9 files changed, 725 insertions, 0 deletions
diff --git a/src/runtime/debug/debug.s b/src/runtime/debug/debug.s
new file mode 100644
index 0000000..6aae33a
--- /dev/null
+++ b/src/runtime/debug/debug.s
@@ -0,0 +1,9 @@
+// Copyright 2013 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.
+
+// Nothing to see here.
+// This file exists so that the go command knows that parts of the
+// package are implemented in C, so that it does not instruct the
+// Go compiler to complain about extern declarations.
+// The actual implementations are in package runtime.
diff --git a/src/runtime/debug/garbage.go b/src/runtime/debug/garbage.go
new file mode 100644
index 0000000..00f92c3
--- /dev/null
+++ b/src/runtime/debug/garbage.go
@@ -0,0 +1,175 @@
+// Copyright 2013 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 debug
+
+import (
+	"runtime"
+	"sort"
+	"time"
+)
+
+// GCStats collect information about recent garbage collections.
+type GCStats struct {
+	LastGC         time.Time       // time of last collection
+	NumGC          int64           // number of garbage collections
+	PauseTotal     time.Duration   // total pause for all collections
+	Pause          []time.Duration // pause history, most recent first
+	PauseEnd       []time.Time     // pause end times history, most recent first
+	PauseQuantiles []time.Duration
+}
+
+// ReadGCStats reads statistics about garbage collection into stats.
+// The number of entries in the pause history is system-dependent;
+// stats.Pause slice will be reused if large enough, reallocated otherwise.
+// ReadGCStats may use the full capacity of the stats.Pause slice.
+// If stats.PauseQuantiles is non-empty, ReadGCStats fills it with quantiles
+// summarizing the distribution of pause time. For example, if
+// len(stats.PauseQuantiles) is 5, it will be filled with the minimum,
+// 25%, 50%, 75%, and maximum pause times.
+func ReadGCStats(stats *GCStats) {
+	// Create a buffer with space for at least two copies of the
+	// pause history tracked by the runtime. One will be returned
+	// to the caller and the other will be used as transfer buffer
+	// for end times history and as a temporary buffer for
+	// computing quantiles.
+	const maxPause = len(((*runtime.MemStats)(nil)).PauseNs)
+	if cap(stats.Pause) < 2*maxPause+3 {
+		stats.Pause = make([]time.Duration, 2*maxPause+3)
+	}
+
+	// readGCStats fills in the pause and end times histories (up to
+	// maxPause entries) and then three more: Unix ns time of last GC,
+	// number of GC, and total pause time in nanoseconds. Here we
+	// depend on the fact that time.Duration's native unit is
+	// nanoseconds, so the pauses and the total pause time do not need
+	// any conversion.
+	readGCStats(&stats.Pause)
+	n := len(stats.Pause) - 3
+	stats.LastGC = time.Unix(0, int64(stats.Pause[n]))
+	stats.NumGC = int64(stats.Pause[n+1])
+	stats.PauseTotal = stats.Pause[n+2]
+	n /= 2 // buffer holds pauses and end times
+	stats.Pause = stats.Pause[:n]
+
+	if cap(stats.PauseEnd) < maxPause {
+		stats.PauseEnd = make([]time.Time, 0, maxPause)
+	}
+	stats.PauseEnd = stats.PauseEnd[:0]
+	for _, ns := range stats.Pause[n : n+n] {
+		stats.PauseEnd = append(stats.PauseEnd, time.Unix(0, int64(ns)))
+	}
+
+	if len(stats.PauseQuantiles) > 0 {
+		if n == 0 {
+			for i := range stats.PauseQuantiles {
+				stats.PauseQuantiles[i] = 0
+			}
+		} else {
+			// There's room for a second copy of the data in stats.Pause.
+			// See the allocation at the top of the function.
