runtime: platform-independent faketime support

This adds a platform-independent implementation of nacl's faketime
support. It can be enabled by setting the faketime build tag.

Updates #30439.

Change-Id: Iee097004d56d796e6d2bfdd303a092c067ade87e
Reviewed-on: https://go-review.googlesource.com/c/go/+/192740
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 101 insertions, 0 deletions

diff --git a/src/runtime/time_fake.go b/src/runtime/time_fake.go
new file mode 100644
index 0000000000..bef3a65873
--- /dev/null
+++ b/src/runtime/time_fake.go
@@ -0,0 +1,101 @@
+// Copyright 2019 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.
+
+// +build faketime
+// +build !nacl
+// +build !windows
+
+// Faketime isn't currently supported on Windows. This would require:
+//
+// 1. Shadowing time_now, which is implemented in assembly on Windows.
+//    Since that's exported directly to the time package from runtime
+//    assembly, this would involve moving it from sys_windows_*.s into
+//    its own assembly files build-tagged with !faketime and using the
+//    implementation of time_now from timestub.go in faketime mode.
+//
+// 2. Modifying syscall.Write to call syscall.faketimeWrite,
+//    translating the Stdout and Stderr handles into FDs 1 and 2.
+//    (See CL 192739 PS 3.)
+
+package runtime
+
+import "unsafe"
+
+// faketime is the simulated time in nanoseconds since 1970 for the
+// playground.
+var faketime int64 = 1257894000000000000
+
+var faketimeState struct {
+	lock mutex
+
+	// lastfaketime is the last faketime value written to fd 1 or 2.
+	lastfaketime int64
+
+	// lastfd is the fd to which lastfaketime was written.
+	//
+	// Subsequent writes to the same fd may use the same
+	// timestamp, but the timestamp must increase if the fd
+	// changes.
+	lastfd uintptr
+}
+
+//go:nosplit
+func nanotime() int64 {
+	return faketime
+}
+
+func walltime() (sec int64, nsec int32) {
+	return faketime / 1000000000, int32(faketime % 1000000000)
+}
+
+func write(fd uintptr, p unsafe.Pointer, n int32) int32 {
+	if !(fd == 1 || fd == 2) {
+		// Do an ordinary write.
+		return write1(fd, p, n)
+	}
+
+	// Write with the playback header.
+
+	// First, lock to avoid interleaving writes.
+	lock(&faketimeState.lock)
+
+	// If the current fd doesn't match the fd of the previous write,
+	// ensure that the timestamp is strictly greater. That way, we can
+	// recover the original order even if we read the fds separately.
+	t := faketimeState.lastfaketime
+	if fd != faketimeState.lastfd {
+		t++
+		faketimeState.lastfd = fd
+	}
+	if faketime > t {
+		t = faketime
+	}
+	faketimeState.lastfaketime = t
+
+	// Playback header: 0 0 P B <8-byte time> <4-byte data length> (big endian)
+	var buf [4 + 8 + 4]byte
+	buf[2] = 'P'
+	buf[3] = 'B'
+	tu := uint64(t)
+	buf[4] = byte(tu >> (7 * 8))
+	buf[5] = byte(tu >> (6 * 8))
+	buf[6] = byte(tu >> (5 * 8))
+	buf[7] = byte(tu >> (4 * 8))
+	buf[8] = byte(tu >> (3 * 8))
+	buf[9] = byte(tu >> (2 * 8))
+	buf[10] = byte(tu >> (1 * 8))
+	buf[11] = byte(tu >> (0 * 8))
+	nu := uint32(n)
+	buf[12] = byte(nu >> (3 * 8))
+	buf[13] = byte(nu >> (2 * 8))
+	buf[14] = byte(nu >> (1 * 8))
+	buf[15] = byte(nu >> (0 * 8))
+	write1(fd, unsafe.Pointer(&buf[0]), int32(len(buf)))
+
+	// Write actual data.
+	res := write1(fd, p, n)
+
+	unlock(&faketimeState.lock)
+	return res
+}