Source file src/internal/poll/fd_unix.go

Documentation: internal/poll

     1  // Copyright 2017 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build unix || (js && wasm) || wasip1
     6  
     7  package poll
     8  
     9  import (
    10  	"internal/itoa"
    11  	"internal/syscall/unix"
    12  	"io"
    13  	"sync/atomic"
    14  	"syscall"
    15  )
    16  
    17  // FD is a file descriptor. The net and os packages use this type as a
    18  // field of a larger type representing a network connection or OS file.
    19  type FD struct {
    20  	// Lock sysfd and serialize access to Read and Write methods.
    21  	fdmu fdMutex
    22  
    23  	// System file descriptor. Immutable until Close.
    24  	Sysfd int
    25  
    26  	// Platform dependent state of the file descriptor.
    27  	SysFile
    28  
    29  	// I/O poller.
    30  	pd pollDesc
    31  
    32  	// Semaphore signaled when file is closed.
    33  	csema uint32
    34  
    35  	// Non-zero if this file has been set to blocking mode.
    36  	isBlocking uint32
    37  
    38  	// Whether this is a streaming descriptor, as opposed to a
    39  	// packet-based descriptor like a UDP socket. Immutable.
    40  	IsStream bool
    41  
    42  	// Whether a zero byte read indicates EOF. This is false for a
    43  	// message based socket connection.
    44  	ZeroReadIsEOF bool
    45  
    46  	// Whether this is a file rather than a network socket.
    47  	isFile bool
    48  }
    49  
    50  // Init initializes the FD. The Sysfd field should already be set.
    51  // This can be called multiple times on a single FD.
    52  // The net argument is a network name from the net package (e.g., "tcp"),
    53  // or "file".
    54  // Set pollable to true if fd should be managed by runtime netpoll.
    55  func (fd *FD) Init(net string, pollable bool) error {
    56  	fd.SysFile.init()
    57  
    58  	// We don't actually care about the various network types.
    59  	if net == "file" {
    60  		fd.isFile = true
    61  	}
    62  	if !pollable {
    63  		fd.isBlocking = 1
    64  		return nil
    65  	}
    66  	err := fd.pd.init(fd)
    67  	if err != nil {
    68  		// If we could not initialize the runtime poller,
    69  		// assume we are using blocking mode.
    70  		fd.isBlocking = 1
    71  	}
    72  	return err
    73  }
    74  
    75  // Destroy closes the file descriptor. This is called when there are
    76  // no remaining references.
    77  func (fd *FD) destroy() error {
    78  	// Poller may want to unregister fd in readiness notification mechanism,
    79  	// so this must be executed before CloseFunc.
    80  	fd.pd.close()
    81  
    82  	err := fd.SysFile.destroy(fd.Sysfd)
    83  
    84  	fd.Sysfd = -1
    85  	runtime_Semrelease(&fd.csema)
    86  	return err
    87  }
    88  
    89  // Close closes the FD. The underlying file descriptor is closed by the
    90  // destroy method when there are no remaining references.
    91  func (fd *FD) Close() error {
    92  	if !fd.fdmu.increfAndClose() {
    93  		return errClosing(fd.isFile)
    94  	}
    95  
    96  	// Unblock any I/O.  Once it all unblocks and returns,
    97  	// so that it cannot be referring to fd.sysfd anymore,
    98  	// the final decref will close fd.sysfd. This should happen
    99  	// fairly quickly, since all the I/O is non-blocking, and any
   100  	// attempts to block in the pollDesc will return errClosing(fd.isFile).
   101  	fd.pd.evict()
   102  
   103  	// The call to decref will call destroy if there are no other
   104  	// references.
   105  	err := fd.decref()
   106  
   107  	// Wait until the descriptor is closed. If this was the only
   108  	// reference, it is already closed. Only wait if the file has
   109  	// not been set to blocking mode, as otherwise any current I/O
   110  	// may be blocking, and that would block the Close.
   111  	// No need for an atomic read of isBlocking, increfAndClose means
   112  	// we have exclusive access to fd.
   113  	if fd.isBlocking == 0 {
   114  		runtime_Semacquire(&fd.csema)
   115  	}
   116  
   117  	return err
   118  }
   119  
   120  // SetBlocking puts the file into blocking mode.
