aboutsummaryrefslogtreecommitdiffstats
path: root/contrib/go/_std_1.22/src/net/net.go
blob: c434c96bf899afcb8f3c646f478520b70bf9a75f (plain) (blame)
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
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package net provides a portable interface for network I/O, including
TCP/IP, UDP, domain name resolution, and Unix domain sockets.

Although the package provides access to low-level networking
primitives, most clients will need only the basic interface provided
by the [Dial], [Listen], and Accept functions and the associated
[Conn] and [Listener] interfaces. The crypto/tls package uses
the same interfaces and similar Dial and Listen functions.

The Dial function connects to a server:

	conn, err := net.Dial("tcp", "golang.org:80")
	if err != nil {
		// handle error
	}
	fmt.Fprintf(conn, "GET / HTTP/1.0\r\n\r\n")
	status, err := bufio.NewReader(conn).ReadString('\n')
	// ...

The Listen function creates servers:

	ln, err := net.Listen("tcp", ":8080")
	if err != nil {
		// handle error
	}
	for {
		conn, err := ln.Accept()
		if err != nil {
			// handle error
		}
		go handleConnection(conn)
	}

# Name Resolution

The method for resolving domain names, whether indirectly with functions like Dial
or directly with functions like [LookupHost] and [LookupAddr], varies by operating system.

On Unix systems, the resolver has two options for resolving names.
It can use a pure Go resolver that sends DNS requests directly to the servers
listed in /etc/resolv.conf, or it can use a cgo-based resolver that calls C
library routines such as getaddrinfo and getnameinfo.

By default the pure Go resolver is used, because a blocked DNS request consumes
only a goroutine, while a blocked C call consumes an operating system thread.
When cgo is available, the cgo-based resolver is used instead under a variety of
conditions: on systems that do not let programs make direct DNS requests (OS X),
when the LOCALDOMAIN environment variable is present (even if empty),
when the RES_OPTIONS or HOSTALIASES environment variable is non-empty,
when the ASR_CONFIG environment variable is non-empty (OpenBSD only),
when /etc/resolv.conf or /etc/nsswitch.conf specify the use of features that the
Go resolver does not implement, and when the name being looked up ends in .local
or is an mDNS name.

The resolver decision can be overridden by setting the netdns value of the
GODEBUG environment variable (see package runtime) to go or cgo, as in:

	export GODEBUG=netdns=go    # force pure Go resolver
	export GODEBUG=netdns=cgo   # force native resolver (cgo, win32)

The decision can also be forced while building the Go source tree
by setting the netgo or netcgo build tag.

A numeric netdns setting, as in GODEBUG=netdns=1, causes the resolver
to print debugging information about its decisions.
To force a particular resolver while also printing debugging information,
join the two settings by a plus sign, as in GODEBUG=netdns=go+1.

On macOS, if Go code that uses the net package is built with
-buildmode=c-archive, linking the resulting archive into a C program
requires passing -lresolv when linking the C code.

On Plan 9, the resolver always accesses /net/cs and /net/dns.

On Windows, in Go 1.18.x and earlier, the resolver always used C
library functions, such as GetAddrInfo and DnsQuery.
*/
package net

import (
	"context"
	"errors"
	"internal/poll"
	"io"
	"os"
	"sync"
	"syscall"
	"time"
)

// Addr represents a network end point address.
//
// The two methods [Addr.Network] and [Addr.String] conventionally return strings
// that can be passed as the arguments to [Dial], but the exact form
// and meaning of the strings is up to the implementation.
type Addr interface {
	Network() string // name of the network (for example, "tcp", "udp")
	String() string  // string form of address (for example, "192.0.2.1:25", "[2001:db8::1]:80")
}

// Conn is a generic stream-oriented network connection.
//
// Multiple goroutines may invoke methods on a Conn simultaneously.
type Conn interface {
	// Read reads data from the connection.
	// Read can be made to time out and return an error after a fixed
	// time limit; see SetDeadline and SetReadDeadline.
	Read(b []byte) (n int, err error)

