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>

src/runtime/os_nacl.go

@@ -289,6 +289,9 @@ func closeonexec(int32)                                   {}
 // gsignalStack is unused on nacl.
 type gsignalStack struct{}
 
+// nacl fake time support - time in nanoseconds since 1970
+var faketime int64
+
 var writelock uint32 // test-and-set spin lock for write
 
 // lastfaketime stores the last faketime value written to fd 1 or 2.

src/runtime/testdata/testfaketime/faketime.go

+// 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.
+
+// Test faketime support. This is its own test program because we have
+// to build it with custom build tags and hence want to minimize
+// dependencies.
+
+package main
+
+import (
+	"os"
+	"time"
+)
+
+func main() {
+	println("line 1")
+	// Stream switch, increments time
+	os.Stdout.WriteString("line 2\n")
+	os.Stdout.WriteString("line 3\n")
+	// Stream switch, increments time
+	os.Stderr.WriteString("line 4\n")
+	// Time jump
+	time.Sleep(1 * time.Second)
+	os.Stdout.WriteString("line 5\n")
+	// Print the current time.
+	os.Stdout.WriteString(time.Now().UTC().Format(time.RFC3339))
+}

src/runtime/time.go

@@ -71,9 +71,6 @@ type timersBucket struct {
 	t            []*timer
 }
 
-// nacl fake time support - time in nanoseconds since 1970
-var faketime int64
-
 // Package time APIs.
 // Godoc uses the comments in package time, not these.
 

src/runtime/time_fake.go

+// 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
+}

src/runtime/time_nofake.go

@@ -2,12 +2,19 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !faketime
 // +build !nacl
 
 package runtime
 
 import "unsafe"
 
+// faketime is the simulated time in nanoseconds since 1970 for the
+// playground.
+//
+// Zero means not to use faketime.
+var faketime int64
+
 //go:nosplit
 func nanotime() int64 {
 	return nanotime1()

src/runtime/time_test.go

+// 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.
+
+package runtime_test
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"internal/testenv"
+	"os/exec"
+	"reflect"
+	"runtime"
+	"testing"
+)
+
+func TestFakeTime(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("faketime not supported on windows")
+	}
+
+	t.Parallel()
+
+	exe, err := buildTestProg(t, "testfaketime", "-tags=faketime")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var stdout, stderr bytes.Buffer
+	cmd := exec.Command(exe)
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	err = testenv.CleanCmdEnv(cmd).Run()
+	if err != nil {
+		t.Fatalf("exit status: %v\n%s", err, stderr.String())
+	}
+
+	t.Logf("raw stdout: %q", stdout.String())
+	t.Logf("raw stderr: %q", stdout.String())
+
+	f1, err1 := parseFakeTime(stdout.Bytes())
+	if err1 != nil {
+		t.Fatal(err1)
+	}
+	f2, err2 := parseFakeTime(stderr.Bytes())
+	if err2 != nil {
+		t.Fatal(err2)
+	}
+
+	const time0 = 1257894000000000000
+	got := [][]fakeTimeFrame{f1, f2}
+	var want = [][]fakeTimeFrame{{
+		{time0 + 1, "line 2\n"},
+		{time0 + 1, "line 3\n"},
+		{time0 + 1e9, "line 5\n"},
+		{time0 + 1e9, "2009-11-10T23:00:01Z"},
+	}, {
+		{time0, "line 1\n"},
+		{time0 + 2, "line 4\n"},
+	}}
+	if !reflect.DeepEqual(want, got) {
+		t.Fatalf("want %v, got %v", want, got)
+	}
+}
+
+type fakeTimeFrame struct {
+	time uint64
+	data string
+}
+
+func parseFakeTime(x []byte) ([]fakeTimeFrame, error) {
+	var frames []fakeTimeFrame
+	for len(x) != 0 {
+		if len(x) < 4+8+4 {
+			return nil, errors.New("truncated header")
+		}
+		const magic = "\x00\x00PB"
+		if string(x[:len(magic)]) != magic {
+			return nil, errors.New("bad magic")
+		}
+		x = x[len(magic):]
+		time := binary.BigEndian.Uint64(x)
+		x = x[8:]
+		dlen := binary.BigEndian.Uint32(x)
+		x = x[4:]
+		data := string(x[:dlen])
+		x = x[dlen:]
+		frames = append(frames, fakeTimeFrame{time, data})
+	}
+	return frames, nil
+}