+			sorted := stats.Pause[n : n+n]
+			copy(sorted, stats.Pause)
+			sort.Slice(sorted, func(i, j int) bool { return sorted[i] < sorted[j] })
+			nq := len(stats.PauseQuantiles) - 1
+			for i := 0; i < nq; i++ {
+				stats.PauseQuantiles[i] = sorted[len(sorted)*i/nq]
+			}
+			stats.PauseQuantiles[nq] = sorted[len(sorted)-1]
+		}
+	}
+}
+
+// SetGCPercent sets the garbage collection target percentage:
+// a collection is triggered when the ratio of freshly allocated data
+// to live data remaining after the previous collection reaches this percentage.
+// SetGCPercent returns the previous setting.
+// The initial setting is the value of the GOGC environment variable
+// at startup, or 100 if the variable is not set.
+// A negative percentage disables garbage collection.
+func SetGCPercent(percent int) int {
+	return int(setGCPercent(int32(percent)))
+}
+
+// FreeOSMemory forces a garbage collection followed by an
+// attempt to return as much memory to the operating system
+// as possible. (Even if this is not called, the runtime gradually
+// returns memory to the operating system in a background task.)
+func FreeOSMemory() {
+	freeOSMemory()
+}
+
+// SetMaxStack sets the maximum amount of memory that
+// can be used by a single goroutine stack.
+// If any goroutine exceeds this limit while growing its stack,
+// the program crashes.
+// SetMaxStack returns the previous setting.
+// The initial setting is 1 GB on 64-bit systems, 250 MB on 32-bit systems.
+// There may be a system-imposed maximum stack limit regardless
+// of the value provided to SetMaxStack.
+//
+// SetMaxStack is useful mainly for limiting the damage done by
+// goroutines that enter an infinite recursion. It only limits future
+// stack growth.
+func SetMaxStack(bytes int) int {
+	return setMaxStack(bytes)
+}
+
+// SetMaxThreads sets the maximum number of operating system
+// threads that the Go program can use. If it attempts to use more than
+// this many, the program crashes.
+// SetMaxThreads returns the previous setting.
+// The initial setting is 10,000 threads.
+//
+// The limit controls the number of operating system threads, not the number
+// of goroutines. A Go program creates a new thread only when a goroutine
+// is ready to run but all the existing threads are blocked in system calls, cgo calls,
+// or are locked to other goroutines due to use of runtime.LockOSThread.
+//
+// SetMaxThreads is useful mainly for limiting the damage done by
+// programs that create an unbounded number of threads. The idea is
+// to take down the program before it takes down the operating system.
+func SetMaxThreads(threads int) int {
+	return setMaxThreads(threads)
+}
+
+// SetPanicOnFault controls the runtime's behavior when a program faults
+// at an unexpected (non-nil) address. Such faults are typically caused by
+// bugs such as runtime memory corruption, so the default response is to crash
+// the program. Programs working with memory-mapped files or unsafe
+// manipulation of memory may cause faults at non-nil addresses in less
+// dramatic situations; SetPanicOnFault allows such programs to request
+// that the runtime trigger only a panic, not a crash.
+// The runtime.Error that the runtime panics with may have an additional method:
+//     Addr() uintptr
+// If that method exists, it returns the memory address which triggered the fault.
+// The results of Addr are best-effort and the veracity of the result
+// may depend on the platform.
+// SetPanicOnFault applies only to the current goroutine.
+// It returns the previous setting.
+func SetPanicOnFault(enabled bool) bool {
+	return setPanicOnFault(enabled)
+}
+
+// WriteHeapDump writes a description of the heap and the objects in
+// it to the given file descriptor.
+//
+// WriteHeapDump suspends the execution of all goroutines until the heap
+// dump is completely written.  Thus, the file descriptor must not be
+// connected to a pipe or socket whose other end is in the same Go
+// process; instead, use a temporary file or network socket.
+//
+// The heap dump format is defined at https://golang.org/s/go15heapdump.
+func WriteHeapDump(fd uintptr)
+
+// SetTraceback sets the amount of detail printed by the runtime in
+// the traceback it prints before exiting due to an unrecovered panic
+// or an internal runtime error.
+// The level argument takes the same values as the GOTRACEBACK
+// environment variable. For example, SetTraceback("all") ensure
+// that the program prints all goroutines when it crashes.