	// Write writes data to the connection.
	// Write can be made to time out and return an error after a fixed
	// time limit; see SetDeadline and SetWriteDeadline.
	Write(b []byte) (n int, err error)

	// Close closes the connection.
	// Any blocked Read or Write operations will be unblocked and return errors.
	Close() error

	// LocalAddr returns the local network address, if known.
	LocalAddr() Addr

	// RemoteAddr returns the remote network address, if known.
	RemoteAddr() Addr

	// SetDeadline sets the read and write deadlines associated
	// with the connection. It is equivalent to calling both
	// SetReadDeadline and SetWriteDeadline.
	//
	// A deadline is an absolute time after which I/O operations
	// fail instead of blocking. The deadline applies to all future
	// and pending I/O, not just the immediately following call to
	// Read or Write. After a deadline has been exceeded, the
	// connection can be refreshed by setting a deadline in the future.
	//
	// If the deadline is exceeded a call to Read or Write or to other
	// I/O methods will return an error that wraps os.ErrDeadlineExceeded.
	// This can be tested using errors.Is(err, os.ErrDeadlineExceeded).
	// The error's Timeout method will return true, but note that there
	// are other possible errors for which the Timeout method will
	// return true even if the deadline has not been exceeded.
	//
	// An idle timeout can be implemented by repeatedly extending
	// the deadline after successful Read or Write calls.
	//
	// A zero value for t means I/O operations will not time out.
	SetDeadline(t time.Time) error

	// SetReadDeadline sets the deadline for future Read calls
	// and any currently-blocked Read call.
	// A zero value for t means Read will not time out.
	SetReadDeadline(t time.Time) error

	// SetWriteDeadline sets the deadline for future Write calls
	// and any currently-blocked Write call.
	// Even if write times out, it may return n > 0, indicating that
	// some of the data was successfully written.
	// A zero value for t means Write will not time out.
	SetWriteDeadline(t time.Time) error
}

type conn struct {
	fd *netFD
}

func (c *conn) ok() bool { return c != nil && c.fd != nil }

// Implementation of the Conn interface.

// Read implements the Conn Read method.
func (c *conn) Read(b []byte) (int, error) {
	if !c.ok() {
		return 0, syscall.EINVAL
	}
	n, err := c.fd.Read(b)
	if err != nil && err != io.EOF {
		err = &OpError{Op: "read", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return n, err
}

// Write implements the Conn Write method.
func (c *conn) Write(b []byte) (int, error) {
	if !c.ok() {
		return 0, syscall.EINVAL
	}
	n, err := c.fd.Write(b)
	if err != nil {
		err = &OpError{Op: "write", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return n, err
}

// Close closes the connection.
func (c *conn) Close() error {
	if !c.ok() {
		return syscall.EINVAL
	}
	err := c.fd.Close()
	if err != nil {
		err = &OpError{Op: "close", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return err
}

// LocalAddr returns the local network address.
// The Addr returned is shared by all invocations of LocalAddr, so
// do not modify it.
func (c *conn) LocalAddr() Addr {
	if !c.ok() {
		return nil
	}
	return c.fd.laddr
}

// RemoteAddr returns the remote network address.
// The Addr returned is shared by all invocations of RemoteAddr, so
// do not modify it.
func (c *conn) RemoteAddr() Addr {
	if !c.ok() {
		return nil
	}
	return c.fd.raddr
}

// SetDeadline implements the Conn SetDeadline method.
func (c *conn) SetDeadline(t time.Time) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := c.fd.SetDeadline(t); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetReadDeadline implements the Conn SetReadDeadline method.
func (c *conn) SetReadDeadline(t time.Time) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := c.fd.SetReadDeadline(t); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetWriteDeadline implements the Conn SetWriteDeadline method.
func (c *conn) SetWriteDeadline(t time.Time) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := c.fd.SetWriteDeadline(t); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetReadBuffer sets the size of the operating system's
// receive buffer associated with the connection.
func (c *conn) SetReadBuffer(bytes int) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := setReadBuffer(c.fd, bytes); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetWriteBuffer sets the size of the operating system's
// transmit buffer associated with the connection.
func (c *conn) SetWriteBuffer(bytes int) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := setWriteBuffer(c.fd, bytes); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// File returns a copy of the underlying [os.File].
// It is the caller's responsibility to close f when finished.
// Closing c does not affect f, and closing f does not affect c.
//
// The returned os.File's file descriptor is different from the connection's.
// Attempting to change properties of the original using this duplicate
// may or may not have the desired effect.
func (c *conn) File() (f *os.File, err error) {
	f, err = c.fd.dup()
	if err != nil {
		err = &OpError{Op: "file", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return
}