   121  func (fd *FD) SetBlocking() error {
   122  	if err := fd.incref(); err != nil {
   123  		return err
   124  	}
   125  	defer fd.decref()
   126  	// Atomic store so that concurrent calls to SetBlocking
   127  	// do not cause a race condition. isBlocking only ever goes
   128  	// from 0 to 1 so there is no real race here.
   129  	atomic.StoreUint32(&fd.isBlocking, 1)
   130  	return syscall.SetNonblock(fd.Sysfd, false)
   131  }
   132  
   133  // Darwin and FreeBSD can't read or write 2GB+ files at a time,
   134  // even on 64-bit systems.
   135  // The same is true of socket implementations on many systems.
   136  // See golang.org/issue/7812 and golang.org/issue/16266.
   137  // Use 1GB instead of, say, 2GB-1, to keep subsequent reads aligned.
   138  const maxRW = 1 << 30
   139  
   140  // Read implements io.Reader.
   141  func (fd *FD) Read(p []byte) (int, error) {
   142  	if err := fd.readLock(); err != nil {
   143  		return 0, err
   144  	}
   145  	defer fd.readUnlock()
   146  	if len(p) == 0 {
   147  		// If the caller wanted a zero byte read, return immediately
   148  		// without trying (but after acquiring the readLock).
   149  		// Otherwise syscall.Read returns 0, nil which looks like
   150  		// io.EOF.
   151  		// TODO(bradfitz): make it wait for readability? (Issue 15735)
   152  		return 0, nil
   153  	}
   154  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   155  		return 0, err
   156  	}
   157  	if fd.IsStream && len(p) > maxRW {
   158  		p = p[:maxRW]
   159  	}
   160  	for {
   161  		n, err := ignoringEINTRIO(syscall.Read, fd.Sysfd, p)
   162  		if err != nil {
   163  			n = 0
   164  			if err == syscall.EAGAIN && fd.pd.pollable() {
   165  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   166  					continue
   167  				}
   168  			}
   169  		}
   170  		err = fd.eofError(n, err)
   171  		return n, err
   172  	}
   173  }
   174  
   175  // Pread wraps the pread system call.
   176  func (fd *FD) Pread(p []byte, off int64) (int, error) {
   177  	// Call incref, not readLock, because since pread specifies the
   178  	// offset it is independent from other reads.
   179  	// Similarly, using the poller doesn't make sense for pread.
   180  	if err := fd.incref(); err != nil {
   181  		return 0, err
   182  	}
   183  	if fd.IsStream && len(p) > maxRW {
   184  		p = p[:maxRW]
   185  	}
   186  	var (
   187  		n   int
   188  		err error
   189  	)
   190  	for {
   191  		n, err = syscall.Pread(fd.Sysfd, p, off)
   192  		if err != syscall.EINTR {
   193  			break
   194  		}
   195  	}
   196  	if err != nil {
   197  		n = 0
   198  	}
   199  	fd.decref()
   200  	err = fd.eofError(n, err)
   201  	return n, err
   202  }
   203  
   204  // ReadFrom wraps the recvfrom network call.
   205  func (fd *FD) ReadFrom(p []byte) (int, syscall.Sockaddr, error) {
   206  	if err := fd.readLock(); err != nil {
   207  		return 0, nil, err
   208  	}
   209  	defer fd.readUnlock()
   210  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   211  		return 0, nil, err
   212  	}
   213  	for {
   214  		n, sa, err := syscall.Recvfrom(fd.Sysfd, p, 0)
   215  		if err != nil {
   216  			if err == syscall.EINTR {
   217  				continue
   218  			}
   219  			n = 0
   220  			if err == syscall.EAGAIN && fd.pd.pollable() {
   221  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   222  					continue
   223  				}
   224  			}
   225  		}
   226  		err = fd.eofError(n, err)
   227  		return n, sa, err
   228  	}
   229  }
   230  
   231  // ReadFromInet4 wraps the recvfrom network call for IPv4.
   232  func (fd *FD) ReadFromInet4(p []byte, from *syscall.SockaddrInet4) (int, error) {
   233  	if err := fd.readLock(); err != nil {
   234  		return 0, err
   235  	}
   236  	defer fd.readUnlock()
   237  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   238  		return 0, err
   239  	}
   240  	for {
   241  		n, err := unix.RecvfromInet4(fd.Sysfd, p, 0, from)
   242  		if err != nil {
   243  			if err == syscall.EINTR {
   244  				continue
   245  			}
   246  			n = 0
   247  			if err == syscall.EAGAIN && fd.pd.pollable() {
   248  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   249  					continue
   250  				}
   251  			}
   252  		}
   253  		err = fd.eofError(n, err)
   254  		return n, err
   255  	}
   256  }
   257  
   258  // ReadFromInet6 wraps the recvfrom network call for IPv6.