+// See the package runtime documentation for details.
+// If SetTraceback is called with a level lower than that of the
+// environment variable, the call is ignored.
+func SetTraceback(level string)
diff --git a/src/runtime/debug/garbage_test.go b/src/runtime/debug/garbage_test.go
new file mode 100644
index 0000000..69e769e
--- /dev/null
+++ b/src/runtime/debug/garbage_test.go
@@ -0,0 +1,193 @@
+// Copyright 2013 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 debug_test
+
+import (
+	"internal/testenv"
+	"runtime"
+	. "runtime/debug"
+	"testing"
+	"time"
+)
+
+func TestReadGCStats(t *testing.T) {
+	defer SetGCPercent(SetGCPercent(-1))
+
+	var stats GCStats
+	var mstats runtime.MemStats
+	var min, max time.Duration
+
+	// First ReadGCStats will allocate, second should not,
+	// especially if we follow up with an explicit garbage collection.
+	stats.PauseQuantiles = make([]time.Duration, 10)
+	ReadGCStats(&stats)
+	runtime.GC()
+
+	// Assume these will return same data: no GC during ReadGCStats.
+	ReadGCStats(&stats)
+	runtime.ReadMemStats(&mstats)
+
+	if stats.NumGC != int64(mstats.NumGC) {
+		t.Errorf("stats.NumGC = %d, but mstats.NumGC = %d", stats.NumGC, mstats.NumGC)
+	}
+	if stats.PauseTotal != time.Duration(mstats.PauseTotalNs) {
+		t.Errorf("stats.PauseTotal = %d, but mstats.PauseTotalNs = %d", stats.PauseTotal, mstats.PauseTotalNs)
+	}
+	if stats.LastGC.UnixNano() != int64(mstats.LastGC) {
+		t.Errorf("stats.LastGC.UnixNano = %d, but mstats.LastGC = %d", stats.LastGC.UnixNano(), mstats.LastGC)
+	}
+	n := int(mstats.NumGC)
+	if n > len(mstats.PauseNs) {
+		n = len(mstats.PauseNs)
+	}
+	if len(stats.Pause) != n {
+		t.Errorf("len(stats.Pause) = %d, want %d", len(stats.Pause), n)
+	} else {
+		off := (int(mstats.NumGC) + len(mstats.PauseNs) - 1) % len(mstats.PauseNs)
+		for i := 0; i < n; i++ {
+			dt := stats.Pause[i]
+			if dt != time.Duration(mstats.PauseNs[off]) {
+				t.Errorf("stats.Pause[%d] = %d, want %d", i, dt, mstats.PauseNs[off])
+			}
+			if max < dt {
+				max = dt
+			}
+			if min > dt || i == 0 {
+				min = dt
+			}
+			off = (off + len(mstats.PauseNs) - 1) % len(mstats.PauseNs)
+		}
+	}
+
+	q := stats.PauseQuantiles
+	nq := len(q)
+	if q[0] != min || q[nq-1] != max {
+		t.Errorf("stats.PauseQuantiles = [%d, ..., %d], want [%d, ..., %d]", q[0], q[nq-1], min, max)
+	}
+
+	for i := 0; i < nq-1; i++ {
+		if q[i] > q[i+1] {
+			t.Errorf("stats.PauseQuantiles[%d]=%d > stats.PauseQuantiles[%d]=%d", i, q[i], i+1, q[i+1])
+		}
+	}
+
+	// compare memory stats with gc stats:
+	if len(stats.PauseEnd) != n {
+		t.Fatalf("len(stats.PauseEnd) = %d, want %d", len(stats.PauseEnd), n)
+	}
+	off := (int(mstats.NumGC) + len(mstats.PauseEnd) - 1) % len(mstats.PauseEnd)
+	for i := 0; i < n; i++ {
+		dt := stats.PauseEnd[i]
+		if dt.UnixNano() != int64(mstats.PauseEnd[off]) {
+			t.Errorf("stats.PauseEnd[%d] = %d, want %d", i, dt.UnixNano(), mstats.PauseEnd[off])
+		}
+		off = (off + len(mstats.PauseEnd) - 1) % len(mstats.PauseEnd)
+	}
+}
+
+var big = make([]byte, 1<<20)
+
+func TestFreeOSMemory(t *testing.T) {
+	var ms1, ms2 runtime.MemStats
+
+	if big == nil {
+		t.Skip("test is not reliable when run multiple times")
+	}
+	big = nil
+	runtime.GC()
+	runtime.ReadMemStats(&ms1)
+	FreeOSMemory()
+	runtime.ReadMemStats(&ms2)
+	if ms1.HeapReleased >= ms2.HeapReleased {
+		t.Errorf("released before=%d; released after=%d; did not go up", ms1.HeapReleased, ms2.HeapReleased)
+	}
+}
+
+var (
+	setGCPercentBallast interface{}
+	setGCPercentSink    interface{}
+)
+
+func TestSetGCPercent(t *testing.T) {
+	testenv.SkipFlaky(t, 20076)
+
+	// Test that the variable is being set and returned correctly.
