X Unfortunately it crashes in GC even with weak.Ref reworked to be one word instead of two

    ---- 8< ----
    (z-dev) kirr@deca:~/src/wendelin/wendelin.core/wcfs$ time ./gogccrash 2>&1

    done, crashes:
    BUG/2/core

    Завершено

    real    28m54,016s
    user    30m27,968s
    sys     7m35,028s

    (z-dev) kirr@deca:~/src/wendelin/wendelin.core/wcfs$ cat BUG/2/log

    ...

    runtime: full=0xc0001f10000005 next=205 jobs=204 nDataRoots=1 nBSSRoots=1 nSpanRoots=16 nStackRoots=184
    panic: non-empty mark queue after concurrent mark
    fatal error: panic on system stack

    ...
    ---- 8< ----

wcfs/.gitignore

 /wcfs
+/wcfs.test

wcfs/go.mod

 module lab.nexedi.com/nexedi/wendelin.core/wcfs
 
-go 1.14
+go 1.18
 
 require (
 	github.com/kisielk/og-rek v1.0.1-0.20180928202415-8b25c4cefd6c
 	lab.nexedi.com/kirr/go123 v0.0.0-20200916121347-316617668e12
 	lab.nexedi.com/kirr/neo/go v0.0.0-20201012044742-28494187df87
 )
+
+// indirects
+require (
+	github.com/cznic/strutil v0.0.0-20181122101858-275e90344537
+	github.com/DataDog/czlib v0.0.0-20160811164712-4bc9a24e37f2
+	github.com/fsnotify/fsnotify v1.4.10-0.20200417215612-7f4cf4dd2b52
+	github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b
+	github.com/philhofer/fwd v1.0.0
+	github.com/pkg/errors v0.9.1
+	github.com/someonegg/gocontainer v1.0.0
+	github.com/shamaton/msgpack v1.1.1
+	github.com/tinylib/msgp v1.1.3-0.20200327023543-e88e92c0ccca
+	golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208
+)

wcfs/vendor/lab.nexedi.com/kirr/neo/go/zodb/connection.go

@@ -117,7 +117,7 @@ type LiveCache struct {
 	//
 	// NOTE2 finalizers don't run on when they are attached to an object in cycle.
 	// Hopefully we don't have cycles with BTree/Bucket.
-	objtab map[Oid]*weak.Ref // oid -> weak.Ref(IPersistent)
+	objtab map[Oid]*weak.Ref[Persistent] // oid -> weak.Ref(IPersistent)
 
 	// hooks for application to influence live caching decisions.
 	control LiveCacheControl
@@ -184,7 +184,7 @@ func newConnection(db *DB, at Tid) *Connection {
 		at:    at,
 		cache: LiveCache{
 			pinned: make(map[Oid]IPersistent),
-			objtab: make(map[Oid]*weak.Ref),
+			objtab: make(map[Oid]*weak.Ref[Persistent]),
 		},
 	}
 }
@@ -231,7 +231,7 @@ func (cache *LiveCache) Get(oid Oid) IPersistent {
 	wobj := cache.objtab[oid]
 	if wobj != nil {
 		if xobj := wobj.Get(); xobj != nil {
-			obj = xobj.(IPersistent)
+			obj = xobj.instance
 		}
 	}
 
@@ -252,7 +252,7 @@ func (cache *LiveCache) setNew(oid Oid, obj IPersistent) {
 		cache.pinned[oid] = obj
 		// XXX assert .objtab[oid] == nil ?
 	} else {
-		cache.objtab[oid] = weak.NewRef(obj)
+		cache.objtab[oid] = weak.NewRef(obj.persistent())
 		// XXX assert .pinned[oid] == nil ?
 	}
 }
@@ -264,7 +264,7 @@ func (cache *LiveCache) forEach(f func(IPersistent)) {
 	}
 	for _, wobj := range cache.objtab {
 		if xobj := wobj.Get(); xobj != nil {
-			f(xobj.(IPersistent))
+			f(xobj.instance)
 		}
 	}
 }
@@ -280,7 +280,7 @@ func (cache *LiveCache) SetControl(c LiveCacheControl) {
 
 	// reclassify all objects
 	c2 := *cache
-	cache.objtab = make(map[Oid]*weak.Ref)
+	cache.objtab = make(map[Oid]*weak.Ref[Persistent])
 	cache.pinned = make(map[Oid]IPersistent)
 	c2.forEach(func(obj IPersistent) {
 		cache.setNew(obj.POid(), obj)

wcfs/vendor/lab.nexedi.com/kirr/neo/go/zodb/internal/weak/weak.go

@@ -31,9 +31,10 @@ import (
 	"sync/atomic"
 	"time"
 	"unsafe"
-	"math/rand"
+//	"math/rand"
 )
 
+/*
 // iface is how Go runtime represents an interface.
 //
 // NOTE layout must be synchronized to Go runtime representation.
@@ -42,6 +43,7 @@ type iface struct {
 	typ  uintptr // type
 	data uintptr // data
 }
+*/
 
 // weakRefState represents current state of an object Ref points to.
 type weakRefState int32
@@ -60,8 +62,10 @@ const (
 // There must be no more than 1 weak reference to any object.
 // Weak references must not be attached to an object on which runtime.SetFinalizer is also used.
 // Weak references must not be copied.
-type Ref struct {
-	iface
+type Ref[T any] struct {
+//	iface
+	// XXX only one word + reference to go memory model
+        iptr uintptr // XXX NOTE correctness depends on non-moving property of Go GC
 