// PacketConn is a generic packet-oriented network connection.
//
// Multiple goroutines may invoke methods on a PacketConn simultaneously.
type PacketConn interface {
	// ReadFrom reads a packet from the connection,
	// copying the payload into p. It returns the number of
	// bytes copied into p and the return address that
	// was on the packet.
	// It returns the number of bytes read (0 <= n <= len(p))
	// and any error encountered. Callers should always process
	// the n > 0 bytes returned before considering the error err.
	// ReadFrom can be made to time out and return an error after a
	// fixed time limit; see SetDeadline and SetReadDeadline.
	ReadFrom(p []byte) (n int, addr Addr, err error)

	// WriteTo writes a packet with payload p to addr.
	// WriteTo can be made to time out and return an Error after a
	// fixed time limit; see SetDeadline and SetWriteDeadline.
	// On packet-oriented connections, write timeouts are rare.
	WriteTo(p []byte, addr Addr) (n int, err error)

	// Close closes the connection.
	// Any blocked ReadFrom or WriteTo operations will be unblocked and return errors.
	Close() error

	// LocalAddr returns the local network address, if known.
	LocalAddr() Addr

	// SetDeadline sets the read and write deadlines associated
	// with the connection. It is equivalent to calling both
	// SetReadDeadline and SetWriteDeadline.
	//
	// A deadline is an absolute time after which I/O operations
	// fail instead of blocking. The deadline applies to all future
	// and pending I/O, not just the immediately following call to
	// Read or Write. After a deadline has been exceeded, the
	// connection can be refreshed by setting a deadline in the future.
	//
	// If the deadline is exceeded a call to Read or Write or to other
	// I/O methods will return an error that wraps os.ErrDeadlineExceeded.
	// This can be tested using errors.Is(err, os.ErrDeadlineExceeded).
	// The error's Timeout method will return true, but note that there
	// are other possible errors for which the Timeout method will
	// return true even if the deadline has not been exceeded.
	//
	// An idle timeout can be implemented by repeatedly extending
	// the deadline after successful ReadFrom or WriteTo calls.
	//
	// A zero value for t means I/O operations will not time out.
	SetDeadline(t time.Time) error

	// SetReadDeadline sets the deadline for future ReadFrom calls
	// and any currently-blocked ReadFrom call.
	// A zero value for t means ReadFrom will not time out.
	SetReadDeadline(t time.Time) error

	// SetWriteDeadline sets the deadline for future WriteTo calls
	// and any currently-blocked WriteTo call.
	// Even if write times out, it may return n > 0, indicating that
	// some of the data was successfully written.
	// A zero value for t means WriteTo will not time out.
	SetWriteDeadline(t time.Time) error
}

var listenerBacklogCache struct {
	sync.Once
	val int
}

// listenerBacklog is a caching wrapper around maxListenerBacklog.
func listenerBacklog() int {
	listenerBacklogCache.Do(func() { listenerBacklogCache.val = maxListenerBacklog() })
	return listenerBacklogCache.val
}

// A Listener is a generic network listener for stream-oriented protocols.
//
// Multiple goroutines may invoke methods on a Listener simultaneously.
type Listener interface {
	// Accept waits for and returns the next connection to the listener.
	Accept() (Conn, error)

	// Close closes the listener.
	// Any blocked Accept operations will be unblocked and return errors.
	Close() error

	// Addr returns the listener's network address.
	Addr() Addr
}

// An Error represents a network error.
type Error interface {
	error
	Timeout() bool // Is the error a timeout?

