | FreeBSD    | [kqueue]  |
/// | Android    | [epoll]   |
///
/// On all supported platforms, socket operations are handled by using the
/// system selector. Platform specific extensions (e.g. [`EventedFd`]) allow
/// accessing other features provided by individual system selectors. For
/// example, Linux's [`signalfd`] feature can be used by registering the FD with
/// `Poll` via [`EventedFd`].
///
/// On all platforms except windows, a call to [`Poll::poll`] is mostly just a
/// direct call to the system selector. However, [IOCP] uses a completion model
/// instead of a readiness model. In this case, `Poll` must adapt the completion
/// model Mio's API. While non-trivial, the bridge layer is still quite
/// efficient. The most expensive part being calls to `read` and `write` require
/// data to be copied into an intermediate buffer before it is passed to the
/// kernel.
///
/// Notifications generated by [`SetReadiness`] are handled by an internal
/// readiness queue. A single call to [`Poll::poll`] will collect events from
/// both from the system selector and the internal readiness queue.
///
/// [epoll]: http://man7.org/linux/man-pages/man7/epoll.7.html
/// [kqueue]: https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
/// [IOCP]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365198(v=vs.85).aspx
/// [`signalfd`]: http://man7.org/linux/man-pages/man2/signalfd.2.html
/// [`EventedFd`]: unix/struct.EventedFd.html
/// [`SetReadiness`]: struct.SetReadiness.html
/// [`Poll::poll`]: struct.Poll.html#method.poll
pub struct Poll {
    // Platform specific IO selector
    selector: sys::Selector,

    // Custom readiness queue
    readiness_queue: ReadinessQueue,

    // Use an atomic to first check if a full lock will be required. This is a
    // fast-path check for single threaded cases avoiding the extra syscall
    lock_state: AtomicUsize,

    // Sequences concurrent calls to `Poll::poll`
    lock: Mutex<()>,

    // Wakeup the next waiter
    condvar: Condvar,
}

/// Handle to a user space `Poll` registration.
///
/// `Registration` allows implementing [`Evented`] for types that cannot work
/// with the [system selector]. A `Registration` is always paired with a
/// `SetReadiness`, which allows updating the registration's readiness state.
/// When [`set_readiness`] is called and the `Registration` is associated with a
/// [`Poll`] instance, a readiness event will be created and eventually returned
/// by [`poll`].
///
/// A `Registration` / `SetReadiness` pair is created by calling
/// [`Registration::new2`]. At this point, the registration is not being
/// monitored by a [`Poll`] instance, so calls to `set_readiness` will not
/// result in any readiness notifications.
///
/// `Registration` implements [`Evented`], so it can be used with [`Poll`] using
/// the same [`register`], [`reregister`], and [`deregister`] functions used
/// with TCP, UDP, etc... types. Once registered with [`Poll`], readiness state
/// changes result in readiness events being dispatched to the [`Poll`] instance
/// with which `Registration` is registered.
///
/// **Note**, before using `Registration` be sure to read the
/// [`set_readiness`] documentation and the [portability] notes. The
/// guarantees offered by `Registration` may be weaker than expected.
///
/// For high level documentation, see [`Poll`].
///
/// # Examples
///
/// ```
/// use mio::{Ready, Registration, Poll, PollOpt, Token};
/// use mio::event::Evented;
///
/// use std::io;
/// use std::time::Instant;
/// use std::thread;
///
/// pub struct Deadline {
///     when: Instant,
///     registration: Registration,
/// }
///
/// impl Deadline {
///     pub fn new(when: Instant) -> Deadline {
///         let (registration, set_readiness) = Registration::new2();
///
///         thread::spawn(move || {
///             let now = Instant::now();
///
///             if now < when {
///                 thread::sleep(when - now);
///             }
///
///             set_readiness.set_readiness(Ready::readable());
///         });
///
///         Deadline {
///             when: when,
///             registration: registration,
///         }
///     }
///
///     pub fn is_elapsed(&self) -> bool {
///         Instant::now() >= self.when
///     }
/// }
///
/// impl Evented for Deadline {
///     fn register(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt)
///         -> io::Result<()>
///     {
///         self.registration.register(poll, token, interest, opts)
///     }
///
///     fn reregister(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt)
///         -> io::Result<()>
///     {
///         self.registration.reregister(poll, token, interest, opts)
///     }
///
///     fn deregister(&self, poll: &Poll) -> io::Result<()> {
///         self.registration.deregister(poll)
///     }
/// }
/// ```
///
/// [system selector]: struct.Poll.html#implementation-notes
/// [`Poll`]: struct.Poll.html
/// [`Registration::new2`]: struct.Registration.html#method.new2
/// [`Evented`]: event/trait.Evented.html
/// [`set_readiness`]: struct.SetReadiness.html#method.set_readiness
/// [`register`]: struct.Poll.html#method.register
/// [`reregister`]: struct.Poll.html#method.reregister
/// [`reregister`]: struct.Poll.html#method.reregister
pub struct Registration {
    inner: RegistrationInner,
}

unsafe impl Send for Registration {}
unsafe impl Sync for Registration {}

/// Updates the readiness state of the associated `Registration`.
///
/// See [`Registration`] for more documentation on using `SetReadiness` and
/// [`Poll`] for high level polling documentation.
///
/// [`Poll`]: struct.Poll.html
/// [`Registration`]: struct.Registration.html
#[derive(Clone)]
pub struct SetReadiness {
    inner: RegistrationInner,
}

unsafe impl Send for SetReadiness {}
unsafe impl Sync for SetReadiness {}

/// Used to associate an IO type with a Selector
#[derive(Debug)]
pub struct SelectorId {
    id: AtomicUsize,
}

struct RegistrationInner {
    // Unsafe pointer to the registration's node. The node is ref counted. This
    // cannot "simply" be tracked by an Arc because `Poll::poll` has an implicit
    // handle though it isn't stored anywhere. In other words, `Poll::poll`
    // needs to decrement the ref count before the node is freed.
    node: *mut ReadinessNode,
}

#[derive(Clone)]
struct ReadinessQueue {
    inner: Arc<ReadinessQueueInner>,
}

unsafe impl Send for ReadinessQueue {}
unsafe impl Sync for ReadinessQueue {}

struct ReadinessQueueInner {
    // Used to wake up `Poll` when readiness is set in another thread.
    awakener: sys::Awakener,

    // Head of the MPSC queue used to signal readiness to `Poll::poll`.
    head_readiness: AtomicPtr<ReadinessNode>,

    // Tail of the readiness queue.
    //
    // Only accessed by Poll::poll. Coordination will be handled by the poll fn
    tail_readiness: UnsafeCell<*mut ReadinessNode>,

    // Fake readiness node used to punctuate the end of the readiness queue.
    // Before attempting to read from the queue, this node is inserted in order
    // to partition the queue between nodes that are "owned" by the dequeue end
    // and nodes that will be pushed on by producers.
    end_marker: Box<ReadinessNode>,

    // Similar to `end_marker`, but this node signals to producers that `Poll`
    // has gone to sleep and must be woken up.
    sleep_marker: Box<ReadinessNode>,

    // Similar to `end_marker`, but the node signals that the queue is closed.
    // This happens when `ReadyQueue` is dropped and signals to producers that
    // the nodes should no longer be pushed into the queue.
    closed_marker: Box<ReadinessNode>,
}

/// Node shared by a `Registration` / `SetReadiness` pair as well as the node
/// queued into the MPSC channel.
struct ReadinessNode {
    // Node state, see struct docs for `ReadinessState`
    //
    // This variable is the primary point of coordination between all the
    // various threads concurrently accessing the node.
    state: AtomicState,

    // The registration token cannot fit into the `state` variable, so it is
    // broken out here. In order to atomically update both the state and token
    // we have to jump through a few hoops.
    //
    // First, `state` includes `token_read_pos` and `token_write_pos`. These can
    // either be 0, 1, or 2 which represent a token slot. `token_write_pos` is
    // the token slot that contains the most up to date registration token.
    // `token_read_pos` is the token slot that `poll` is currently reading from.
    //
    // When a call to `update` includes a different token than the one currently
    // associated with the registration (token_write_pos), first an unused token
    // slot is found. The unused slot is the one not represented by
    // `token_read_pos` OR `token_write_pos`. The new token is written to this
    // slot, then `state` is updated with the new `token_write_pos` value. This
    // requires that there is only a *single* concurrent call to `update`.
    //
    // When `poll` reads a node state, it checks that `token_read_pos` matches
    // `token_write_pos`. If they do not match, then it atomically updates
    // `state` such that `token_read_pos` is set to `token_write_pos`. It will
    // then read the token at the newly updated `token_read_pos`.
    token_0: UnsafeCell<Token>,
    token_1: UnsafeCell<Token>,
    token_2: UnsafeCell<Token>,

