time_fake.go

     1  // Copyright 2019 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 faketime && !windows
     6  
     7  // Faketime isn't currently supported on Windows. This would require
     8  // modifying syscall.Write to call syscall.faketimeWrite,
     9  // translating the Stdout and Stderr handles into FDs 1 and 2.
    10  // (See CL 192739 PS 3.)
    11  
    12  package runtime
    13  
    14  import "unsafe"
    15  
    16  // faketime is the simulated time in nanoseconds since 1970 for the
    17  // playground.
    18  var faketime int64 = 1257894000000000000
    19  
    20  var faketimeState struct {
    21  	lock mutex
    22  
    23  	// lastfaketime is the last faketime value written to fd 1 or 2.
    24  	lastfaketime int64
    25  
    26  	// lastfd is the fd to which lastfaketime was written.
    27  	//
    28  	// Subsequent writes to the same fd may use the same
    29  	// timestamp, but the timestamp must increase if the fd
    30  	// changes.
    31  	lastfd uintptr
    32  }
    33  
    34  //go:linkname nanotime
    35  //go:nosplit
    36  func nanotime() int64 {
    37  	return faketime
    38  }
    39  
    40  //go:linkname time_now time.now
    41  func time_now() (sec int64, nsec int32, mono int64) {
    42  	return faketime / 1e9, int32(faketime % 1e9), faketime
    43  }
    44  
    45  // write is like the Unix write system call.
    46  // We have to avoid write barriers to avoid potential deadlock
    47  // on write calls.
    48  //
    49  //go:nowritebarrierrec
    50  func write(fd uintptr, p unsafe.Pointer, n int32) int32 {
    51  	if !(fd == 1 || fd == 2) {
    52  		// Do an ordinary write.
    53  		return write1(fd, p, n)
    54  	}
    55  
    56  	// Write with the playback header.
    57  
    58  	// First, lock to avoid interleaving writes.
    59  	lock(&faketimeState.lock)
    60  
    61  	// If the current fd doesn't match the fd of the previous write,
    62  	// ensure that the timestamp is strictly greater. That way, we can
    63  	// recover the original order even if we read the fds separately.
    64  	t := faketimeState.lastfaketime
    65  	if fd != faketimeState.lastfd {
    66  		t++
    67  		faketimeState.lastfd = fd
    68  	}
    69  	if faketime > t {
    70  		t = faketime
    71  	}
    72  	faketimeState.lastfaketime = t
    73  
    74  	// Playback header: 0 0 P B <8-byte time> <4-byte data length> (big endian)
    75  	var buf [4 + 8 + 4]byte
    76  	buf[2] = 'P'
    77  	buf[3] = 'B'
    78  	tu := uint64(t)
    79  	buf[4] = byte(tu >> (7 * 8))
    80  	buf[5] = byte(tu >> (6 * 8))
    81  	buf[6] = byte(tu >> (5 * 8))
    82  	buf[7] = byte(tu >> (4 * 8))
    83  	buf[8] = byte(tu >> (3 * 8))
    84  	buf[9] = byte(tu >> (2 * 8))
    85  	buf[10] = byte(tu >> (1 * 8))
    86  	buf[11] = byte(tu >> (0 * 8))
    87  	nu := uint32(n)
    88  	buf[12] = byte(nu >> (3 * 8))
    89  	buf[13] = byte(nu >> (2 * 8))
    90  	buf[14] = byte(nu >> (1 * 8))
    91  	buf[15] = byte(nu >> (0 * 8))
    92  	write1(fd, unsafe.Pointer(&buf[0]), int32(len(buf)))
    93  
    94  	// Write actual data.
    95  	res := write1(fd, p, n)
    96  
    97  	unlock(&faketimeState.lock)
    98  	return res
    99  }
   100
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