Fix StopTheWorld crash due to change in runtime.stopTheWorld signature:

In Go1.21 stopTheWorld changed from accepting reason as string into
accepting reason as uint8 code:

    https://github.com/golang/go/commit/b1aadd034c1f
    https://golang.org/cl/494495

which leads to the following crash if reason is still passed as string:

    fatal error: index out of range

    goroutine 6 [running]:
    runtime.throw({0x531aad?, 0x0?})
    	/home/kirr/src/tools/go/go/src/runtime/panic.go:1077 +0x5c fp=0xc000042608 sp=0xc0000425d8 pc=0x4365bc
    runtime.panicCheck1(0x4785af?, {0x531aad, 0x12})
    	/home/kirr/src/tools/go/go/src/runtime/panic.go:58 +0x94 fp=0xc000042628 sp=0xc000042608 pc=0x434034
    runtime.goPanicIndex(0x8d, 0x11)
    	/home/kirr/src/tools/go/go/src/runtime/panic.go:113 +0x2e fp=0xc000042668 sp=0xc000042628 pc=0x4340ee
    runtime.stwReason.String(...)
    	/home/kirr/src/tools/go/go/src/runtime/proc.go:1217
    runtime.stopTheWorld(0x8d)
    	/home/kirr/src/tools/go/go/src/runtime/proc.go:1260 +0xe6 fp=0xc0000426c8 sp=0xc000042668 pc=0x43b3a6
    lab.nexedi.com/kirr/go123/tracing/internal/xruntime.StopTheWorld(...)
    	/home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/tracing/internal/xruntime/runtime.go:40
    lab.nexedi.com/kirr/go123/tracing/internal/xruntime.TestStartStopTheWorld(0xc000007860)
    	/home/kirr/src/neo/src/lab.nexedi.com/kirr/go123/tracing/internal/xruntime/runtime_test.go:75 +0x139 fp=0xc000042770 sp=0xc0000426c8 pc=0x4fa0f9
    testing.tRunner(0xc000007860, 0x53a8b8)
    	/home/kirr/src/tools/go/go/src/testing/testing.go:1595 +0xff fp=0xc0000427c0 sp=0xc000042770 pc=0x4bbd9f
    testing.(*T).Run.func1()
    	/home/kirr/src/tools/go/go/src/testing/testing.go:1648 +0x25 fp=0xc0000427e0 sp=0xc0000427c0 pc=0x4bcd25
    runtime.goexit()
    	/home/kirr/src/tools/go/go/src/runtime/asm_amd64.s:1650 +0x1 fp=0xc0000427e8 sp=0xc0000427e0 pc=0x468901
    created by testing.(*T).Run in goroutine 1
    	/home/kirr/src/tools/go/go/src/testing/testing.go:1648 +0x3ad

-> Fix it by accounting that STW reason is uint8 on go1.21 and passing it
to runtime correspondingly.

Sadly we cannot express arbitrary string reason into fixed list of
reasons that runtime and stdlib tracing are aware of, so we always use
"unknown" for custom stop-the-world requests.

Generate g for today's state of Go 1.21 (go1.21.0-15-g70aa116c4a).
Compared to Go1.20 new field parentGoid was added and tracing-related
fields were moved to gTraceState struct.

Regenerated files stay without changes for Go1.20 and previous releases.

---- 8< ----
diff --git a/zruntime_g_go1.20.go b/zruntime_g_go1.21.go
index 99f483a..c55d865 100644
--- a/zruntime_g_go1.20.go
+++ b/zruntime_g_go1.21.go
@@ -1,7 +1,7 @@
 // Code generated by g_typedef; DO NOT EDIT.