    // Used when the node is queued in the readiness linked list. Accessing
    // this field requires winning the "queue" lock
    next_readiness: AtomicPtr<ReadinessNode>,

    // Ensures that there is only one concurrent call to `update`.
    //
    // Each call to `update` will attempt to swap `update_lock` from `false` to
    // `true`. If the CAS succeeds, the thread has obtained the update lock. If
    // the CAS fails, then the `update` call returns immediately and the update
    // is discarded.
    update_lock: AtomicBool,

    // Pointer to Arc<ReadinessQueueInner>
    readiness_queue: AtomicPtr<()>,

    // Tracks the number of `ReadyRef` pointers
    ref_count: AtomicUsize,
}

/// Stores the ReadinessNode state in an AtomicUsize. This wrapper around the
/// atomic variable handles encoding / decoding `ReadinessState` values.
struct AtomicState {
    inner: AtomicUsize,
}

const MASK_2: usize = 4 - 1;
const MASK_4: usize = 16 - 1;
const QUEUED_MASK: usize = 1 << QUEUED_SHIFT;
const DROPPED_MASK: usize = 1 << DROPPED_SHIFT;

const READINESS_SHIFT: usize = 0;
const INTEREST_SHIFT: usize = 4;
const POLL_OPT_SHIFT: usize = 8;
const TOKEN_RD_SHIFT: usize = 12;
const TOKEN_WR_SHIFT: usize = 14;
const QUEUED_SHIFT: usize = 16;
const DROPPED_SHIFT: usize = 17;

/// Tracks all state for a single `ReadinessNode`. The state is packed into a
/// `usize` variable from low to high bit as follows:
///
/// 4 bits: Registration current readiness
/// 4 bits: Registration interest
/// 4 bits: Poll options
/// 2 bits: Token position currently being read from by `poll`
/// 2 bits: Token position last written to by `update`
/// 1 bit:  Queued flag, set when node is being pushed into MPSC queue.
/// 1 bit:  Dropped flag, set when all `Registration` handles have been dropped.
#[derive(Debug, Copy, Clone, Eq, PartialEq)]
struct ReadinessState(usize);

/// Returned by `dequeue_node`. Represents the different states as described by
/// the queue documentation on 1024cores.net.
enum Dequeue {
    Data(*mut ReadinessNode),
    Empty,
    Inconsistent,
}

const AWAKEN: Token = Token(usize::MAX);
const MAX_REFCOUNT: usize = (isize::MAX) as usize;

/*
 *
 * ===== Poll =====
 *
 */

impl Poll {
    /// Return a new `Poll` handle.
    ///
    /// This function will make a syscall to the operating system to create the
    /// system selector. If this syscall fails, `Poll::new` will return with the
    /// error.
    ///
    /// See [struct] level docs for more details.
    ///
    /// [struct]: struct.Poll.html
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Poll, Events};
    /// use std::time::Duration;
    ///
    /// let poll = match Poll::new() {
    ///     Ok(poll) => poll,
    ///     Err(e) => panic!("failed to create Poll instance; err={:?}", e),
    /// };
    ///
    /// // Create a structure to receive polled events
    /// let mut events = Events::with_capacity(1024);
    ///
    /// // Wait for events, but none will be received because no `Evented`
    /// // handles have been registered with this `Poll` instance.
    /// let n = poll.poll(&mut events, Some(Duration::from_millis(500)))?;
    /// assert_eq!(n, 0);
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    pub fn new() -> io::Result<Poll> {
        is_send::<Poll>();
        is_sync::<Poll>();

        let poll = Poll {
            selector: sys::Selector::new()?,
            readiness_queue: ReadinessQueue::new()?,
            lock_state: AtomicUsize::new(0),
            lock: Mutex::new(()),
            condvar: Condvar::new(),
        };

        // Register the notification wakeup FD with the IO poller
        poll.readiness_queue.inner.awakener.register(&poll, AWAKEN, Ready::readable(), PollOpt::edge())?;

        Ok(poll)
    }

    /// Register an `Evented` handle with the `Poll` instance.
    ///
    /// Once registered, the `Poll` instance will monitor the `Evented` handle
    /// for readiness state changes. When it notices a state change, it will
    /// return a readiness event for the handle the next time [`poll`] is
    /// called.
    ///
    /// See the [`struct`] docs for a high level overview.
    ///
    /// # Arguments
    ///
    /// `handle: &E: Evented`: This is the handle that the `Poll` instance
    /// should monitor for readiness state changes.
    ///
    /// `token: Token`: The caller picks a token to associate with the socket.
    /// When [`poll`] returns an event for the handle, this token is included.
    /// This allows the caller to map the event to its handle. The token
    /// associated with the `Evented` handle can be changed at any time by
    /// calling [`reregister`].
    ///
    /// `token` cannot be `Token(usize::MAX)` as it is reserved for internal
    /// usage.
    ///
    /// See documentation on [`Token`] for an example showing how to pick
    /// [`Token`] values.
    ///
    /// `interest: Ready`: Specifies which operations `Poll` should monitor for
    /// readiness. `Poll` will only return readiness events for operations
    /// specified by this argument.
    ///
    /// If a socket is registered with [`readable`] interest and the socket
    /// becomes writable, no event will be returned from [`poll`].
    ///
    /// The readiness interest for an `Evented` handle can be changed at any
    /// time by calling [`reregister`].
    ///
    /// `opts: PollOpt`: Specifies the registration options. The most common
    /// options being [`level`] for level-triggered events, [`edge`] for
    /// edge-triggered events, and [`oneshot`].
    ///
    /// The registration options for an `Evented` handle can be changed at any
    /// time by calling [`reregister`].
    ///
    /// # Notes
    ///
    /// Unless otherwise specified, the caller should assume that once an
    /// `Evented` handle is registered with a `Poll` instance, it is bound to
    /// that `Poll` instance for the lifetime of the `Evented` handle. This
    /// remains true even if the `Evented` handle is deregistered from the poll
    /// instance using [`deregister`].
    ///
    /// This function is **thread safe**. It can be called concurrently from
    /// multiple threads.
    ///
    /// [`struct`]: #
    /// [`reregister`]: #method.reregister
    /// [`deregister`]: #method.deregister
    /// [`poll`]: #method.poll
    /// [`level`]: struct.PollOpt.html#method.level
    /// [`edge`]: struct.PollOpt.html#method.edge
    /// [`oneshot`]: struct.PollOpt.html#method.oneshot
    /// [`Token`]: struct.Token.html
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll, Ready, PollOpt, Token};
    /// use mio::net::TcpStream;
    /// use std::time::{Duration, Instant};
    ///
    /// let poll = Poll::new()?;
    /// let socket = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
    ///
    /// // Register the socket with `poll`
    /// poll.register(&socket, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
    ///
    /// let mut events = Events::with_capacity(1024);
    /// let start = Instant::now();
    /// let timeout = Duration::from_millis(500);
    ///
    /// loop {
    ///     let elapsed = start.elapsed();
    ///
    ///     if elapsed >= timeout {
    ///         // Connection timed out
    ///         return Ok(());
    ///     }
    ///
    ///     let remaining = timeout - elapsed;
    ///     poll.poll(&mut events, Some(remaining))?;
    ///
    ///     for event in &events {
    ///         if event.token() == Token(0) {
    ///             // Something (probably) happened on the socket.
    ///             return Ok(());
    ///         }
    ///     }
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    pub fn register<E: ?Sized>(&self, handle: &E, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()>
        where E: Evented
    {
        validate_args(token)?;

        /*
         * Undefined behavior:
         * - Reusing a token with a different `Evented` without deregistering
         * (or closing) the original `Evented`.
         */
        trace!("registering with poller");

        // Register interests for this socket
        handle.register(self, token, interest, opts)?;