+	old := SetGCPercent(123)
+	new := SetGCPercent(old)
+	if new != 123 {
+		t.Errorf("SetGCPercent(123); SetGCPercent(x) = %d, want 123", new)
+	}
+
+	// Test that the percentage is implemented correctly.
+	defer func() {
+		SetGCPercent(old)
+		setGCPercentBallast, setGCPercentSink = nil, nil
+	}()
+	SetGCPercent(100)
+	runtime.GC()
+	// Create 100 MB of live heap as a baseline.
+	const baseline = 100 << 20
+	var ms runtime.MemStats
+	runtime.ReadMemStats(&ms)
+	setGCPercentBallast = make([]byte, baseline-ms.Alloc)
+	runtime.GC()
+	runtime.ReadMemStats(&ms)
+	if abs64(baseline-int64(ms.Alloc)) > 10<<20 {
+		t.Fatalf("failed to set up baseline live heap; got %d MB, want %d MB", ms.Alloc>>20, baseline>>20)
+	}
+	// NextGC should be ~200 MB.
+	const thresh = 20 << 20 // TODO: Figure out why this is so noisy on some builders
+	if want := int64(2 * baseline); abs64(want-int64(ms.NextGC)) > thresh {
+		t.Errorf("NextGC = %d MB, want %d±%d MB", ms.NextGC>>20, want>>20, thresh>>20)
+	}
+	// Create some garbage, but not enough to trigger another GC.
+	for i := 0; i < int(1.2*baseline); i += 1 << 10 {
+		setGCPercentSink = make([]byte, 1<<10)
+	}
+	setGCPercentSink = nil
+	// Adjust GOGC to 50. NextGC should be ~150 MB.
+	SetGCPercent(50)
+	runtime.ReadMemStats(&ms)
+	if want := int64(1.5 * baseline); abs64(want-int64(ms.NextGC)) > thresh {
+		t.Errorf("NextGC = %d MB, want %d±%d MB", ms.NextGC>>20, want>>20, thresh>>20)
+	}
+
+	// Trigger a GC and get back to 100 MB live with GOGC=100.
+	SetGCPercent(100)
+	runtime.GC()
+	// Raise live to 120 MB.
+	setGCPercentSink = make([]byte, int(0.2*baseline))
+	// Lower GOGC to 10. This must force a GC.
+	runtime.ReadMemStats(&ms)
+	ngc1 := ms.NumGC
+	SetGCPercent(10)
+	// It may require an allocation to actually force the GC.
+	setGCPercentSink = make([]byte, 1<<20)
+	runtime.ReadMemStats(&ms)
+	ngc2 := ms.NumGC
+	if ngc1 == ngc2 {
+		t.Errorf("expected GC to run but it did not")
+	}
+}
+
+func abs64(a int64) int64 {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+func TestSetMaxThreadsOvf(t *testing.T) {
+	// Verify that a big threads count will not overflow the int32
+	// maxmcount variable, causing a panic (see Issue 16076).