-//go:build go1.20 && !go1.21
-// +build go1.20,!go1.21
+//go:build go1.21 && !go1.22
+// +build go1.21,!go1.22

 package xruntime

@@ -69,20 +69,17 @@ type g struct {
 	parkingOnChan atomic.Bool

 	raceignore    int8  // ignore race detection events
-	sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about this goroutine
 	tracking      bool  // whether we're tracking this G for sched latency statistics
 	trackingSeq   uint8 // used to decide whether to track this G
 	trackingStamp int64 // timestamp of when the G last started being tracked
 	runnableTime  int64 // the amount of time spent runnable, cleared when running, only used when tracking
-	sysexitticks   int64    // cputicks when syscall has returned (for tracing)
-	traceseq       uint64   // trace event sequencer
-	tracelastp     puintptr // last P emitted an event for this goroutine
 	lockedm       muintptr
 	sig           uint32
 	writebuf      []byte
 	sigcode0      uintptr
 	sigcode1      uintptr
 	sigpc         uintptr
+	parentGoid    uint64          // goid of goroutine that created this goroutine
 	gopc          uintptr         // pc of go statement that created this goroutine
 	ancestors     *[]ancestorInfo // ancestor information goroutine(s) that created this goroutine (only used if debug.tracebackancestors)
 	startpc       uintptr         // pc of goroutine function
@@ -97,6 +94,9 @@ type g struct {
 	// current in-progress goroutine profile
 	goroutineProfiled goroutineProfileStateHolder

+	// Per-G tracer state.
+	trace gTraceState
+
 	// gcAssistBytes is this G's GC assist credit in terms of
 	// bytes allocated. If this is positive, then the G has credit
 	// to allocate gcAssistBytes bytes without assisting. If this
@@ -202,6 +202,13 @@ type ancestorInfo struct {
 	gopc uintptr   // pc of go statement that created this goroutine
 }
 type goroutineProfileStateHolder atomic.Uint32
+type gTraceState struct {
+	sysExitTime        traceTime // timestamp when syscall has returned
+	tracedSyscallEnter bool      // syscall or cgo was entered while trace was enabled or StartTrace has emitted EvGoInSyscall about this goroutine
+	seq                uint64    // trace event sequencer
+	lastP              puintptr  // last P emitted an event for this goroutine
+}
+type traceTime uint64
 type uintreg uint          // FIXME wrong on amd64p32
 type m struct{}            // FIXME stub
 type sudog struct{}        // FIXME stub

Generate g for today's state of Go 1.20 (go1.20-0-gde4748c47c).
Compared to Go1.19 goid is changed from int64 -> to uint64, and that
more variables, that are used atomically, now use atomic.X types.

Regenerated files stay without changes for Go1.19 and previous releases.

---- 8< ----
diff --git a/zruntime_g_go1.19.go b/zruntime_g_go1.20.go
index 707aadd..99f483a 100644
--- a/zruntime_g_go1.19.go
+++ b/zruntime_g_go1.20.go
@@ -1,7 +1,7 @@
 // Code generated by g_typedef; DO NOT EDIT.

-//go:build go1.19 && !go1.20
-// +build go1.19,!go1.20
+//go:build go1.20 && !go1.21
+// +build go1.20,!go1.21

 package xruntime

@@ -39,9 +39,9 @@ type g struct {
 	// 3. By debugCallWrap to pass parameters to a new goroutine because allocating a
 	//    closure in the runtime is forbidden.
 	param        unsafe.Pointer
-	atomicstatus uint32
+	atomicstatus atomic.Uint32
 	stackLock    uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
-	goid         int64
+	goid         uint64
 	schedlink    guintptr
 	waitsince    int64      // approx time when the g become blocked
 	waitreason   waitReason // if status==Gwaiting
@@ -65,14 +65,14 @@ type g struct {
 	activeStackChans bool
 	// parkingOnChan indicates that the goroutine is about to
 	// park on a chansend or chanrecv. Used to signal an unsafe point
-	// for stack shrinking. It's a boolean value, but is updated atomically.
-	parkingOnChan uint8
+	// for stack shrinking.
+	parkingOnChan atomic.Bool

 	raceignore     int8     // ignore race detection events
 	sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about this goroutine
 	tracking       bool     // whether we're tracking this G for sched latency statistics
 	trackingSeq    uint8    // used to decide whether to track this G
-	runnableStamp  int64    // timestamp of when the G last became runnable, only used when tracking
+	trackingStamp  int64    // timestamp of when the G last started being tracked
 	runnableTime   int64    // the amount of time spent runnable, cleared when running, only used when tracking
 	sysexitticks   int64    // cputicks when syscall has returned (for tracing)
 	traceseq       uint64   // trace event sequencer
@@ -91,7 +91,7 @@ type g struct {
 	cgoCtxt        []uintptr      // cgo traceback context
 	labels         unsafe.Pointer // profiler labels
 	timer          *timer         // cached timer for time.Sleep
-	selectDone     uint32         // are we participating in a select and did someone win the race?
+	selectDone     atomic.Uint32  // are we participating in a select and did someone win the race?