        Ok(())
    }

    /// Re-register an `Evented` handle with the `Poll` instance.
    ///
    /// Re-registering an `Evented` handle allows changing the details of the
    /// registration. Specifically, it allows updating the associated `token`,
    /// `interest`, and `opts` specified in previous `register` and `reregister`
    /// calls.
    ///
    /// The `reregister` arguments fully override the previous values. In other
    /// words, if a socket is registered with [`readable`] interest and the call
    /// to `reregister` specifies [`writable`], then read interest is no longer
    /// requested for the handle.
    ///
    /// The `Evented` handle must have previously been registered with this
    /// instance of `Poll` otherwise the call to `reregister` will return with
    /// an error.
    ///
    /// `token` cannot be `Token(usize::MAX)` as it is reserved for internal
    /// usage.
    ///
    /// See the [`register`] documentation for details about the function
    /// arguments and see the [`struct`] docs for a high level overview of
    /// polling.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Poll, Ready, PollOpt, Token};
    /// use mio::net::TcpStream;
    ///
    /// let poll = Poll::new()?;
    /// let socket = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
    ///
    /// // Register the socket with `poll`, requesting readable
    /// poll.register(&socket, Token(0), Ready::readable(), PollOpt::edge())?;
    ///
    /// // Reregister the socket specifying a different token and write interest
    /// // instead. `PollOpt::edge()` must be specified even though that value
    /// // is not being changed.
    /// poll.reregister(&socket, Token(2), Ready::writable(), PollOpt::edge())?;
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// [`struct`]: #
    /// [`register`]: #method.register
    /// [`readable`]: struct.Ready.html#method.readable
    /// [`writable`]: struct.Ready.html#method.writable
    pub fn reregister<E: ?Sized>(&self, handle: &E, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()>
        where E: Evented
    {
        validate_args(token)?;

        trace!("registering with poller");

        // Register interests for this socket
        handle.reregister(self, token, interest, opts)?;

        Ok(())
    }

    /// Deregister an `Evented` handle with the `Poll` instance.
    ///
    /// When an `Evented` handle is deregistered, the `Poll` instance will
    /// no longer monitor it for readiness state changes. Unlike disabling
    /// handles with [`oneshot`], deregistering clears up any internal resources
    /// needed to track the handle.
    ///
    /// A handle can be passed back to `register` after it has been
    /// deregistered; however, it must be passed back to the **same** `Poll`
    /// instance.
    ///
    /// `Evented` handles are automatically deregistered when they are dropped.
    /// It is common to never need to explicitly call `deregister`.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll, Ready, PollOpt, Token};
    /// use mio::net::TcpStream;
    /// use std::time::Duration;
    ///
    /// let poll = Poll::new()?;
    /// let socket = TcpStream::connect(&"216.58.193.100:80".parse()?)?;
    ///
    /// // Register the socket with `poll`
    /// poll.register(&socket, Token(0), Ready::readable(), PollOpt::edge())?;
    ///
    /// poll.deregister(&socket)?;
    ///
    /// let mut events = Events::with_capacity(1024);
    ///
    /// // Set a timeout because this poll should never receive any events.
    /// let n = poll.poll(&mut events, Some(Duration::from_secs(1)))?;
    /// assert_eq!(0, n);
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    pub fn deregister<E: ?Sized>(&self, handle: &E) -> io::Result<()>
        where E: Evented
    {
        trace!("deregistering handle with poller");

        // Deregister interests for this socket
        handle.deregister(self)?;

        Ok(())
    }

    /// Wait for readiness events
    ///
    /// Blocks the current thread and waits for readiness events for any of the
    /// `Evented` handles that have been registered with this `Poll` instance.
    /// The function will block until either at least one readiness event has
    /// been received or `timeout` has elapsed. A `timeout` of `None` means that
    /// `poll` will block until a readiness event has been received.
    ///
    /// The supplied `events` will be cleared and newly received readiness events
    /// will be pushed onto the end. At most `events.capacity()` events will be
    /// returned. If there are further pending readiness events, they will be
    /// returned on the next call to `poll`.
    ///
    /// A single call to `poll` may result in multiple readiness events being
    /// returned for a single `Evented` handle. For example, if a TCP socket
    /// becomes both readable and writable, it may be possible for a single
    /// readiness event to be returned with both [`readable`] and [`writable`]
    /// readiness **OR** two separate events may be returned, one with
    /// [`readable`] set and one with [`writable`] set.
    ///
    /// Note that the `timeout` will be rounded up to the system clock
    /// granularity (usually 1ms), and kernel scheduling delays mean that
    /// the blocking interval may be overrun by a small amount.
    ///
    /// `poll` returns the number of readiness events that have been pushed into
    /// `events` or `Err` when an error has been encountered with the system
    /// selector.
    ///
    /// See the [struct] level documentation for a higher level discussion of
    /// polling.
    ///
    /// [`readable`]: struct.Ready.html#method.readable
    /// [`writable`]: struct.Ready.html#method.writable
    /// [struct]: #
    ///
    /// # Examples
    ///
    /// A basic example -- establishing a `TcpStream` connection.
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll, Ready, PollOpt, Token};
    /// use mio::net::TcpStream;
    ///
    /// use std::net::{TcpListener, SocketAddr};
    /// use std::thread;
    ///
    /// // Bind a server socket to connect to.
    /// let addr: SocketAddr = "127.0.0.1:0".parse()?;
    /// let server = TcpListener::bind(&addr)?;
    /// let addr = server.local_addr()?.clone();
    ///
    /// // Spawn a thread to accept the socket
    /// thread::spawn(move || {
    ///     let _ = server.accept();
    /// });
    ///
    /// // Construct a new `Poll` handle as well as the `Events` we'll store into
    /// let poll = Poll::new()?;
    /// let mut events = Events::with_capacity(1024);
    ///
    /// // Connect the stream
    /// let stream = TcpStream::connect(&addr)?;
    ///
    /// // Register the stream with `Poll`
    /// poll.register(&stream, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
    ///
    /// // Wait for the socket to become ready. This has to happens in a loop to
    /// // handle spurious wakeups.
    /// loop {
    ///     poll.poll(&mut events, None)?;
    ///
    ///     for event in &events {
    ///         if event.token() == Token(0) && event.readiness().is_writable() {
    ///             // The socket connected (probably, it could still be a spurious
    ///             // wakeup)
    ///             return Ok(());
    ///         }
    ///     }
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// [struct]: #
    pub fn poll(&self, events: &mut Events, mut timeout: Option<Duration>) -> io::Result<usize> {
        let zero = Some(Duration::from_millis(0));

        // At a high level, the synchronization strategy is to acquire access to
        // the critical section by transitioning the atomic from unlocked ->
        // locked. If the attempt fails, the thread will wait on the condition
        // variable.
        //
        // # Some more detail
        //
        // The `lock_state` atomic usize combines:
        //
        // - locked flag, stored in the least significant bit
        // - number of waiting threads, stored in the rest of the bits.
        //
        // When a thread transitions the locked flag from 0 -> 1, it has
        // obtained access to the critical section.
        //
        // When entering `poll`, a compare-and-swap from 0 -> 1 is attempted.
        // This is a fast path for the case when there are no concurrent calls
        // to poll, which is very common.
        //
        // On failure, the mutex is locked, and the thread attempts to increment
        // the number of waiting threads component of `lock_state`. If this is
        // successfully done while the locked flag is set, then the thread can
        // wait on the condition variable.
        //
        // When a thread exits the critical section, it unsets the locked flag.
        // If there are any waiters, which is atomically determined while
        // unsetting the locked flag, then the condvar is notified.

        let mut curr = self.lock_state.compare_and_swap(0, 1, SeqCst);

        if 0 != curr {
            // Enter slower path
            let mut lock = self.lock.lock().unwrap();
            let mut inc = false;

            loop {
                if curr & 1 == 0 {
                    // The lock is currently free, attempt to grab it
                    let mut next = curr | 1;

                    if inc {
                        // The waiter count has previously been incremented, so
                        // decrement it here
                        next -= 2;
                    }

                    let actual = self.lock_state.compare_and_swap(curr, next, SeqCst);

                    if actual != curr {
                        curr = actual;
                        continue;
                    }

                    // Lock acquired, break from the loop
                    break;
                }

                if timeout == zero {
                    if inc {
                        self.lock_state.fetch_sub(2, SeqCst);
                    }

                    return Ok(0);
                }

                // The lock is currently held, so wait for it to become
                // free. If the waiter count hasn't been incremented yet, do
                // so now
                if !inc {
                    let next = curr.checked_add(2).expect("overflow");
                    let actual = self.lock_state.compare_and_swap(curr, next, SeqCst);

                    if actual != curr {
                        curr = actual;
                        continue;
                    }

                    // Track that the waiter count has been incremented for
                    // this thread and fall through to the condvar waiting
                    inc = true;
                }

                lock = match timeout {
                    Some(to) => {
                        let now = Instant::now();

                        // Wait to be notified
                        let (l, _) = self.condvar.wait_timeout(lock, to).unwrap();

                        // See how much time was elapsed in the wait
                        let elapsed = now.elapsed();

                        // Update `timeout` to reflect how much time is left to
                        // wait.
                        if elapsed >= to {
                            timeout = zero;
                        } else {
                            // Update the timeout
                            timeout = Some(to - elapsed);
                        }