+	//
+	// This can only happen when ints are 64 bits, since on platforms
+	// with 32 bit ints SetMaxThreads (which takes an int parameter)
+	// cannot be given anything that will overflow an int32.
+	//
+	// Call SetMaxThreads with 1<<31, but only on 64 bit systems.
+	nt := SetMaxThreads(1 << (30 + ^uint(0)>>63))
+	SetMaxThreads(nt) // restore previous value
+}
diff --git a/src/runtime/debug/heapdump_test.go b/src/runtime/debug/heapdump_test.go
new file mode 100644
index 0000000..768934d
--- /dev/null
+++ b/src/runtime/debug/heapdump_test.go
@@ -0,0 +1,69 @@
+// Copyright 2014 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 debug_test
+
+import (
+	"os"
+	"runtime"
+	. "runtime/debug"
+	"testing"
+)
+
+func TestWriteHeapDumpNonempty(t *testing.T) {
+	if runtime.GOOS == "js" {
+		t.Skipf("WriteHeapDump is not available on %s.", runtime.GOOS)
+	}
+	f, err := os.CreateTemp("", "heapdumptest")
+	if err != nil {
+		t.Fatalf("TempFile failed: %v", err)
+	}
+	defer os.Remove(f.Name())
+	defer f.Close()
+	WriteHeapDump(f.Fd())
+	fi, err := f.Stat()
+	if err != nil {
+		t.Fatalf("Stat failed: %v", err)
+	}
+	const minSize = 1
+	if size := fi.Size(); size < minSize {
+		t.Fatalf("Heap dump size %d bytes, expected at least %d bytes", size, minSize)
+	}
+}
+
+type Obj struct {
+	x, y int
+}
+
+func objfin(x *Obj) {
+	//println("finalized", x)
+}
+
+func TestWriteHeapDumpFinalizers(t *testing.T) {
+	if runtime.GOOS == "js" {
+		t.Skipf("WriteHeapDump is not available on %s.", runtime.GOOS)
+	}
+	f, err := os.CreateTemp("", "heapdumptest")
+	if err != nil {
+		t.Fatalf("TempFile failed: %v", err)
+	}
+	defer os.Remove(f.Name())
+	defer f.Close()
+
+	// bug 9172: WriteHeapDump couldn't handle more than one finalizer
+	println("allocating objects")
+	x := &Obj{}
+	runtime.SetFinalizer(x, objfin)
+	y := &Obj{}
+	runtime.SetFinalizer(y, objfin)
+
+	// Trigger collection of x and y, queueing of their finalizers.
+	println("starting gc")
+	runtime.GC()
+
+	// Make sure WriteHeapDump doesn't fail with multiple queued finalizers.
+	println("starting dump")
+	WriteHeapDump(f.Fd())
+	println("done dump")
+}
diff --git a/src/runtime/debug/mod.go b/src/runtime/debug/mod.go
new file mode 100644
index 0000000..0381bdc
--- /dev/null
+++ b/src/runtime/debug/mod.go
@@ -0,0 +1,114 @@
+// Copyright 2018 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 debug
+
+import (
+	"strings"
+)
+
+// exported from runtime
+func modinfo() string
+
+// ReadBuildInfo returns the build information embedded
+// in the running binary. The information is available only
+// in binaries built with module support.
+func ReadBuildInfo() (info *BuildInfo, ok bool) {
+	return readBuildInfo(modinfo())
+}
+
+// BuildInfo represents the build information read from
+// the running binary.
+type BuildInfo struct {
+	Path string    // The main package path
+	Main Module    // The module containing the main package
+	Deps []*Module // Module dependencies
+}
+
+// Module represents a module.