   259  func (fd *FD) ReadFromInet6(p []byte, from *syscall.SockaddrInet6) (int, error) {
   260  	if err := fd.readLock(); err != nil {
   261  		return 0, err
   262  	}
   263  	defer fd.readUnlock()
   264  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   265  		return 0, err
   266  	}
   267  	for {
   268  		n, err := unix.RecvfromInet6(fd.Sysfd, p, 0, from)
   269  		if err != nil {
   270  			if err == syscall.EINTR {
   271  				continue
   272  			}
   273  			n = 0
   274  			if err == syscall.EAGAIN && fd.pd.pollable() {
   275  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   276  					continue
   277  				}
   278  			}
   279  		}
   280  		err = fd.eofError(n, err)
   281  		return n, err
   282  	}
   283  }
   284  
   285  // ReadMsg wraps the recvmsg network call.
   286  func (fd *FD) ReadMsg(p []byte, oob []byte, flags int) (int, int, int, syscall.Sockaddr, error) {
   287  	if err := fd.readLock(); err != nil {
   288  		return 0, 0, 0, nil, err
   289  	}
   290  	defer fd.readUnlock()
   291  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   292  		return 0, 0, 0, nil, err
   293  	}
   294  	for {
   295  		n, oobn, sysflags, sa, err := syscall.Recvmsg(fd.Sysfd, p, oob, flags)
   296  		if err != nil {
   297  			if err == syscall.EINTR {
   298  				continue
   299  			}
   300  			// TODO(dfc) should n and oobn be set to 0
   301  			if err == syscall.EAGAIN && fd.pd.pollable() {
   302  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   303  					continue
   304  				}
   305  			}
   306  		}
   307  		err = fd.eofError(n, err)
   308  		return n, oobn, sysflags, sa, err
   309  	}
   310  }
   311  
   312  // ReadMsgInet4 is ReadMsg, but specialized for syscall.SockaddrInet4.
   313  func (fd *FD) ReadMsgInet4(p []byte, oob []byte, flags int, sa4 *syscall.SockaddrInet4) (int, int, int, error) {
   314  	if err := fd.readLock(); err != nil {
   315  		return 0, 0, 0, err
   316  	}
   317  	defer fd.readUnlock()
   318  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   319  		return 0, 0, 0, err
   320  	}
   321  	for {
   322  		n, oobn, sysflags, err := unix.RecvmsgInet4(fd.Sysfd, p, oob, flags, sa4)
   323  		if err != nil {
   324  			if err == syscall.EINTR {
   325  				continue
   326  			}
   327  			// TODO(dfc) should n and oobn be set to 0
   328  			if err == syscall.EAGAIN && fd.pd.pollable() {
   329  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   330  					continue
   331  				}
   332  			}
   333  		}
   334  		err = fd.eofError(n, err)
   335  		return n, oobn, sysflags, err
   336  	}
   337  }
   338  
   339  // ReadMsgInet6 is ReadMsg, but specialized for syscall.SockaddrInet6.
   340  func (fd *FD) ReadMsgInet6(p []byte, oob []byte, flags int, sa6 *syscall.SockaddrInet6) (int, int, int, error) {
   341  	if err := fd.readLock(); err != nil {
   342  		return 0, 0, 0, err
   343  	}
   344  	defer fd.readUnlock()
   345  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   346  		return 0, 0, 0, err
   347  	}
   348  	for {
   349  		n, oobn, sysflags, err := unix.RecvmsgInet6(fd.Sysfd, p, oob, flags, sa6)
   350  		if err != nil {
   351  			if err == syscall.EINTR {
   352  				continue
   353  			}
   354  			// TODO(dfc) should n and oobn be set to 0
   355  			if err == syscall.EAGAIN && fd.pd.pollable() {
   356  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   357  					continue
   358  				}
   359  			}
   360  		}
   361  		err = fd.eofError(n, err)
   362  		return n, oobn, sysflags, err
   363  	}
   364  }
   365  
   366  // Write implements io.Writer.