	// Deprecated: Temporary errors are not well-defined.
	// Most "temporary" errors are timeouts, and the few exceptions are surprising.
	// Do not use this method.
	Temporary() bool
}

// Various errors contained in OpError.
var (
	// For connection setup operations.
	errNoSuitableAddress = errors.New("no suitable address found")

	// For connection setup and write operations.
	errMissingAddress = errors.New("missing address")

	// For both read and write operations.
	errCanceled         = canceledError{}
	ErrWriteToConnected = errors.New("use of WriteTo with pre-connected connection")
)

// canceledError lets us return the same error string we have always
// returned, while still being Is context.Canceled.
type canceledError struct{}

func (canceledError) Error() string { return "operation was canceled" }

func (canceledError) Is(err error) bool { return err == context.Canceled }

// mapErr maps from the context errors to the historical internal net
// error values.
func mapErr(err error) error {
	switch err {
	case context.Canceled:
		return errCanceled
	case context.DeadlineExceeded:
		return errTimeout
	default:
		return err
	}
}

// OpError is the error type usually returned by functions in the net
// package. It describes the operation, network type, and address of
// an error.
type OpError struct {
	// Op is the operation which caused the error, such as
	// "read" or "write".
	Op string

	// Net is the network type on which this error occurred,
	// such as "tcp" or "udp6".
	Net string

	// For operations involving a remote network connection, like
	// Dial, Read, or Write, Source is the corresponding local
	// network address.
	Source Addr

	// Addr is the network address for which this error occurred.
	// For local operations, like Listen or SetDeadline, Addr is
	// the address of the local endpoint being manipulated.
	// For operations involving a remote network connection, like
	// Dial, Read, or Write, Addr is the remote address of that
	// connection.
	Addr Addr

	// Err is the error that occurred during the operation.
	// The Error method panics if the error is nil.
	Err error
}

func (e *OpError) Unwrap() error { return e.Err }

func (e *OpError) Error() string {
	if e == nil {
		return "<nil>"
	}
	s := e.Op
	if e.Net != "" {
		s += " " + e.Net
	}
	if e.Source != nil {
		s += " " + e.Source.String()
	}
	if e.Addr != nil {
		if e.Source != nil {
			s += "->"
		} else {
			s += " "
		}
		s += e.Addr.String()
	}
	s += ": " + e.Err.Error()
	return s
}

var (
	// aLongTimeAgo is a non-zero time, far in the past, used for
	// immediate cancellation of dials.
	aLongTimeAgo = time.Unix(1, 0)

	// noDeadline and noCancel are just zero values for
	// readability with functions taking too many parameters.
	noDeadline = time.Time{}
	noCancel   = (chan struct{})(nil)
)

type timeout interface {
	Timeout() bool
}

func (e *OpError) Timeout() bool {
	if ne, ok := e.Err.(*os.SyscallError); ok {
		t, ok := ne.Err.(timeout)
		return ok && t.Timeout()
	}
	t, ok := e.Err.(timeout)
	return ok && t.Timeout()
}

type temporary interface {
	Temporary() bool
}

func (e *OpError) Temporary() bool {
	// Treat ECONNRESET and ECONNABORTED as temporary errors when
	// they come from calling accept. See issue 6163.
	if e.Op == "accept" && isConnError(e.Err) {
		return true
	}

	if ne, ok := e.Err.(*os.SyscallError); ok {
		t, ok := ne.Err.(temporary)
		return ok && t.Temporary()
	}
	t, ok := e.Err.(temporary)
	return ok && t.Temporary()
}

// A ParseError is the error type of literal network address parsers.
type ParseError struct {
	// Type is the type of string that was expected, such as
	// "IP address", "CIDR address".
	Type string