                        l
                    }
                    None => {
                        self.condvar.wait(lock).unwrap()
                    }
                };

                // Reload the state
                curr = self.lock_state.load(SeqCst);

                // Try to lock again...
            }
        }

        let ret = self.poll2(events, timeout);

        // Release the lock
        if 1 != self.lock_state.fetch_and(!1, Release) {
            // Acquire the mutex
            let _lock = self.lock.lock().unwrap();

            // There is at least one waiting thread, so notify one
            self.condvar.notify_one();
        }

        ret
    }

    #[inline]
    fn poll2(&self, events: &mut Events, timeout: Option<Duration>) -> io::Result<usize> {
        // Compute the timeout value passed to the system selector. If the
        // readiness queue has pending nodes, we still want to poll the system
        // selector for new events, but we don't want to block the thread to
        // wait for new events.
        let timeout = if timeout == Some(Duration::from_millis(0)) {
            // If blocking is not requested, then there is no need to prepare
            // the queue for sleep
            timeout
        } else if self.readiness_queue.prepare_for_sleep() {
            // The readiness queue is empty. The call to `prepare_for_sleep`
            // inserts `sleep_marker` into the queue. This signals to any
            // threads setting readiness that the `Poll::poll` is going to
            // sleep, so the awakener should be used.
            timeout
        } else {
            // The readiness queue is not empty, so do not block the thread.
            Some(Duration::from_millis(0))
        };

        // First get selector events
        let res = self.selector.select(&mut events.inner, AWAKEN, timeout);

        if res? {
            // Some awakeners require reading from a FD.
            self.readiness_queue.inner.awakener.cleanup();
        }

        // Poll custom event queue
        self.readiness_queue.poll(&mut events.inner);

        // Return number of polled events
        Ok(events.len())
    }
}

fn validate_args(token: Token) -> io::Result<()> {
    if token == AWAKEN {
        return Err(io::Error::new(io::ErrorKind::Other, "invalid token"));
    }

    Ok(())
}

impl fmt::Debug for Poll {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("Poll")
            .finish()
    }
}

#[cfg(all(unix, not(target_os = "fuchsia")))]
impl AsRawFd for Poll {
    fn as_raw_fd(&self) -> RawFd {
        self.selector.as_raw_fd()
    }
}

/// A collection of readiness events.
///
/// `Events` is passed as an argument to [`Poll::poll`] and will be used to
/// receive any new readiness events received since the last poll. Usually, a
/// single `Events` instance is created at the same time as a [`Poll`] and
/// reused on each call to [`Poll::poll`].
///
/// See [`Poll`] for more documentation on polling.
///
/// # Examples
///
/// ```
/// # use std::error::Error;
/// # fn try_main() -> Result<(), Box<Error>> {
/// use mio::{Events, Poll};
/// use std::time::Duration;
///
/// let mut events = Events::with_capacity(1024);
/// let poll = Poll::new()?;
///
/// assert_eq!(0, events.len());
///
/// // Register `Evented` handles with `poll`
///
/// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
///
/// for event in &events {
///     println!("event={:?}", event);
/// }
/// #     Ok(())
/// # }
/// #
/// # fn main() {
/// #     try_main().unwrap();
/// # }
/// ```
///
/// [`Poll::poll`]: struct.Poll.html#method.poll
/// [`Poll`]: struct.Poll.html
pub struct Events {
    inner: sys::Events,
}

/// [`Events`] iterator.
///
/// This struct is created by the [`iter`] method on [`Events`].
///
/// # Examples
///
/// ```
/// # use std::error::Error;
/// # fn try_main() -> Result<(), Box<Error>> {
/// use mio::{Events, Poll};
/// use std::time::Duration;
///
/// let mut events = Events::with_capacity(1024);
/// let poll = Poll::new()?;
///
/// // Register handles with `poll`
///
/// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
///
/// for event in events.iter() {
///     println!("event={:?}", event);
/// }
/// #     Ok(())
/// # }
/// #
/// # fn main() {
/// #     try_main().unwrap();
/// # }
/// ```
///
/// [`Events`]: struct.Events.html
/// [`iter`]: struct.Events.html#method.iter
#[derive(Debug, Clone)]
pub struct Iter<'a> {
    inner: &'a Events,
    pos: usize,
}

/// Owned [`Events`] iterator.
///
/// This struct is created by the `into_iter` method on [`Events`].
///
/// # Examples
///
/// ```
/// # use std::error::Error;
/// # fn try_main() -> Result<(), Box<Error>> {
/// use mio::{Events, Poll};
/// use std::time::Duration;
///
/// let mut events = Events::with_capacity(1024);
/// let poll = Poll::new()?;
///
/// // Register handles with `poll`
///
/// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
///
/// for event in events {
///     println!("event={:?}", event);
/// }
/// #     Ok(())
/// # }
/// #
/// # fn main() {
/// #     try_main().unwrap();
/// # }
/// ```
/// [`Events`]: struct.Events.html
#[derive(Debug)]
pub struct IntoIter {
    inner: Events,
    pos: usize,
}

impl Events {
    /// Return a new `Events` capable of holding up to `capacity` events.
    ///
    /// # Examples
    ///
    /// ```
    /// use mio::Events;
    ///
    /// let events = Events::with_capacity(1024);
    ///
    /// assert_eq!(1024, events.capacity());
    /// ```
    pub fn with_capacity(capacity: usize) -> Events {
        Events {
            inner: sys::Events::with_capacity(capacity),
        }
    }

    /// Returns the `Event` at the given index, or `None` if the index is out of
    /// bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll};
    /// use std::time::Duration;
    ///
    /// let mut events = Events::with_capacity(1024);
    /// let poll = Poll::new()?;
    ///
    /// // Register handles with `poll`
    ///
    /// let n = poll.poll(&mut events, Some(Duration::from_millis(100)))?;
    ///
    /// for i in 0..n {
    ///     println!("event={:?}", events.get(i).unwrap());
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    pub fn get(&self, idx: usize) -> Option<Event> {
        self.inner.get(idx)
    }

    /// Returns the number of `Event` values currently in `self`.
    ///
    /// # Examples
    ///
    /// ```
    /// use mio::Events;
    ///
    /// let events = Events::with_capacity(1024);
    ///
    /// assert_eq!(0, events.len());
    /// ```
    pub fn len(&self) -> usize {
        self.inner.len()
    }

    /// Returns the number of `Event` values that `self` can hold.
    ///
    /// ```
    /// use mio::Events;
    ///
    /// let events = Events::with_capacity(1024);
    ///
    /// assert_eq!(1024, events.capacity());
    /// ```
    pub fn capacity(&self) -> usize {
        self.inner.capacity()
    }

    /// Returns `true` if `self` contains no `Event` values.
    ///
    /// # Examples
    ///
    /// ```
    /// use mio::Events;
    ///
    /// let events = Events::with_capacity(1024);
    ///
    /// assert!(events.is_empty());
    /// ```
    pub fn is_empty(&self) -> bool {
        self.inner.is_empty()
    }

    /// Returns an iterator over the `Event` values.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Poll};
    /// use std::time::Duration;
    ///
    /// let mut events = Events::with_capacity(1024);
    /// let poll = Poll::new()?;
    ///
    /// // Register handles with `poll`
    ///
    /// poll.poll(&mut events, Some(Duration::from_millis(100)))?;
    ///
    /// for event in events.iter() {
    ///     println!("event={:?}", event);
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    pub fn iter(&self) -> Iter {
        Iter {
            inner: self,
            pos: 0
        }
    }
}

impl<'a> IntoIterator for &'a Events {
    type Item = Event;
    type IntoIter = Iter<'a>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

impl<'a> Iterator for Iter<'a> {
    type Item = Event;

    fn next(&mut self) -> Option<Event> {
        let ret = self.inner.get(self.pos);
        self.pos += 1;
        ret
    }
}

impl IntoIterator for Events {
    type Item = Event;
    type IntoIter = IntoIter;

    fn into_iter(self) -> Self::IntoIter {
        IntoIter {
            inner: self,
            pos: 0,
        }
    }
}

impl Iterator for IntoIter {
    type Item = Event;

    fn next(&mut self) -> Option<Event> {
        let ret = self.inner.get(self.pos);
        self.pos += 1;
        ret
    }
}

impl fmt::Debug for Events {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Events")
            .field("len", &self.len())
            .field("capacity", &self.capacity())
            .finish()
    }
}