   367  func (fd *FD) Write(p []byte) (int, error) {
   368  	if err := fd.writeLock(); err != nil {
   369  		return 0, err
   370  	}
   371  	defer fd.writeUnlock()
   372  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   373  		return 0, err
   374  	}
   375  	var nn int
   376  	for {
   377  		max := len(p)
   378  		if fd.IsStream && max-nn > maxRW {
   379  			max = nn + maxRW
   380  		}
   381  		n, err := ignoringEINTRIO(syscall.Write, fd.Sysfd, p[nn:max])
   382  		if n > 0 {
   383  			if n > max-nn {
   384  				// This can reportedly happen when using
   385  				// some VPN software. Issue #61060.
   386  				// If we don't check this we will panic
   387  				// with slice bounds out of range.
   388  				// Use a more informative panic.
   389  				panic("invalid return from write: got " + itoa.Itoa(n) + " from a write of " + itoa.Itoa(max-nn))
   390  			}
   391  			nn += n
   392  		}
   393  		if nn == len(p) {
   394  			return nn, err
   395  		}
   396  		if err == syscall.EAGAIN && fd.pd.pollable() {
   397  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
   398  				continue
   399  			}
   400  		}
   401  		if err != nil {
   402  			return nn, err
   403  		}
   404  		if n == 0 {
   405  			return nn, io.ErrUnexpectedEOF
   406  		}
   407  	}
   408  }
   409  
   410  // Pwrite wraps the pwrite system call.
   411  func (fd *FD) Pwrite(p []byte, off int64) (int, error) {
   412  	// Call incref, not writeLock, because since pwrite specifies the
   413  	// offset it is independent from other writes.
   414  	// Similarly, using the poller doesn't make sense for pwrite.
   415  	if err := fd.incref(); err != nil {
   416  		return 0, err
   417  	}
   418  	defer fd.decref()
   419  	var nn int
   420  	for {
   421  		max := len(p)
   422  		if fd.IsStream && max-nn > maxRW {
   423  			max = nn + maxRW
   424  		}
   425  		n, err := syscall.Pwrite(fd.Sysfd, p[nn:max], off+int64(nn))
   426  		if err == syscall.EINTR {
   427  			continue
   428  		}
   429  		if n > 0 {
   430  			nn += n
   431  		}
   432  		if nn == len(p) {
   433  			return nn, err
   434  		}
   435  		if err != nil {
   436  			return nn, err
   437  		}
   438  		if n == 0 {
   439  			return nn, io.ErrUnexpectedEOF
   440  		}
   441  	}
   442  }
   443  
   444  // WriteToInet4 wraps the sendto network call for IPv4 addresses.
   445  func (fd *FD) WriteToInet4(p []byte, sa *syscall.SockaddrInet4) (int, error) {
   446  	if err := fd.writeLock(); err != nil {
   447  		return 0, err
   448  	}
   449  	defer fd.writeUnlock()
   450  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   451  		return 0, err
   452  	}
   453  	for {
   454  		err := unix.SendtoInet4(fd.Sysfd, p, 0, sa)
   455  		if err == syscall.EINTR {
   456  			continue
   457  		}
   458  		if err == syscall.EAGAIN && fd.pd.pollable() {
   459  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
   460  				continue
   461  			}
   462  		}
   463  		if err != nil {
   464  			return 0, err
   465  		}
   466  		return len(p), nil
   467  	}
   468  }
   469  
   470  // WriteToInet6 wraps the sendto network call for IPv6 addresses.
   471  func (fd *FD) WriteToInet6(p []byte, sa *syscall.SockaddrInet6) (int, error) {
   472  	if err := fd.writeLock(); err != nil {
   473  		return 0, err
   474  	}
   475  	defer fd.writeUnlock()
   476  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   477  		return 0, err
   478  	}
   479  	for {
   480  		err := unix.SendtoInet6(fd.Sysfd, p, 0, sa)
   481  		if err == syscall.EINTR {
   482  			continue
   483  		}
   484  		if err == syscall.EAGAIN && fd.pd.pollable() {
   485  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
   486  				continue
   487  			}
   488  		}
   489  		if err != nil {
   490  			return 0, err
   491  		}
   492  		return len(p), nil
   493  	}
   494  }
   495  
   496  // WriteTo wraps the sendto network call.