+type Module struct {
+	Path    string  // module path
+	Version string  // module version
+	Sum     string  // checksum
+	Replace *Module // replaced by this module
+}
+
+func readBuildInfo(data string) (*BuildInfo, bool) {
+	if len(data) < 32 {
+		return nil, false
+	}
+	data = data[16 : len(data)-16]
+
+	const (
+		pathLine = "path\t"
+		modLine  = "mod\t"
+		depLine  = "dep\t"
+		repLine  = "=>\t"
+	)
+
+	readEntryFirstLine := func(elem []string) (Module, bool) {
+		if len(elem) != 2 && len(elem) != 3 {
+			return Module{}, false
+		}
+		sum := ""
+		if len(elem) == 3 {
+			sum = elem[2]
+		}
+		return Module{
+			Path:    elem[0],
+			Version: elem[1],
+			Sum:     sum,
+		}, true
+	}
+
+	var (
+		info = &BuildInfo{}
+		last *Module
+		line string
+		ok   bool
+	)
+	// Reverse of cmd/go/internal/modload.PackageBuildInfo
+	for len(data) > 0 {
+		i := strings.IndexByte(data, '\n')
+		if i < 0 {
+			break
+		}
+		line, data = data[:i], data[i+1:]
+		switch {
+		case strings.HasPrefix(line, pathLine):
+			elem := line[len(pathLine):]
+			info.Path = elem
+		case strings.HasPrefix(line, modLine):
+			elem := strings.Split(line[len(modLine):], "\t")
+			last = &info.Main
+			*last, ok = readEntryFirstLine(elem)
+			if !ok {
+				return nil, false
+			}
+		case strings.HasPrefix(line, depLine):
+			elem := strings.Split(line[len(depLine):], "\t")
+			last = new(Module)
+			info.Deps = append(info.Deps, last)
+			*last, ok = readEntryFirstLine(elem)
+			if !ok {
+				return nil, false
+			}
+		case strings.HasPrefix(line, repLine):
+			elem := strings.Split(line[len(repLine):], "\t")
+			if len(elem) != 3 {
+				return nil, false
+			}
+			if last == nil {
+				return nil, false
+			}
+			last.Replace = &Module{
+				Path:    elem[0],
+				Version: elem[1],
+				Sum:     elem[2],
+			}
+			last = nil
+		}
+	}
+	return info, true
+}
diff --git a/src/runtime/debug/panic_test.go b/src/runtime/debug/panic_test.go
new file mode 100644
index 0000000..b67a3de
--- /dev/null
+++ b/src/runtime/debug/panic_test.go
@@ -0,0 +1,53 @@
+// Copyright 2020 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 aix darwin dragonfly freebsd linux netbsd openbsd
+
+// TODO: test on Windows?
+
+package debug_test
+
+import (
+	"runtime"
+	"runtime/debug"
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+func TestPanicOnFault(t *testing.T) {
+	if runtime.GOARCH == "s390x" {
+		t.Skip("s390x fault addresses are missing the low order bits")
+	}
+	if runtime.GOOS == "ios" {
+		t.Skip("iOS doesn't provide fault addresses")
+	}
+	m, err := syscall.Mmap(-1, 0, 0x1000, syscall.PROT_READ /* Note: no PROT_WRITE */, syscall.MAP_SHARED|syscall.MAP_ANON)
+	if err != nil {
+		t.Fatalf("can't map anonymous memory: %s", err)
+	}
+	defer syscall.Munmap(m)
+	old := debug.SetPanicOnFault(true)
+	defer debug.SetPanicOnFault(old)
+	const lowBits = 0x3e7
+	defer func() {
+		r := recover()
+		if r == nil {
+			t.Fatalf("write did not fault")
+		}
+		type addressable interface {
+			Addr() uintptr
+		}
+		a, ok := r.(addressable)
+		if !ok {
+			t.Fatalf("fault does not contain address")
+		}
+		want := uintptr(unsafe.Pointer(&m[lowBits]))
+		got := a.Addr()
+		if got != want {
+			t.Fatalf("fault address %x, want %x", got, want)
+		}
+	}()
+	m[lowBits] = 1 // will fault
+}
diff --git a/src/runtime/debug/stack.go b/src/runtime/debug/stack.go
new file mode 100644
index 0000000..5d810af
--- /dev/null
+++ b/src/runtime/debug/stack.go
@@ -0,0 +1,30 @@
+// Copyright 2011 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 debug contains facilities for programs to debug themselves while
+// they are running.