// ===== Accessors for internal usage =====

pub fn selector(poll: &Poll) -> &sys::Selector {
    &poll.selector
}

/*
 *
 * ===== Registration =====
 *
 */

// TODO: get rid of this, windows depends on it for now
#[allow(dead_code)]
pub fn new_registration(poll: &Poll, token: Token, ready: Ready, opt: PollOpt)
        -> (Registration, SetReadiness)
{
    Registration::new_priv(poll, token, ready, opt)
}

impl Registration {
    /// Create and return a new `Registration` and the associated
    /// `SetReadiness`.
    ///
    /// See [struct] documentation for more detail and [`Poll`]
    /// for high level documentation on polling.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Ready, Registration, Poll, PollOpt, Token};
    /// use std::thread;
    ///
    /// let (registration, set_readiness) = Registration::new2();
    ///
    /// thread::spawn(move || {
    ///     use std::time::Duration;
    ///     thread::sleep(Duration::from_millis(500));
    ///
    ///     set_readiness.set_readiness(Ready::readable());
    /// });
    ///
    /// let poll = Poll::new()?;
    /// poll.register(&registration, Token(0), Ready::readable() | Ready::writable(), PollOpt::edge())?;
    ///
    /// let mut events = Events::with_capacity(256);
    ///
    /// loop {
    ///     poll.poll(&mut events, None);
    ///
    ///     for event in &events {
    ///         if event.token() == Token(0) && event.readiness().is_readable() {
    ///             return Ok(());
    ///         }
    ///     }
    /// }
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    /// [struct]: #
    /// [`Poll`]: struct.Poll.html
    pub fn new2() -> (Registration, SetReadiness) {
        // Allocate the registration node. The new node will have `ref_count`
        // set to 2: one SetReadiness, one Registration.
        let node = Box::into_raw(Box::new(ReadinessNode::new(
                    ptr::null_mut(), Token(0), Ready::empty(), PollOpt::empty(), 2)));

        let registration = Registration {
            inner: RegistrationInner {
                node: node,
            },
        };

        let set_readiness = SetReadiness {
            inner: RegistrationInner {
                node: node,
            },
        };

        (registration, set_readiness)
    }

    #[deprecated(since = "0.6.5", note = "use `new2` instead")]
    #[cfg(feature = "with-deprecated")]
    #[doc(hidden)]
    pub fn new(poll: &Poll, token: Token, interest: Ready, opt: PollOpt)
        -> (Registration, SetReadiness)
    {
        Registration::new_priv(poll, token, interest, opt)
    }

    // TODO: Get rid of this (windows depends on it for now)
    fn new_priv(poll: &Poll, token: Token, interest: Ready, opt: PollOpt)
        -> (Registration, SetReadiness)
    {
        is_send::<Registration>();
        is_sync::<Registration>();
        is_send::<SetReadiness>();
        is_sync::<SetReadiness>();

        // Clone handle to the readiness queue, this bumps the ref count
        let queue = poll.readiness_queue.inner.clone();

        // Convert to a *mut () pointer
        let queue: *mut () = unsafe { mem::transmute(queue) };

        // Allocate the registration node. The new node will have `ref_count`
        // set to 3: one SetReadiness, one Registration, and one Poll handle.
        let node = Box::into_raw(Box::new(ReadinessNode::new(
                    queue, token, interest, opt, 3)));

        let registration = Registration {
            inner: RegistrationInner {
                node: node,
            },
        };

        let set_readiness = SetReadiness {
            inner: RegistrationInner {
                node: node,
            },
        };

        (registration, set_readiness)
    }

    #[deprecated(since = "0.6.5", note = "use `Evented` impl")]
    #[cfg(feature = "with-deprecated")]
    #[doc(hidden)]
    pub fn update(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
        self.inner.update(poll, token, interest, opts)
    }

    #[deprecated(since = "0.6.5", note = "use `Evented` impl")]
    #[cfg(feature = "with-deprecated")]
    #[doc(hidden)]
    pub fn deregister(&self, poll: &Poll) -> io::Result<()> {
        self.inner.update(poll, Token(0), Ready::empty(), PollOpt::empty())
    }
}

impl Evented for Registration {
    fn register(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
        self.inner.update(poll, token, interest, opts)
    }

    fn reregister(&self, poll: &Poll, token: Token, interest: Ready, opts: PollOpt) -> io::Result<()> {
        self.inner.update(poll, token, interest, opts)
    }

    fn deregister(&self, poll: &Poll) -> io::Result<()> {
        self.inner.update(poll, Token(0), Ready::empty(), PollOpt::empty())
    }
}

impl Drop for Registration {
    fn drop(&mut self) {
        // `flag_as_dropped` toggles the `dropped` flag and notifies
        // `Poll::poll` to release its handle (which is just decrementing
        // the ref count).
        if self.inner.state.flag_as_dropped() {
            // Can't do anything if the queuing fails
            let _ = self.inner.enqueue_with_wakeup();
        }
    }
}

impl fmt::Debug for Registration {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("Registration")
            .finish()
    }
}

impl SetReadiness {
    /// Returns the registration's current readiness.
    ///
    /// # Note
    ///
    /// There is no guarantee that `readiness` establishes any sort of memory
    /// ordering. Any concurrent data access must be synchronized using another
    /// strategy.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Registration, Ready};
    ///
    /// let (registration, set_readiness) = Registration::new2();
    ///
    /// assert!(set_readiness.readiness().is_empty());
    ///
    /// set_readiness.set_readiness(Ready::readable())?;
    /// assert!(set_readiness.readiness().is_readable());
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    pub fn readiness(&self) -> Ready {
        self.inner.readiness()
    }

    /// Set the registration's readiness
    ///
    /// If the associated `Registration` is registered with a [`Poll`] instance
    /// and has requested readiness events that include `ready`, then a future
    /// call to [`Poll::poll`] will receive a readiness event representing the
    /// readiness state change.
    ///
    /// # Note
    ///
    /// There is no guarantee that `readiness` establishes any sort of memory
    /// ordering. Any concurrent data access must be synchronized using another
    /// strategy.
    ///
    /// There is also no guarantee as to when the readiness event will be
    /// delivered to poll. A best attempt will be made to make the delivery in a
    /// "timely" fashion. For example, the following is **not** guaranteed to
    /// work:
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Events, Registration, Ready, Poll, PollOpt, Token};
    ///
    /// let poll = Poll::new()?;
    /// let (registration, set_readiness) = Registration::new2();
    ///
    /// poll.register(&registration,
    ///               Token(0),
    ///               Ready::readable(),
    ///               PollOpt::edge())?;
    ///
    /// // Set the readiness, then immediately poll to try to get the readiness
    /// // event
    /// set_readiness.set_readiness(Ready::readable())?;
    ///
    /// let mut events = Events::with_capacity(1024);
    /// poll.poll(&mut events, None)?;
    ///
    /// // There is NO guarantee that the following will work. It is possible
    /// // that the readiness event will be delivered at a later time.
    /// let event = events.get(0).unwrap();
    /// assert_eq!(event.token(), Token(0));
    /// assert!(event.readiness().is_readable());
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// # Examples
    ///
    /// A simple example, for a more elaborate example, see the [`Evented`]
    /// documentation.
    ///
    /// ```
    /// # use std::error::Error;
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// use mio::{Registration, Ready};
    ///
    /// let (registration, set_readiness) = Registration::new2();
    ///
    /// assert!(set_readiness.readiness().is_empty());
    ///
    /// set_readiness.set_readiness(Ready::readable())?;
    /// assert!(set_readiness.readiness().is_readable());
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    ///
    /// [`Registration`]: struct.Registration.html
    /// [`Evented`]: event/trait.Evented.html#examples
    /// [`Poll`]: struct.Poll.html
    /// [`Poll::poll`]: struct.Poll.html#method.poll
    pub fn set_readiness(&self, ready: Ready) -> io::Result<()> {
        self.inner.set_readiness(ready)
    }
}

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

impl RegistrationInner {
    /// Get the registration's readiness.
    fn readiness(&self) -> Ready {
        self.state.load(Relaxed).readiness()
    }

    /// Set the registration's readiness.
    ///
    /// This function can be called concurrently by an arbitrary number of
    /// SetReadiness handles.
    fn set_readiness(&self, ready: Ready) -> io::Result<()> {
        // Load the current atomic state.
        let mut state = self.state.load(Acquire);
        let mut next;

        loop {
            next = state;

            if state.is_dropped() {
                // Node is dropped, no more notifications
                return Ok(());
            }

            // Update the readiness
            next.set_readiness(ready);