 	// goroutineProfiled indicates the status of this goroutine's stack for the
 	// current in-progress goroutine profile
@@ -190,7 +190,7 @@ type timer struct {
 	nextwhen int64

 	// The status field holds one of the values below.
-	status uint32
+	status atomic.Uint32
 }
 type guintptr uintptr
 type puintptr uintptr
@@ -198,7 +198,7 @@ type muintptr uintptr
 type waitReason uint8
 type ancestorInfo struct {
 	pcs  []uintptr // pcs from the stack of this goroutine
-	goid int64     // goroutine id of this goroutine; original goroutine possibly dead
+	goid uint64    // goroutine id of this goroutine; original goroutine possibly dead
 	gopc uintptr   // pc of go statement that created this goroutine
 }
 type goroutineProfileStateHolder atomic.Uint32

Else if a project, that uses go123, is compiled by go1.18 or go1.19,
then go complains that go123 uses `any`, but the language, that go123
requires in it's go.mod, is only 1.14:

    kirr@deca:~/src/wendelin/wendelin.core/wcfs$ go build
    # lab.nexedi.com/kirr/go123/tracing/internal/xruntime
    /home/kirr/go/pkg/mod/lab.nexedi.com/kirr/go123@v0.0.0-20221005052354-179529a7/tracing/internal/xruntime/zruntime_g_go1.19.go:114:12: predeclared any requires go1.18 or later (-lang was set to go1.14; check go.mod)
    /home/kirr/go/pkg/mod/lab.nexedi.com/kirr/go123@v0.0.0-20221005052354-179529a7/tracing/internal/xruntime/zruntime_g_go1.19.go:184:14: predeclared any requires go1.18 or later (-lang was set to go1.14; check go.mod)
    /home/kirr/go/pkg/mod/lab.nexedi.com/kirr/go123@v0.0.0-20221005052354-179529a7/tracing/internal/xruntime/zruntime_g_go1.19.go:185:9: predeclared any requires go1.18 or later (-lang was set to go1.14; check go.mod)

-> Fix that by expanding `any` to what it is aliased to:

    $ go doc builtin.any
    package builtin // import "builtin"

    type any = interface{}
        any is an alias for interface{} and is equivalent to interface{} in all
        ways.

Go123 wants to support older systems, and so we do not want to bump
go123's required language, at least for now.

For the reference: they started to use `any` somewhere in go1.18 (see
070bfdbb "tracing/runtime: Refresh for Go1.18").

Generate g for today's state of Go 1.19 (go1.19.2-0-g895664482c).
Compared to Go1.18 the only change is that new field `goroutineProfiled`
is added to g struct.

Regenerated files stay without changes for Go1.18 and previous releases.

---- 8< ----
diff --git a/zruntime_g_go1.18.go b/zruntime_g_go1.19.go
index 76f905d..3b33cbe 100644
--- a/zruntime_g_go1.18.go
+++ b/zruntime_g_go1.19.go
@@ -1,10 +1,11 @@
 // Code generated by g_typedef; DO NOT EDIT.

-// +build go1.18,!go1.19
+// +build go1.19,!go1.20

 package xruntime

 import "unsafe"
+import "sync/atomic"

 type g struct {
 	// Stack parameters.
@@ -91,6 +92,10 @@ type g struct {
 	timer          *timer         // cached timer for time.Sleep
 	selectDone     uint32         // are we participating in a select and did someone win the race?

+	// goroutineProfiled indicates the status of this goroutine's stack for the
+	// current in-progress goroutine profile
+	goroutineProfiled goroutineProfileStateHolder
+
 	// gcAssistBytes is this G's GC assist credit in terms of
 	// bytes allocated. If this is positive, then the G has credit
 	// to allocate gcAssistBytes bytes without assisting. If this
@@ -195,6 +200,7 @@ type ancestorInfo struct {
 	goid int64     // goroutine id of this goroutine; original goroutine possibly dead
 	gopc uintptr   // pc of go statement that created this goroutine
 }
+type goroutineProfileStateHolder atomic.Uint32
 type uintreg  uint	// FIXME wrong on amd64p32
 type m struct {}		// FIXME stub
 type sudog struct {}	// FIXME stub

Regenerate for Go 1.18 release (go1.18-0-g4aa1efed48)

The only change is interface{} -> any.