+package debug
+
+import (
+	"os"
+	"runtime"
+)
+
+// PrintStack prints to standard error the stack trace returned by runtime.Stack.
+func PrintStack() {
+	os.Stderr.Write(Stack())
+}
+
+// Stack returns a formatted stack trace of the goroutine that calls it.
+// It calls runtime.Stack with a large enough buffer to capture the entire trace.
+func Stack() []byte {
+	buf := make([]byte, 1024)
+	for {
+		n := runtime.Stack(buf, false)
+		if n < len(buf) {
+			return buf[:n]
+		}
+		buf = make([]byte, 2*len(buf))
+	}
+}
diff --git a/src/runtime/debug/stack_test.go b/src/runtime/debug/stack_test.go
new file mode 100644
index 0000000..9376e82
--- /dev/null
+++ b/src/runtime/debug/stack_test.go
@@ -0,0 +1,65 @@
+// Copyright 2011 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 debug_test
+
+import (
+	. "runtime/debug"
+	"strings"
+	"testing"
+)
+
+type T int
+
+func (t *T) ptrmethod() []byte {
+	return Stack()
+}
+func (t T) method() []byte {
+	return t.ptrmethod()
+}
+
+/*
+	The traceback should look something like this, modulo line numbers and hex constants.
+	Don't worry much about the base levels, but check the ones in our own package.
+
+		goroutine 10 [running]:
+		runtime/debug.Stack(0x0, 0x0, 0x0)
+			/Users/r/go/src/runtime/debug/stack.go:28 +0x80
+		runtime/debug.(*T).ptrmethod(0xc82005ee70, 0x0, 0x0, 0x0)
+			/Users/r/go/src/runtime/debug/stack_test.go:15 +0x29
+		runtime/debug.T.method(0x0, 0x0, 0x0, 0x0)
+			/Users/r/go/src/runtime/debug/stack_test.go:18 +0x32
+		runtime/debug.TestStack(0xc8201ce000)
+			/Users/r/go/src/runtime/debug/stack_test.go:37 +0x38
+		testing.tRunner(0xc8201ce000, 0x664b58)
+			/Users/r/go/src/testing/testing.go:456 +0x98
+		created by testing.RunTests
+			/Users/r/go/src/testing/testing.go:561 +0x86d
+*/
+func TestStack(t *testing.T) {
+	b := T(0).method()
+	lines := strings.Split(string(b), "\n")
+	if len(lines) < 6 {
+		t.Fatal("too few lines")
+	}
+	n := 0
+	frame := func(line, code string) {
+		check(t, lines[n], code)
+		n++
+		check(t, lines[n], line)
+		n++
+	}
+	n++
+	frame("src/runtime/debug/stack.go", "runtime/debug.Stack")
+	frame("src/runtime/debug/stack_test.go", "runtime/debug_test.(*T).ptrmethod")
+	frame("src/runtime/debug/stack_test.go", "runtime/debug_test.T.method")
+	frame("src/runtime/debug/stack_test.go", "runtime/debug_test.TestStack")
+	frame("src/testing/testing.go", "")
+}
+
+func check(t *testing.T, line, has string) {
+	if !strings.Contains(line, has) {
+		t.Errorf("expected %q in %q", has, line)
+	}
+}
diff --git a/src/runtime/debug/stubs.go b/src/runtime/debug/stubs.go
new file mode 100644
index 0000000..2cba136
--- /dev/null
+++ b/src/runtime/debug/stubs.go
@@ -0,0 +1,17 @@
+// Copyright 2014 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 debug
+
+import (
+	"time"
+)
+
+// Implemented in package runtime.
+func readGCStats(*[]time.Duration)
+func freeOSMemory()
+func setMaxStack(int) int
+func setGCPercent(int32) int32
+func setPanicOnFault(bool) bool
+func setMaxThreads(int) int