            // If the readiness is not blank, try to obtain permission to
            // push the node into the readiness queue.
            if !next.effective_readiness().is_empty() {
                next.set_queued();
            }

            let actual = self.state.compare_and_swap(state, next, AcqRel);

            if state == actual {
                break;
            }

            state = actual;
        }

        if !state.is_queued() && next.is_queued() {
            // We toggled the queued flag, making us responsible for queuing the
            // node in the MPSC readiness queue.
            self.enqueue_with_wakeup()?;
        }

        Ok(())
    }

    /// Update the registration details associated with the node
    fn update(&self, poll: &Poll, token: Token, interest: Ready, opt: PollOpt) -> io::Result<()> {
        // First, ensure poll instances match
        //
        // Load the queue pointer, `Relaxed` is sufficient here as only the
        // pointer is being operated on. The actual memory is guaranteed to be
        // visible the `poll: &Poll` ref passed as an argument to the function.
        let mut queue = self.readiness_queue.load(Relaxed);
        let other: &*mut () = unsafe { mem::transmute(&poll.readiness_queue.inner) };
        let other = *other;

        debug_assert!(mem::size_of::<Arc<ReadinessQueueInner>>() == mem::size_of::<*mut ()>());

        if queue.is_null() {
            // Attempt to set the queue pointer. `Release` ordering synchronizes
            // with `Acquire` in `ensure_with_wakeup`.
            let actual = self.readiness_queue.compare_and_swap(
                queue, other, Release);

            if actual.is_null() {
                // The CAS succeeded, this means that the node's ref count
                // should be incremented to reflect that the `poll` function
                // effectively owns the node as well.
                //
                // `Relaxed` ordering used for the same reason as in
                // RegistrationInner::clone
                self.ref_count.fetch_add(1, Relaxed);

                // Note that the `queue` reference stored in our
                // `readiness_queue` field is intended to be a strong reference,
                // so now that we've successfully claimed the reference we bump
                // the refcount here.
                //
                // Down below in `release_node` when we deallocate this
                // `RegistrationInner` is where we'll transmute this back to an
                // arc and decrement the reference count.
                mem::forget(poll.readiness_queue.clone());
            } else {
                // The CAS failed, another thread set the queue pointer, so ensure
                // that the pointer and `other` match
                if actual != other {
                    return Err(io::Error::new(io::ErrorKind::Other, "registration handle associated with another `Poll` instance"));
                }
            }

            queue = other;
        } else if queue != other {
            return Err(io::Error::new(io::ErrorKind::Other, "registration handle associated with another `Poll` instance"));
        }

        unsafe {
            let actual = &poll.readiness_queue.inner as *const _ as *const usize;
            debug_assert_eq!(queue as usize, *actual);
        }

        // The `update_lock` atomic is used as a flag ensuring only a single
        // thread concurrently enters the `update` critical section. Any
        // concurrent calls to update are discarded. If coordinated updates are
        // required, the Mio user is responsible for handling that.
        //
        // Acquire / Release ordering is used on `update_lock` to ensure that
        // data access to the `token_*` variables are scoped to the critical
        // section.

        // Acquire the update lock.
        if self.update_lock.compare_and_swap(false, true, Acquire) {
            // The lock is already held. Discard the update
            return Ok(());
        }

        // Relaxed ordering is acceptable here as the only memory that needs to
        // be visible as part of the update are the `token_*` variables, and
        // ordering has already been handled by the `update_lock` access.
        let mut state = self.state.load(Relaxed);
        let mut next;

        // Read the current token, again this memory has been ordered by the
        // acquire on `update_lock`.
        let curr_token_pos = state.token_write_pos();
        let curr_token = unsafe { self::token(self, curr_token_pos) };

        let mut next_token_pos = curr_token_pos;

        // If the `update` call is changing the token, then compute the next
        // available token slot and write the token there.
        //
        // Note that this computation is happening *outside* of the
        // compare-and-swap loop. The update lock ensures that only a single
        // thread could be mutating the write_token_position, so the
        // `next_token_pos` will never need to be recomputed even if
        // `token_read_pos` concurrently changes. This is because
        // `token_read_pos` can ONLY concurrently change to the current value of
        // `token_write_pos`, so `next_token_pos` will always remain valid.
        if token != curr_token {
            next_token_pos = state.next_token_pos();

            // Update the token
            match next_token_pos {
                0 => unsafe { *self.token_0.get() = token },
                1 => unsafe { *self.token_1.get() = token },
                2 => unsafe { *self.token_2.get() = token },
                _ => unreachable!(),
            }
        }

        // Now enter the compare-and-swap loop
        loop {
            next = state;

            // The node is only dropped once all `Registration` handles are
            // dropped. Only `Registration` can call `update`.
            debug_assert!(!state.is_dropped());

            // Update the write token position, this will also release the token
            // to Poll::poll.
            next.set_token_write_pos(next_token_pos);

            // Update readiness and poll opts
            next.set_interest(interest);
            next.set_poll_opt(opt);

            // If there is effective readiness, the node will need to be queued
            // for processing. This exact behavior is still TBD, so we are
            // conservative for now and always fire.
            //
            // See https://github.com/carllerche/mio/issues/535.
            if !next.effective_readiness().is_empty() {
                next.set_queued();
            }

            // compare-and-swap the state values. Only `Release` is needed here.
            // The `Release` ensures that `Poll::poll` will see the token
            // update and the update function doesn't care about any other
            // memory visibility.
            let actual = self.state.compare_and_swap(state, next, Release);

            if actual == state {
                break;
            }

            // CAS failed, but `curr_token_pos` should not have changed given
            // that we still hold the update lock.
            debug_assert_eq!(curr_token_pos, actual.token_write_pos());

            state = actual;
        }

        // Release the lock
        self.update_lock.store(false, Release);

        if !state.is_queued() && next.is_queued() {
            // We are responsible for enqueing the node.
            enqueue_with_wakeup(queue, self)?;
        }

        Ok(())
    }
}

impl ops::Deref for RegistrationInner {
    type Target = ReadinessNode;

    fn deref(&self) -> &ReadinessNode {
        unsafe { &*self.node }
    }
}

impl Clone for RegistrationInner {
    fn clone(&self) -> RegistrationInner {
        // Using a relaxed ordering is alright here, as knowledge of the
        // original reference prevents other threads from erroneously deleting
        // the object.
        //
        // As explained in the [Boost documentation][1], Increasing the
        // reference counter can always be done with memory_order_relaxed: New
        // references to an object can only be formed from an existing
        // reference, and passing an existing reference from one thread to
        // another must already provide any required synchronization.
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        let old_size = self.ref_count.fetch_add(1, Relaxed);

        // However we need to guard against massive refcounts in case someone
        // is `mem::forget`ing Arcs. If we don't do this the count can overflow
        // and users will use-after free. We racily saturate to `isize::MAX` on
        // the assumption that there aren't ~2 billion threads incrementing
        // the reference count at once. This branch will never be taken in
        // any realistic program.
        //
        // We abort because such a program is incredibly degenerate, and we
        // don't care to support it.
        if old_size & !MAX_REFCOUNT != 0 {
            // TODO: This should really abort the process
            panic!();
        }

        RegistrationInner {
            node: self.node.clone(),
        }
    }
}

impl Drop for RegistrationInner {
    fn drop(&mut self) {
        // Only handles releasing from `Registration` and `SetReadiness`
        // handles. Poll has to call this itself.
        release_node(self.node);
    }
}

/*
 *
 * ===== ReadinessQueue =====
 *
 */

impl ReadinessQueue {
    /// Create a new `ReadinessQueue`.
    fn new() -> io::Result<ReadinessQueue> {
        is_send::<Self>();
        is_sync::<Self>();

        let end_marker = Box::new(ReadinessNode::marker());
        let sleep_marker = Box::new(ReadinessNode::marker());
        let closed_marker = Box::new(ReadinessNode::marker());

        let ptr = &*end_marker as *const _ as *mut _;

        Ok(ReadinessQueue {
            inner: Arc::new(ReadinessQueueInner {
                awakener: sys::Awakener::new()?,
                head_readiness: AtomicPtr::new(ptr),
                tail_readiness: UnsafeCell::new(ptr),
                end_marker: end_marker,
                sleep_marker: sleep_marker,
                closed_marker: closed_marker,
            })
        })
    }

    /// Poll the queue for new events
    fn poll(&self, dst: &mut sys::Events) {
        // `until` is set with the first node that gets re-enqueued due to being
        // set to have level-triggered notifications. This prevents an infinite
        // loop where `Poll::poll` will keep dequeuing nodes it enqueues.
        let mut until = ptr::null_mut();