 	// XXX try to do without mutex and only with atomics
 	mu    sync.Mutex
@@ -71,12 +75,13 @@ type Ref struct {
 // NewRef creates new weak reference pointing to obj.
 //
 // TODO + onrelease callback?
-func NewRef(obj interface{}) *Ref {
+func NewRef[T any](obj *T) *Ref[T] {
 	// since starting from ~ Go1.4 the GC is precise, we can save interface
 	// pointers to uintptr and that won't prevent GC from garbage
 	// collecting the object.
-	w := &Ref{
-		iface: *(*iface)(unsafe.Pointer(&obj)),
+	w := &Ref[T]{
+//		iface: *(*iface)(unsafe.Pointer(&obj)),
+		iptr:  (uintptr)(unsafe.Pointer(obj)),
 		state: objLive,
 	}
 
@@ -85,10 +90,11 @@ func NewRef(obj interface{}) *Ref {
 	return w
 }
 
-func (w *Ref) release(obj interface{}) {
-	ifobj := *(*iface)(unsafe.Pointer(&obj))
-	if w.iface != ifobj {
-		panic(fmt.Sprintf("weak: release: w.iface != obj;  w.iface=%x  obj=%x", w.iface, ifobj))
+func (w *Ref[T]) release(obj *T) {
+//	ifobj := *(*iface)(unsafe.Pointer(&obj))
+	iptr := (uintptr)(unsafe.Pointer(obj))
+	if w.iptr != iptr {
+		panic(fmt.Sprintf("weak: release: w.iptr != obj;  w.iptr=%x  obj=%x", w.iptr, iptr))
 	}
 
 	// GC decided that the object is no longer reachable and
@@ -115,7 +121,7 @@ var xxx uint64
 //
 // If original object is still alive - it is returned.
 // If not - nil is returned.
-func (w *Ref) Get() (obj interface{}) {
+func (w *Ref[T]) Get() (obj *T) {
 	w.mu.Lock()
 	if w.state != objReleased {
 		w.state = objGot
@@ -123,6 +129,13 @@ func (w *Ref) Get() (obj interface{}) {
 		//time.Sleep(100*time.Nanosecond)
 		// recreate interface{} from saved words.
 		// XXX do writes as pointers so that compiler emits write barriers to notify GC?
+
+		// recreate pointer from saved word.
+		obj = (*T)(unsafe.Pointer(w.iptr))
+
+		atomic.AddUint64(&xxx, 1) // barrier
+		time.Sleep(100*time.Nanosecond)
+/*
 		i := (*iface)(unsafe.Pointer(&obj))
 		//*i = w.iface
 
@@ -150,6 +163,7 @@ func (w *Ref) Get() (obj interface{}) {
 		} else {
 			i.data = w.iface.data
 		}
+*/
 	}
 	w.mu.Unlock()
 

wcfs/vendor/modules.txt

 # github.com/DataDog/czlib v0.0.0-20160811164712-4bc9a24e37f2
 github.com/DataDog/czlib
+## explicit
 # github.com/cznic/strutil v0.0.0-20181122101858-275e90344537
 github.com/cznic/strutil
+## explicit
 # github.com/fsnotify/fsnotify v1.4.10-0.20200417215612-7f4cf4dd2b52
 github.com/fsnotify/fsnotify
+## explicit
 # github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b
 github.com/golang/glog
+## explicit
 # github.com/kisielk/og-rek v1.0.1-0.20180928202415-8b25c4cefd6c
 ## explicit
 github.com/kisielk/og-rek
 # github.com/philhofer/fwd v1.0.0
 github.com/philhofer/fwd
+## explicit
 # github.com/pkg/errors v0.9.1
 github.com/pkg/errors
+## explicit
 # github.com/shamaton/msgpack v1.1.1
+## explicit
 github.com/shamaton/msgpack
 github.com/shamaton/msgpack/def
 github.com/shamaton/msgpack/ext
@@ -23,9 +30,12 @@ github.com/shamaton/msgpack/internal/encoding
 github.com/shamaton/msgpack/time
 # github.com/someonegg/gocontainer v1.0.0
 github.com/someonegg/gocontainer/rbuf
+## explicit
 # github.com/tinylib/msgp v1.1.3-0.20200327023543-e88e92c0ccca
 github.com/tinylib/msgp/msgp
+## explicit
 # golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208
+## explicit
 golang.org/x/sync/errgroup
 # lab.nexedi.com/kirr/go123 v0.0.0-20200916121347-316617668e12
 ## explicit
@@ -48,7 +58,7 @@ lab.nexedi.com/kirr/go123/xruntime
 lab.nexedi.com/kirr/go123/xstrings
 lab.nexedi.com/kirr/go123/xsync
 # lab.nexedi.com/kirr/neo/go v0.0.0-20201012044742-28494187df87
-## explicit
+## explicit; go 1.18
 lab.nexedi.com/kirr/neo/go/internal/log
 lab.nexedi.com/kirr/neo/go/internal/packed
 lab.nexedi.com/kirr/neo/go/internal/task