A deadlock scenario was possible before this patch:

- main thread finished checks, wants to return, goes to deferred wg.Wait();
- that wg.Wait waits for other spawned threads to complete;
- in one of that thread there is extra event being sent to a stream;
- that send is blocked waiting for main thread to receive it, but the
  main thread is waiting in wg.Wait() and only after that it would go to
  call T.closeStreamTab() which marks all the channels as down -> deadlock.

-> Fix it by explicitly detecting deadlocks not only in Recv, but also on Send.

NOTE Contrary to Recv - which is always called from main thread - Send is
always called from non-main threads. T.unsentv and the logic prepared in
the previous patch care to handle that correctly.

If we don't, it won't be automatically the case when Send's will be
adjusted to detect and log deadlocks. And then, if there will be no
ordering, it will be hard to do tests in example_test.go which expects
logging output in particular "main -> everything else" order.

This is possible if the system is incorrectly decomposed into serial
streams. We want to show all pending events in that case, not only the
first random one.

-> Reuse T.mu for serialization.

closeStreamTab changes locking so that acquired t.mu covers whole time
when that function could log anything.

There is no guaranty that the main thread would call Fatal or Fail if a
non-main thread calls fatalfInNonMain. This way it was possible for a
test to succeed if only non-main thread(s) failed somehow.

-> Fix it by marking the test as failed if any non-main thread fails.

Generate g for today state of Go 1.18 (go1.17beta1-2522-g47db3bb443)
Compared to Go1.17 there are small number of changes in _defer struct.

Regenerated files stay without changes for Go1.17 and previous releases.

---- 8< ----
diff --git a/zruntime_g_go1.17.go b/zruntime_g_go1.18.go
index 7a76db6..5e2fafb 100644
--- a/zruntime_g_go1.17.go
+++ b/zruntime_g_go1.18.go
@@ -1,6 +1,6 @@
 // Code generated by g_typedef; DO NOT EDIT.

-// +build go1.17,!go1.18
+// +build go1.18,!go1.19

 package xruntime

@@ -115,7 +115,6 @@ type _panic struct {
 	goexit    bool
 }
 type _defer struct {
-	siz     int32 // includes both arguments and results
 	started bool
 	heap    bool
 	// openDefer indicates that this _defer is for a frame with open-coded
@@ -124,9 +123,9 @@ type _defer struct {
 	openDefer bool
 	sp        uintptr // sp at time of defer
 	pc        uintptr // pc at time of defer
-	fn        *funcval // can be nil for open-coded defers
+	fn        func()  // can be nil for open-coded defers
 	_panic    *_panic // panic that is running defer
-	link      *_defer
+	link      *_defer // next defer on G; can point to either heap or stack!

 	// If openDefer is true, the fields below record values about the stack
 	// frame and associated function that has the open-coded defer(s). sp

Generate g for ~ Go 1.17beta1 (go1.17beta1-2-g770f1de8c5)
Compared to Go1.16 there is one non-cosmetic change to g related to tracking
scheduling latencies:

https://github.com/golang/go/commit/bedfeed54a7a

Regenerated files stay without changes for Go1.16 and previous releases.

---- 8< ----
diff --git a/zruntime_g_go1.16.go b/zruntime_g_go1.17.go
index 9604d6b..7a76db6 100644
--- a/zruntime_g_go1.16.go
+++ b/zruntime_g_go1.17.go
@@ -1,6 +1,6 @@
 // Code generated by g_typedef; DO NOT EDIT.