        'outer:
        while dst.len() < dst.capacity() {
            // Dequeue a node. If the queue is in an inconsistent state, then
            // stop polling. `Poll::poll` will be called again shortly and enter
            // a syscall, which should be enough to enable the other thread to
            // finish the queuing process.
            let ptr = match unsafe { self.inner.dequeue_node(until) } {
                Dequeue::Empty | Dequeue::Inconsistent => break,
                Dequeue::Data(ptr) => ptr,
            };

            let node = unsafe { &*ptr };

            // Read the node state with Acquire ordering. This allows reading
            // the token variables.
            let mut state = node.state.load(Acquire);
            let mut next;
            let mut readiness;
            let mut opt;

            loop {
                // Build up any changes to the readiness node's state and
                // attempt the CAS at the end
                next = state;

                // Given that the node was just read from the queue, the
                // `queued` flag should still be set.
                debug_assert!(state.is_queued());

                // The dropped flag means we need to release the node and
                // perform no further processing on it.
                if state.is_dropped() {
                    // Release the node and continue
                    release_node(ptr);
                    continue 'outer;
                }

                // Process the node
                readiness = state.effective_readiness();
                opt = state.poll_opt();

                if opt.is_edge() {
                    // Mark the node as dequeued
                    next.set_dequeued();

                    if opt.is_oneshot() && !readiness.is_empty() {
                        next.disarm();
                    }
                } else if readiness.is_empty() {
                    next.set_dequeued();
                }

                // Ensure `token_read_pos` is set to `token_write_pos` so that
                // we read the most up to date token value.
                next.update_token_read_pos();

                if state == next {
                    break;
                }

                let actual = node.state.compare_and_swap(state, next, AcqRel);

                if actual == state {
                    break;
                }

                state = actual;
            }

            // If the queued flag is still set, then the node must be requeued.
            // This typically happens when using level-triggered notifications.
            if next.is_queued() {
                if until.is_null() {
                    // We never want to see the node again
                    until = ptr;
                }

                // Requeue the node
                self.inner.enqueue_node(node);
            }

            if !readiness.is_empty() {
                // Get the token
                let token = unsafe { token(node, next.token_read_pos()) };

                // Push the event
                dst.push_event(Event::new(readiness, token));
            }
        }
    }

    /// Prepare the queue for the `Poll::poll` thread to block in the system
    /// selector. This involves changing `head_readiness` to `sleep_marker`.
    /// Returns true if successful and `poll` can block.
    fn prepare_for_sleep(&self) -> bool {
        let end_marker = self.inner.end_marker();
        let sleep_marker = self.inner.sleep_marker();

        let tail = unsafe { *self.inner.tail_readiness.get() };

        // If the tail is currently set to the sleep_marker, then check if the
        // head is as well. If it is, then the queue is currently ready to
        // sleep. If it is not, then the queue is not empty and there should be
        // no sleeping.
        if tail == sleep_marker {
            return self.inner.head_readiness.load(Acquire) == sleep_marker;
        }

        // If the tail is not currently set to `end_marker`, then the queue is
        // not empty.
        if tail != end_marker {
            return false;
        }

        self.inner.sleep_marker.next_readiness.store(ptr::null_mut(), Relaxed);

        let actual = self.inner.head_readiness.compare_and_swap(
            end_marker, sleep_marker, AcqRel);

        debug_assert!(actual != sleep_marker);

        if actual != end_marker {
            // The readiness queue is not empty
            return false;
        }

        // The current tail should be pointing to `end_marker`
        debug_assert!(unsafe { *self.inner.tail_readiness.get() == end_marker });
        // The `end_marker` next pointer should be null
        debug_assert!(self.inner.end_marker.next_readiness.load(Relaxed).is_null());

        // Update tail pointer.
        unsafe { *self.inner.tail_readiness.get() = sleep_marker; }
        true
    }
}

impl Drop for ReadinessQueue {
    fn drop(&mut self) {
        // Close the queue by enqueuing the closed node
        self.inner.enqueue_node(&*self.inner.closed_marker);

        loop {
            // Free any nodes that happen to be left in the readiness queue
            let ptr = match unsafe { self.inner.dequeue_node(ptr::null_mut()) } {
                Dequeue::Empty => break,
                Dequeue::Inconsistent => {
                    // This really shouldn't be possible as all other handles to
                    // `ReadinessQueueInner` are dropped, but handle this by
                    // spinning I guess?
                    continue;
                }
                Dequeue::Data(ptr) => ptr,
            };

            let node = unsafe { &*ptr };

            let state = node.state.load(Acquire);

            debug_assert!(state.is_queued());

            release_node(ptr);
        }
    }
}

impl ReadinessQueueInner {
    fn wakeup(&self) -> io::Result<()> {
        self.awakener.wakeup()
    }

    /// Prepend the given node to the head of the readiness queue. This is done
    /// with relaxed ordering. Returns true if `Poll` needs to be woken up.
    fn enqueue_node_with_wakeup(&self, node: &ReadinessNode) -> io::Result<()> {
        if self.enqueue_node(node) {
            self.wakeup()?;
        }

        Ok(())
    }

    /// Push the node into the readiness queue
    fn enqueue_node(&self, node: &ReadinessNode) -> bool {
        // This is the 1024cores.net intrusive MPSC queue [1] "push" function.
        let node_ptr = node as *const _ as *mut _;

        // Relaxed used as the ordering is "released" when swapping
        // `head_readiness`
        node.next_readiness.store(ptr::null_mut(), Relaxed);

        unsafe {
            let mut prev = self.head_readiness.load(Acquire);

            loop {
                if prev == self.closed_marker() {
                    debug_assert!(node_ptr != self.closed_marker());
                    // debug_assert!(node_ptr != self.end_marker());
                    debug_assert!(node_ptr != self.sleep_marker());

                    if node_ptr != self.end_marker() {
                        // The readiness queue is shutdown, but the enqueue flag was
                        // set. This means that we are responsible for decrementing
                        // the ready queue's ref count
                        debug_assert!(node.ref_count.load(Relaxed) >= 2);
                        release_node(node_ptr);
                    }

                    return false;
                }

                let act = self.head_readiness.compare_and_swap(prev, node_ptr, AcqRel);

                if prev == act {
                    break;
                }

                prev = act;
            }

            debug_assert!((*prev).next_readiness.load(Relaxed).is_null());

            (*prev).next_readiness.store(node_ptr, Release);

            prev == self.sleep_marker()
        }
    }

    /// Must only be called in `poll` or `drop`
    unsafe fn dequeue_node(&self, until: *mut ReadinessNode) -> Dequeue {
        // This is the 1024cores.net intrusive MPSC queue [1] "pop" function
        // with the modifications mentioned at the top of the file.
        let mut tail = *self.tail_readiness.get();
        let mut next = (*tail).next_readiness.load(Acquire);

        if tail == self.end_marker() || tail == self.sleep_marker() || tail == self.closed_marker() {
            if next.is_null() {
                return Dequeue::Empty;
            }

            *self.tail_readiness.get() = next;
            tail = next;
            next = (*next).next_readiness.load(Acquire);
        }

        // Only need to check `until` at this point. `until` is either null,
        // which will never match tail OR it is a node that was pushed by
        // the current thread. This means that either:
        //
        // 1) The queue is inconsistent, which is handled explicitly
        // 2) We encounter `until` at this point in dequeue
        // 3) we will pop a different node
        if tail == until {
            return Dequeue::Empty;
        }

        if !next.is_null() {
            *self.tail_readiness.get() = next;
            return Dequeue::Data(tail);
        }

        if self.head_readiness.load(Acquire) != tail {
            return Dequeue::Inconsistent;
        }

        // Push the stub node
        self.enqueue_node(&*self.end_marker);

        next = (*tail).next_readiness.load(Acquire);

        if !next.is_null() {
            *self.tail_readiness.get() = next;
            return Dequeue::Data(tail);
        }