   497  func (fd *FD) WriteTo(p []byte, sa syscall.Sockaddr) (int, error) {
   498  	if err := fd.writeLock(); err != nil {
   499  		return 0, err
   500  	}
   501  	defer fd.writeUnlock()
   502  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   503  		return 0, err
   504  	}
   505  	for {
   506  		err := syscall.Sendto(fd.Sysfd, p, 0, sa)
   507  		if err == syscall.EINTR {
   508  			continue
   509  		}
   510  		if err == syscall.EAGAIN && fd.pd.pollable() {
   511  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
   512  				continue
   513  			}
   514  		}
   515  		if err != nil {
   516  			return 0, err
   517  		}
   518  		return len(p), nil
   519  	}
   520  }
   521  
   522  // WriteMsg wraps the sendmsg network call.
   523  func (fd *FD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (int, int, error) {
   524  	if err := fd.writeLock(); err != nil {
   525  		return 0, 0, err
   526  	}
   527  	defer fd.writeUnlock()
   528  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   529  		return 0, 0, err
   530  	}
   531  	for {
   532  		n, err := syscall.SendmsgN(fd.Sysfd, p, oob, sa, 0)
   533  		if err == syscall.EINTR {
   534  			continue
   535  		}
   536  		if err == syscall.EAGAIN && fd.pd.pollable() {
   537  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
   538  				continue
   539  			}
   540  		}
   541  		if err != nil {
   542  			return n, 0, err
   543  		}
   544  		return n, len(oob), err
   545  	}
   546  }
   547  
   548  // WriteMsgInet4 is WriteMsg specialized for syscall.SockaddrInet4.
   549  func (fd *FD) WriteMsgInet4(p []byte, oob []byte, sa *syscall.SockaddrInet4) (int, int, error) {
   550  	if err := fd.writeLock(); err != nil {
   551  		return 0, 0, err
   552  	}
   553  	defer fd.writeUnlock()
   554  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   555  		return 0, 0, err
   556  	}
   557  	for {
   558  		n, err := unix.SendmsgNInet4(fd.Sysfd, p, oob, sa, 0)
   559  		if err == syscall.EINTR {
   560  			continue
   561  		}
   562  		if err == syscall.EAGAIN && fd.pd.pollable() {
   563  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
   564  				continue
   565  			}
   566  		}
   567  		if err != nil {
   568  			return n, 0, err
   569  		}
   570  		return n, len(oob), err
   571  	}
   572  }
   573  
   574  // WriteMsgInet6 is WriteMsg specialized for syscall.SockaddrInet6.
   575  func (fd *FD) WriteMsgInet6(p []byte, oob []byte, sa *syscall.SockaddrInet6) (int, int, error) {
   576  	if err := fd.writeLock(); err != nil {
   577  		return 0, 0, err
   578  	}
   579  	defer fd.writeUnlock()
   580  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   581  		return 0, 0, err
   582  	}
   583  	for {
   584  		n, err := unix.SendmsgNInet6(fd.Sysfd, p, oob, sa, 0)
   585  		if err == syscall.EINTR {
   586  			continue
   587  		}
   588  		if err == syscall.EAGAIN && fd.pd.pollable() {
   589  			if err = fd.pd.waitWrite(fd.isFile); err == nil {
   590  				continue
   591  			}
   592  		}
   593  		if err != nil {
   594  			return n, 0, err
   595  		}
   596  		return n, len(oob), err
   597  	}
   598  }
   599  
   600  // Accept wraps the accept network call.
   601  func (fd *FD) Accept() (int, syscall.Sockaddr, string, error) {
   602  	if err := fd.readLock(); err != nil {
   603  		return -1, nil, "", err
   604  	}
   605  	defer fd.readUnlock()
   606  
   607  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   608  		return -1, nil, "", err
   609  	}
   610  	for {
   611  		s, rsa, errcall, err := accept(fd.Sysfd)
   612  		if err == nil {
   613  			return s, rsa, "", err
   614  		}
   615  		switch err {
   616  		case syscall.EINTR:
   617  			continue
   618  		case syscall.EAGAIN:
   619  			if fd.pd.pollable() {
   620  				if err = fd.pd.waitRead(fd.isFile); err == nil {
   621  					continue
   622  				}
   623  			}
   624  		case syscall.ECONNABORTED:
   625  			// This means that a socket on the listen
   626  			// queue was closed before we Accept()ed it;
   627  			// it's a silly error, so try again.
   628  			continue
   629  		}
   630  		return -1, nil, errcall, err
   631  	}
   632  }
   633  
   634  // Fchmod wraps syscall.Fchmod.