	// Text is the malformed text string.
	Text string
}

func (e *ParseError) Error() string { return "invalid " + e.Type + ": " + e.Text }

func (e *ParseError) Timeout() bool   { return false }
func (e *ParseError) Temporary() bool { return false }

type AddrError struct {
	Err  string
	Addr string
}

func (e *AddrError) Error() string {
	if e == nil {
		return "<nil>"
	}
	s := e.Err
	if e.Addr != "" {
		s = "address " + e.Addr + ": " + s
	}
	return s
}

func (e *AddrError) Timeout() bool   { return false }
func (e *AddrError) Temporary() bool { return false }

type UnknownNetworkError string

func (e UnknownNetworkError) Error() string   { return "unknown network " + string(e) }
func (e UnknownNetworkError) Timeout() bool   { return false }
func (e UnknownNetworkError) Temporary() bool { return false }

type InvalidAddrError string

func (e InvalidAddrError) Error() string   { return string(e) }
func (e InvalidAddrError) Timeout() bool   { return false }
func (e InvalidAddrError) Temporary() bool { return false }

// errTimeout exists to return the historical "i/o timeout" string
// for context.DeadlineExceeded. See mapErr.
// It is also used when Dialer.Deadline is exceeded.
// error.Is(errTimeout, context.DeadlineExceeded) returns true.
//
// TODO(iant): We could consider changing this to os.ErrDeadlineExceeded
// in the future, if we make
//
//	errors.Is(os.ErrDeadlineExceeded, context.DeadlineExceeded)
//
// return true.
var errTimeout error = &timeoutError{}

type timeoutError struct{}

func (e *timeoutError) Error() string   { return "i/o timeout" }
func (e *timeoutError) Timeout() bool   { return true }
func (e *timeoutError) Temporary() bool { return true }

func (e *timeoutError) Is(err error) bool {
	return err == context.DeadlineExceeded
}

// DNSConfigError represents an error reading the machine's DNS configuration.
// (No longer used; kept for compatibility.)
type DNSConfigError struct {
	Err error
}

func (e *DNSConfigError) Unwrap() error   { return e.Err }
func (e *DNSConfigError) Error() string   { return "error reading DNS config: " + e.Err.Error() }
func (e *DNSConfigError) Timeout() bool   { return false }
func (e *DNSConfigError) Temporary() bool { return false }

// Various errors contained in DNSError.
var (
	errNoSuchHost = errors.New("no such host")
)

// DNSError represents a DNS lookup error.
type DNSError struct {
	Err         string // description of the error
	Name        string // name looked for
	Server      string // server used
	IsTimeout   bool   // if true, timed out; not all timeouts set this
	IsTemporary bool   // if true, error is temporary; not all errors set this

	// IsNotFound is set to true when the requested name does not
	// contain any records of the requested type (data not found),
	// or the name itself was not found (NXDOMAIN).
	IsNotFound bool
}

func (e *DNSError) Error() string {
	if e == nil {
		return "<nil>"
	}
	s := "lookup " + e.Name
	if e.Server != "" {
		s += " on " + e.Server
	}
	s += ": " + e.Err
	return s
}

// Timeout reports whether the DNS lookup is known to have timed out.
// This is not always known; a DNS lookup may fail due to a timeout
// and return a [DNSError] for which Timeout returns false.
func (e *DNSError) Timeout() bool { return e.IsTimeout }

// Temporary reports whether the DNS error is known to be temporary.
// This is not always known; a DNS lookup may fail due to a temporary
// error and return a [DNSError] for which Temporary returns false.
func (e *DNSError) Temporary() bool { return e.IsTimeout || e.IsTemporary }

// errClosed exists just so that the docs for ErrClosed don't mention
// the internal package poll.
var errClosed = poll.ErrNetClosing

// ErrClosed is the error returned by an I/O call on a network
// connection that has already been closed, or that is closed by
// another goroutine before the I/O is completed. This may be wrapped
// in another error, and should normally be tested using
// errors.Is(err, net.ErrClosed).
var ErrClosed error = errClosed

// noReadFrom can be embedded alongside another type to
// hide the ReadFrom method of that other type.
type noReadFrom struct{}

// ReadFrom hides another ReadFrom method.
// It should never be called.
func (noReadFrom) ReadFrom(io.Reader) (int64, error) {
	panic("can't happen")
}