        Dequeue::Inconsistent
    }

    fn end_marker(&self) -> *mut ReadinessNode {
        &*self.end_marker as *const ReadinessNode as *mut ReadinessNode
    }

    fn sleep_marker(&self) -> *mut ReadinessNode {
        &*self.sleep_marker as *const ReadinessNode as *mut ReadinessNode
    }

    fn closed_marker(&self) -> *mut ReadinessNode {
        &*self.closed_marker as *const ReadinessNode as *mut ReadinessNode
    }
}

impl ReadinessNode {
    /// Return a new `ReadinessNode`, initialized with a ref_count of 3.
    fn new(queue: *mut (),
           token: Token,
           interest: Ready,
           opt: PollOpt,
           ref_count: usize) -> ReadinessNode
    {
        ReadinessNode {
            state: AtomicState::new(interest, opt),
            // Only the first token is set, the others are initialized to 0
            token_0: UnsafeCell::new(token),
            token_1: UnsafeCell::new(Token(0)),
            token_2: UnsafeCell::new(Token(0)),
            next_readiness: AtomicPtr::new(ptr::null_mut()),
            update_lock: AtomicBool::new(false),
            readiness_queue: AtomicPtr::new(queue),
            ref_count: AtomicUsize::new(ref_count),
        }
    }

    fn marker() -> ReadinessNode {
        ReadinessNode {
            state: AtomicState::new(Ready::empty(), PollOpt::empty()),
            token_0: UnsafeCell::new(Token(0)),
            token_1: UnsafeCell::new(Token(0)),
            token_2: UnsafeCell::new(Token(0)),
            next_readiness: AtomicPtr::new(ptr::null_mut()),
            update_lock: AtomicBool::new(false),
            readiness_queue: AtomicPtr::new(ptr::null_mut()),
            ref_count: AtomicUsize::new(0),
        }
    }

    fn enqueue_with_wakeup(&self) -> io::Result<()> {
        let queue = self.readiness_queue.load(Acquire);

        if queue.is_null() {
            // Not associated with a queue, nothing to do
            return Ok(());
        }

        enqueue_with_wakeup(queue, self)
    }
}

fn enqueue_with_wakeup(queue: *mut (), node: &ReadinessNode) -> io::Result<()> {
    debug_assert!(!queue.is_null());
    // This is ugly... but we don't want to bump the ref count.
    let queue: &Arc<ReadinessQueueInner> = unsafe { mem::transmute(&queue) };
    queue.enqueue_node_with_wakeup(node)
}

unsafe fn token(node: &ReadinessNode, pos: usize) -> Token {
    match pos {
        0 => *node.token_0.get(),
        1 => *node.token_1.get(),
        2 => *node.token_2.get(),
        _ => unreachable!(),
    }
}

fn release_node(ptr: *mut ReadinessNode) {
    unsafe {
        // `AcqRel` synchronizes with other `release_node` functions and ensures
        // that the drop happens after any reads / writes on other threads.
        if (*ptr).ref_count.fetch_sub(1, AcqRel) != 1 {
            return;
        }

        let node = Box::from_raw(ptr);

        // Decrement the readiness_queue Arc
        let queue = node.readiness_queue.load(Acquire);

        if queue.is_null() {
            return;
        }

        let _: Arc<ReadinessQueueInner> = mem::transmute(queue);
    }
}

impl AtomicState {
    fn new(interest: Ready, opt: PollOpt) -> AtomicState {
        let state = ReadinessState::new(interest, opt);

        AtomicState {
            inner: AtomicUsize::new(state.into()),
        }
    }

    /// Loads the current `ReadinessState`
    fn load(&self, order: Ordering) -> ReadinessState {
        self.inner.load(order).into()
    }

    /// Stores a state if the current state is the same as `current`.
    fn compare_and_swap(&self, current: ReadinessState, new: ReadinessState, order: Ordering) -> ReadinessState {
        self.inner.compare_and_swap(current.into(), new.into(), order).into()
    }

    // Returns `true` if the node should be queued
    fn flag_as_dropped(&self) -> bool {
        let prev: ReadinessState = self.inner.fetch_or(DROPPED_MASK | QUEUED_MASK, Release).into();
        // The flag should not have been previously set
        debug_assert!(!prev.is_dropped());

        !prev.is_queued()
    }
}

impl ReadinessState {
    // Create a `ReadinessState` initialized with the provided arguments
    #[inline]
    fn new(interest: Ready, opt: PollOpt) -> ReadinessState {
        let interest = event::ready_as_usize(interest);
        let opt = event::opt_as_usize(opt);

        debug_assert!(interest <= MASK_4);
        debug_assert!(opt <= MASK_4);

        let mut val = interest << INTEREST_SHIFT;
        val |= opt << POLL_OPT_SHIFT;

        ReadinessState(val)
    }

    #[inline]
    fn get(&self, mask: usize, shift: usize) -> usize{
        (self.0 >> shift) & mask
    }

    #[inline]
    fn set(&mut self, val: usize, mask: usize, shift: usize) {
        self.0 = (self.0 & !(mask << shift)) | (val << shift)
    }

    /// Get the readiness
    #[inline]
    fn readiness(&self) -> Ready {
        let v = self.get(MASK_4, READINESS_SHIFT);
        event::ready_from_usize(v)
    }

    #[inline]
    fn effective_readiness(&self) -> Ready {
        self.readiness() & self.interest()
    }

    /// Set the readiness
    #[inline]
    fn set_readiness(&mut self, v: Ready) {
        self.set(event::ready_as_usize(v), MASK_4, READINESS_SHIFT);
    }

    /// Get the interest
    #[inline]
    fn interest(&self) -> Ready {
        let v = self.get(MASK_4, INTEREST_SHIFT);
        event::ready_from_usize(v)
    }

    /// Set the interest
    #[inline]
    fn set_interest(&mut self, v: Ready) {
        self.set(event::ready_as_usize(v), MASK_4, INTEREST_SHIFT);
    }

    #[inline]
    fn disarm(&mut self) {
        self.set_interest(Ready::empty());
    }

    /// Get the poll options
    #[inline]
    fn poll_opt(&self) -> PollOpt {
        let v = self.get(MASK_4, POLL_OPT_SHIFT);
        event::opt_from_usize(v)
    }

    /// Set the poll options
    #[inline]
    fn set_poll_opt(&mut self, v: PollOpt) {
        self.set(event::opt_as_usize(v), MASK_4, POLL_OPT_SHIFT);
    }

    #[inline]
    fn is_queued(&self) -> bool {
        self.0 & QUEUED_MASK == QUEUED_MASK
    }

    /// Set the queued flag
    #[inline]
    fn set_queued(&mut self) {
        // Dropped nodes should never be queued
        debug_assert!(!self.is_dropped());
        self.0 |= QUEUED_MASK;
    }

    #[inline]
    fn set_dequeued(&mut self) {
        debug_assert!(self.is_queued());
        self.0 &= !QUEUED_MASK
    }

    #[inline]
    fn is_dropped(&self) -> bool {
        self.0 & DROPPED_MASK == DROPPED_MASK
    }

    #[inline]
    fn token_read_pos(&self) -> usize {
        self.get(MASK_2, TOKEN_RD_SHIFT)
    }

    #[inline]
    fn token_write_pos(&self) -> usize {
        self.get(MASK_2, TOKEN_WR_SHIFT)
    }

    #[inline]
    fn next_token_pos(&self) -> usize {
        let rd = self.token_read_pos();
        let wr = self.token_write_pos();

        match wr {
            0 => {
                match rd {
                    1 => 2,
                    2 => 1,
                    0 => 1,
                    _ => unreachable!(),
                }
            }
            1 => {
                match rd {
                    0 => 2,
                    2 => 0,
                    1 => 2,
                    _ => unreachable!(),
                }
            }
            2 => {
                match rd {
                    0 => 1,
                    1 => 0,
                    2 => 0,
                    _ => unreachable!(),
                }
            }
            _ => unreachable!(),
        }
    }

    #[inline]
    fn set_token_write_pos(&mut self, val: usize) {
        self.set(val, MASK_2, TOKEN_WR_SHIFT);
    }

    #[inline]
    fn update_token_read_pos(&mut self) {
        let val = self.token_write_pos();
        self.set(val, MASK_2, TOKEN_RD_SHIFT);
    }
}

impl From<ReadinessState> for usize {
    fn from(src: ReadinessState) -> usize {
        src.0
    }
}

impl From<usize> for ReadinessState {
    fn from(src: usize) -> ReadinessState {
        ReadinessState(src)
    }
}

fn is_send<T: Send>() {}
fn is_sync<T: Sync>() {}

impl SelectorId {
    pub fn new() -> SelectorId {
        SelectorId {
            id: AtomicUsize::new(0),
        }
    }

    pub fn associate_selector(&self, poll: &Poll) -> io::Result<()> {
        let selector_id = self.id.load(Ordering::SeqCst);

        if selector_id != 0 && selector_id != poll.selector.id() {
            Err(io::Error::new(io::ErrorKind::Other, "socket already registered"))
        } else {
            self.id.store(poll.selector.id(), Ordering::SeqCst);
            Ok(())
        }
    }
}

impl Clone for SelectorId {
    fn clone(&self) -> SelectorId {
        SelectorId {
            id: AtomicUsize::new(self.id.load(Ordering::SeqCst)),
        }
    }
}

#[test]
#[cfg(all(unix, not(target_os = "fuchsia")))]
pub fn as_raw_fd() {
    let poll = Poll::new().unwrap();
    assert!(poll.as_raw_fd() > 0);
}