-// +build go1.16,!go1.17
+// +build go1.17,!go1.18

 package xruntime

@@ -25,7 +25,18 @@ type g struct {
        syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
        syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
        stktopsp  uintptr // expected sp at top of stack, to check in traceback
-       param        unsafe.Pointer // passed parameter on wakeup
+       // param is a generic pointer parameter field used to pass
+       // values in particular contexts where other storage for the
+       // parameter would be difficult to find. It is currently used
+       // in three ways:
+       // 1. When a channel operation wakes up a blocked goroutine, it sets param to
+       //    point to the sudog of the completed blocking operation.
+       // 2. By gcAssistAlloc1 to signal back to its caller that the goroutine completed
+       //    the GC cycle. It is unsafe to do so in any other way, because the goroutine's
+       //    stack may have moved in the meantime.
+       // 3. By debugCallWrap to pass parameters to a new goroutine because allocating a
+       //    closure in the runtime is forbidden.
+       param        unsafe.Pointer
        atomicstatus uint32
        stackLock    uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
        goid         int64
@@ -57,6 +68,10 @@ type g struct {

        raceignore     int8     // ignore race detection events
        sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about this goroutine
+       tracking       bool     // whether we're tracking this G for sched latency statistics
+       trackingSeq    uint8    // used to decide whether to track this G
+       runnableStamp  int64    // timestamp of when the G last became runnable, only used when tracking
+       runnableTime   int64    // the amount of time spent runnable, cleared when running, only used when tracking
        sysexitticks   int64    // cputicks when syscall has returned (for tracing)
        traceseq       uint64   // trace event sequencer
        tracelastp     puintptr // last P emitted an event for this goroutine
@@ -142,7 +157,7 @@ type gobuf struct {
        pc   uintptr
        g    guintptr
        ctxt unsafe.Pointer
-       ret  uintreg
+       ret  uintptr
        lr   uintptr
        bp   uintptr // for framepointer-enabled architectures
 }

Tracetest is a package to verify concurrent systems based on synchronous
tracing. It is used in NEO/go tests and originates from what was
initially explained in https://navytux.spb.ru/~kirr/neo.html in

    """On top of that package tracetest provides infrastructure for
    testing concurrent systems..."""

See top-level package overview + example_test.go for details.

Some NEO/go tests need to start a cluster under tracing and then to
continue running test workload with tracing disabled.

NetTrace now returns Tracer instead of general Networker. The reason we
need this is because in the next patch Tracer will implement methods,
additional to just what is required by Networker, to control itself.

Rename the interface that specifies methods to be implemented by trace
receivers. Reason is that in the next patch we'll need to expose
"Tracer" as the tracing networker itself.

Generate g for ~ Go 1.16beta1 (go1.16beta1-34-g4fd9455882)
Compared to Go1.15 there are only cosmetic changes:

    --- a/zruntime_g_go1.15.go
    +++ b/zruntime_g_go1.16.go
    @@ -1,6 +1,6 @@
     // Code generated by g_typedef; DO NOT EDIT.

    -// +build go1.15,!go1.16
    +// +build go1.16,!go1.17

     package xruntime

    @@ -144,7 +144,7 @@ type gobuf struct {
     	ctxt unsafe.Pointer
     	ret  uintreg
     	lr   uintptr
    -	bp   uintptr // for GOEXPERIMENT=framepointer
    +	bp   uintptr // for framepointer-enabled architectures
     }
     type funcval struct {
     	fn uintptr
    @@ -158,6 +158,8 @@ type timer struct {
     	// Timer wakes up at when, and then at when+period, ... (period > 0 only)
     	// each time calling f(arg, now) in the timer goroutine, so f must be
     	// a well-behaved function and not block.
    +	//
    +	// when must be positive on an active timer.
     	when   int64
     	period int64
     	f      func(interface{}, uintptr)

Regenerated files stay without changes for Go1.15 and previous releases.

When a virtnet network - e.g. lonet - is explicitly used in tests, the
open and close sequence is easy to implement:

    network = lonet.Join(...)
    defer network.Close()

    hostα = network.NewHost("α")
    defer hostα.Close()
    hostβ = network.NewHost("β")
    defer hostβ.Close()
    ...

However in other scenarios - for example when the code wants to call
xnet.Networker factory that creates a networker out of several potential
choices according to given parameters, only interface for network
access-point (e.g. Host in lonet/virtnet case) is returned. The network
itself (virtnet.SubNetwork) is not returned to the user. As it is this
makes it impossible to close the network and release its resources.