// tcpConnWithoutReadFrom implements all the methods of *TCPConn other
// than ReadFrom. This is used to permit ReadFrom to call io.Copy
// without leading to a recursive call to ReadFrom.
type tcpConnWithoutReadFrom struct {
	noReadFrom
	*TCPConn
}

// Fallback implementation of io.ReaderFrom's ReadFrom, when sendfile isn't
// applicable.
func genericReadFrom(c *TCPConn, r io.Reader) (n int64, err error) {
	// Use wrapper to hide existing r.ReadFrom from io.Copy.
	return io.Copy(tcpConnWithoutReadFrom{TCPConn: c}, r)
}

// noWriteTo can be embedded alongside another type to
// hide the WriteTo method of that other type.
type noWriteTo struct{}

// WriteTo hides another WriteTo method.
// It should never be called.
func (noWriteTo) WriteTo(io.Writer) (int64, error) {
	panic("can't happen")
}

// tcpConnWithoutWriteTo implements all the methods of *TCPConn other
// than WriteTo. This is used to permit WriteTo to call io.Copy
// without leading to a recursive call to WriteTo.
type tcpConnWithoutWriteTo struct {
	noWriteTo
	*TCPConn
}

// Fallback implementation of io.WriterTo's WriteTo, when zero-copy isn't applicable.
func genericWriteTo(c *TCPConn, w io.Writer) (n int64, err error) {
	// Use wrapper to hide existing w.WriteTo from io.Copy.
	return io.Copy(w, tcpConnWithoutWriteTo{TCPConn: c})
}

// Limit the number of concurrent cgo-using goroutines, because
// each will block an entire operating system thread. The usual culprit
// is resolving many DNS names in separate goroutines but the DNS
// server is not responding. Then the many lookups each use a different
// thread, and the system or the program runs out of threads.

var threadLimit chan struct{}

var threadOnce sync.Once

func acquireThread() {
	threadOnce.Do(func() {
		threadLimit = make(chan struct{}, concurrentThreadsLimit())
	})
	threadLimit <- struct{}{}
}

func releaseThread() {
	<-threadLimit
}

// buffersWriter is the interface implemented by Conns that support a
// "writev"-like batch write optimization.
// writeBuffers should fully consume and write all chunks from the
// provided Buffers, else it should report a non-nil error.
type buffersWriter interface {
	writeBuffers(*Buffers) (int64, error)
}

// Buffers contains zero or more runs of bytes to write.
//
// On certain machines, for certain types of connections, this is
// optimized into an OS-specific batch write operation (such as
// "writev").
type Buffers [][]byte

var (
	_ io.WriterTo = (*Buffers)(nil)
	_ io.Reader   = (*Buffers)(nil)
)

// WriteTo writes contents of the buffers to w.
//
// WriteTo implements [io.WriterTo] for [Buffers].
//
// WriteTo modifies the slice v as well as v[i] for 0 <= i < len(v),
// but does not modify v[i][j] for any i, j.
func (v *Buffers) WriteTo(w io.Writer) (n int64, err error) {
	if wv, ok := w.(buffersWriter); ok {
		return wv.writeBuffers(v)
	}
	for _, b := range *v {
		nb, err := w.Write(b)
		n += int64(nb)
		if err != nil {
			v.consume(n)
			return n, err
		}
	}
	v.consume(n)
	return n, nil
}

// Read from the buffers.
//
// Read implements [io.Reader] for [Buffers].
//
// Read modifies the slice v as well as v[i] for 0 <= i < len(v),
// but does not modify v[i][j] for any i, j.
func (v *Buffers) Read(p []byte) (n int, err error) {
	for len(p) > 0 && len(*v) > 0 {
		n0 := copy(p, (*v)[0])
		v.consume(int64(n0))
		p = p[n0:]
		n += n0
	}
	if len(*v) == 0 {
		err = io.EOF
	}
	return
}

func (v *Buffers) consume(n int64) {
	for len(*v) > 0 {
		ln0 := int64(len((*v)[0]))
		if ln0 > n {
			(*v)[0] = (*v)[0][n:]
			return
		}
		n -= ln0
		(*v)[0] = nil
		*v = (*v)[1:]
	}
}