   635  func (fd *FD) Fchmod(mode uint32) error {
   636  	if err := fd.incref(); err != nil {
   637  		return err
   638  	}
   639  	defer fd.decref()
   640  	return ignoringEINTR(func() error {
   641  		return syscall.Fchmod(fd.Sysfd, mode)
   642  	})
   643  }
   644  
   645  // Fstat wraps syscall.Fstat
   646  func (fd *FD) Fstat(s *syscall.Stat_t) error {
   647  	if err := fd.incref(); err != nil {
   648  		return err
   649  	}
   650  	defer fd.decref()
   651  	return ignoringEINTR(func() error {
   652  		return syscall.Fstat(fd.Sysfd, s)
   653  	})
   654  }
   655  
   656  // dupCloexecUnsupported indicates whether F_DUPFD_CLOEXEC is supported by the kernel.
   657  var dupCloexecUnsupported atomic.Bool
   658  
   659  // DupCloseOnExec dups fd and marks it close-on-exec.
   660  func DupCloseOnExec(fd int) (int, string, error) {
   661  	if syscall.F_DUPFD_CLOEXEC != 0 && !dupCloexecUnsupported.Load() {
   662  		r0, err := unix.Fcntl(fd, syscall.F_DUPFD_CLOEXEC, 0)
   663  		if err == nil {
   664  			return r0, "", nil
   665  		}
   666  		switch err {
   667  		case syscall.EINVAL, syscall.ENOSYS:
   668  			// Old kernel, or js/wasm (which returns
   669  			// ENOSYS). Fall back to the portable way from
   670  			// now on.
   671  			dupCloexecUnsupported.Store(true)
   672  		default:
   673  			return -1, "fcntl", err
   674  		}
   675  	}
   676  	return dupCloseOnExecOld(fd)
   677  }
   678  
   679  // Dup duplicates the file descriptor.
   680  func (fd *FD) Dup() (int, string, error) {
   681  	if err := fd.incref(); err != nil {
   682  		return -1, "", err
   683  	}
   684  	defer fd.decref()
   685  	return DupCloseOnExec(fd.Sysfd)
   686  }
   687  
   688  // On Unix variants only, expose the IO event for the net code.
   689  
   690  // WaitWrite waits until data can be written to fd.
   691  func (fd *FD) WaitWrite() error {
   692  	return fd.pd.waitWrite(fd.isFile)
   693  }
   694  
   695  // WriteOnce is for testing only. It makes a single write call.
   696  func (fd *FD) WriteOnce(p []byte) (int, error) {
   697  	if err := fd.writeLock(); err != nil {
   698  		return 0, err
   699  	}
   700  	defer fd.writeUnlock()
   701  	return ignoringEINTRIO(syscall.Write, fd.Sysfd, p)
   702  }
   703  
   704  // RawRead invokes the user-defined function f for a read operation.
   705  func (fd *FD) RawRead(f func(uintptr) bool) error {
   706  	if err := fd.readLock(); err != nil {
   707  		return err
   708  	}
   709  	defer fd.readUnlock()
   710  	if err := fd.pd.prepareRead(fd.isFile); err != nil {
   711  		return err
   712  	}
   713  	for {
   714  		if f(uintptr(fd.Sysfd)) {
   715  			return nil
   716  		}
   717  		if err := fd.pd.waitRead(fd.isFile); err != nil {
   718  			return err
   719  		}
   720  	}
   721  }
   722  
   723  // RawWrite invokes the user-defined function f for a write operation.
   724  func (fd *FD) RawWrite(f func(uintptr) bool) error {
   725  	if err := fd.writeLock(); err != nil {
   726  		return err
   727  	}
   728  	defer fd.writeUnlock()
   729  	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
   730  		return err
   731  	}
   732  	for {
   733  		if f(uintptr(fd.Sysfd)) {
   734  			return nil
   735  		}
   736  		if err := fd.pd.waitWrite(fd.isFile); err != nil {
   737  			return err
   738  		}
   739  	}
   740  }
   741  
   742  // ignoringEINTRIO is like ignoringEINTR, but just for IO calls.
   743  func ignoringEINTRIO(fn func(fd int, p []byte) (int, error), fd int, p []byte) (int, error) {
   744  	for {
   745  		n, err := fn(fd, p)
   746  		if err != syscall.EINTR {
   747  			return n, err
   748  		}
   749  	}
   750  }
   751  

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