Instead of adding complexity to such "create-networker" functions to
e.g. change Close of returned networker to also close subnetwork, or
return both networker and a separate close func, let's teach virtnet to
close SubNetwork automatically when last host of this subnetwork is
closed. Make it opt-in since this behaviour is not universally wanted.

This way a factory that is asked to connect to network as e.g. a node on
lonet, could do the following:

    func neonet.Join(...) xnet.Networker {
        ...
        // join lonet "<net>" as host "<node>"
        network = lonet.Join("<net>")
        host = network.NewHost("<node>")
        network.AutoClose() // host.Close will close network
        return host
    }

and network will be closed on host.Close() call.

-> Add SubNetwork.AutoClose to schedule Close to be called after last host on the subnetwork is closed.
-> Mirror this change in virtnet part of lonet.py

Even though there is no need to manually release resources for NetPlain
and NetTLS, for other networkers - for example lonet - releasing
resources is needed. In general programs cannot always use lonet
explicitly, as there might be a factory function that creates a
Networker according to given parameters.

-> Adjust Networker interface to include general Close.
-> Adjust NetPlain to follow added "interrupt-on-close" semantic for uniformity.

Similarly to what we do in package xio (see 7ad867a3 "xio: Add
Reader/Writer/... interfaces - io analogs that add support for
contexts"), add utilities to bind xnet.Listener to context (giving
net.Listener), and, on the other hand, adapt uncontextified net.Listener
to xnet.Listener that handles cancellation.

This utilities are needed because a lot of third-party code wants to
work with net.Listener interface only.

Contrary to xio, we can make it working reliably in both ways (because
Accept already buffers accepting connections starting at OS level).

In the previous patch xnet.Networker was changed: Listen now accepts ctx
and returns xnet.Listener instead of net.Listener.

-> Adapt the code all around to that.

We already handle cancellation in Dial, but Accepting was out of luck
until now. This makes it more difficult for clients to implement and
wrap acceptors where they need to handle cancellations. This also makes
it possible for a test or program to get stuck in Accept loop if it is
not careful enough to manually handle ctx cancel around Accept calls.

-> Fix it in one place - here, in xnet - so that users are offloaded
from all this and can just call Accept(ctx) and rely on underlying
implementation to handle ctx cancel.

This patch:

- introduces xnet.Listener interface, which is like net.Listener, but
  Accept goes with ctx argument.
- changes Networker.Listen signature to return xnet.Listener instead of
  net.Listener. While we are here - changing it - also add ctx argument
  to Listen call itself.
- Adds listenerCtx - which, given net.Listener, provides xnet.Listener
  by wrapping some logic around original.
- Adapts NetPlain, NetTLS and NetTrace to provide updated interface.

We'll fix up everything in other packages to match/use updated interface
in the next patch.

See nexedi/pygolang@21756bd3.

Go1.14.10 and Go1.15.3 changed internal structure of g:

https://github.com/golang/go/commit/878da0bf881b (go1.14)
https://github.com/golang/go/commit/bf79f91d3dc4 (go1.15)

A shorthand for when exception context is just formatted string.
Simplifies

	func doSomething(path string) {
		defer exc.Context(func() interface{} {
			return fmt.Sprintf("doing something %s", path)
		})()
		...

to

	func doSomething(path string) {
		defer exc.Contextf("doing something %s", path)
		...

`go mod init` + build/test that fetched in dependencies.

It was address-of error (the type) instead of &err.

https://github.com/golang/go/issues/36503
https://github.com/golang/go/issues/36448

They are there for 6 months already without being considered for real
and so the progress is very unlikely, but still...

Generate g for ~ Go 1.15beta1 (go1.14beta1-1729-gc0e8e405c0)
Compared to Go1.14 there are no changes.

Regenerated files stay without changes for Go1.14 and previous releases.

Provide race.Enabled for programs to know whether they were built with
race detector or not.

Code originates from https://lab.nexedi.com/kirr/wendelin.core/tree/25c3184d/wcfs/internal/race
See also: https://github.com/golang/go/issues/36477.

So that Go1.13 and golang.org/x/xerrors understand the wrapping:
https://blog.golang.org/go1.13-errors

Preserve Cause for compatibility with github.com/pkg/errors.