erts: Fix ets:select table fixation leak at owner change

Symtom:
ETS table remains fixed after finished ets:select* call.

Problem:
The decision to unfix table after a yielding ets:select*
is based on table ownership, but ownership might have changed
while ets:select* was yielding.

Solution:
Remember and pass along whether table was fixed
when the traversal started.

6 files changed, 268 insertions, 118 deletions

diff --git a/erts/emulator/beam/erl_db.c b/erts/emulator/beam/erl_db.c
index 4132a54934..0a50af4d1a 100644
--- a/erts/emulator/beam/erl_db.c
+++ b/erts/emulator/beam/erl_db.c
@@ -81,14 +81,6 @@ static BIF_RETTYPE db_bif_fail(Process* p, Uint freason,
 /* Get a key from any table structure and a tagged object */
 #define TERM_GETKEY(tb, obj) db_getkey((tb)->common.keypos, (obj)) 
 
- 
-/* How safe are we from double-hits or missed objects
-** when iterating without fixation? */ 
-enum DbIterSafety {
-    ITER_UNSAFE,      /* Must fixate to be safe */
-    ITER_SAFE_LOCKED, /* Safe while table is locked, not between trap calls */
-    ITER_SAFE         /* No need to fixate at all */
-};
 #  define ITERATION_SAFETY(Proc,Tab) \
     ((IS_TREE_TABLE((Tab)->common.status) || IS_CATREE_TABLE((Tab)->common.status) \
       || ONLY_WRITER(Proc,Tab)) ? ITER_SAFE                             \
@@ -195,9 +187,6 @@ static int fixed_tabs_find(DbFixation* first, DbFixation* fix)
 #define ERTS_RBT_WANT_DELETE
 #define ERTS_RBT_WANT_FOREACH
 #define ERTS_RBT_WANT_FOREACH_DESTROY
-#ifdef DEBUG
-# define ERTS_RBT_WANT_LOOKUP
-#endif
 #define ERTS_RBT_UNDEF
 
 #include "erl_rbtree.h"
@@ -2287,6 +2276,7 @@ static BIF_RETTYPE ets_select_delete_trap_1(BIF_ALIST_1)
     Eterm ret;
     Eterm *tptr;
     db_lock_kind_t kind = LCK_WRITE_REC;
+    enum DbIterSafety safety = ITER_SAFE;
 
     CHECK_TABLES();
     ASSERT(is_tuple(a1));
@@ -2296,10 +2286,11 @@ static BIF_RETTYPE ets_select_delete_trap_1(BIF_ALIST_1)
     DB_TRAP_GET_TABLE(tb, tptr[1], DB_WRITE, kind,
                       &ets_select_delete_continue_exp);
 
-    cret = tb->common.meth->db_select_delete_continue(p,tb,a1,&ret);
+    cret = tb->common.meth->db_select_delete_continue(p,tb,a1,&ret,&safety);
 
-    if(!DID_TRAP(p,ret) && ITERATION_SAFETY(p,tb) != ITER_SAFE) {  
-	unfix_table_locked(p, tb, &kind);
+    if(!DID_TRAP(p,ret) && safety != ITER_SAFE) {
+        ASSERT(erts_refc_read(&tb->common.fix_count,1));
+        unfix_table_locked(p, tb, &kind);
     }
 
     db_unlock(tb, kind);
@@ -2337,7 +2328,8 @@ BIF_RETTYPE ets_internal_select_delete_2(BIF_ALIST_2)
     if (safety == ITER_UNSAFE) {
 	local_fix_table(tb);
     }
-    cret = tb->common.meth->db_select_delete(BIF_P, tb, BIF_ARG_1, BIF_ARG_2, &ret);
+    cret = tb->common.meth->db_select_delete(BIF_P, tb, BIF_ARG_1, BIF_ARG_2,
+                                             &ret, safety);
 
     if (DID_TRAP(BIF_P,ret) && safety != ITER_SAFE) {
 	fix_table_locked(BIF_P,tb);
@@ -2729,7 +2721,7 @@ ets_select3(Process* p, DbTable* tb, Eterm tid, Eterm ms, Sint chunk_size)
     cret = tb->common.meth->db_select_chunk(p, tb, tid,
 					    ms, chunk_size,
 					    0 /* not reversed */,
-					    &ret);
+					    &ret, safety);
     if (DID_TRAP(p,ret) && safety != ITER_SAFE) {
 	fix_table_locked(p, tb);
     }
@@ -2756,7 +2748,8 @@ ets_select3(Process* p, DbTable* tb, Eterm tid, Eterm ms, Sint chunk_size)
 }
 
 
-/* We get here instead of in the real BIF when trapping */
+/* Trap here from: ets_select_1/2/3
+ */
 static BIF_RETTYPE ets_select_trap_1(BIF_ALIST_1)
 {
     Process *p = BIF_P;
@@ -2767,6 +2760,7 @@ static BIF_RETTYPE ets_select_trap_1(BIF_ALIST_1)
     Eterm ret;
     Eterm *tptr;
     db_lock_kind_t kind = LCK_READ;
+    enum DbIterSafety safety = ITER_SAFE;
 
     CHECK_TABLES();
 
@@ -2776,11 +2770,13 @@ static BIF_RETTYPE ets_select_trap_1(BIF_ALIST_1)
     DB_TRAP_GET_TABLE(tb, tptr[1], DB_READ, kind,
                       &ets_select_continue_exp);
 
-    cret = tb->common.meth->db_select_continue(p, tb, a1,
-					       &ret);
+    cret = tb->common.meth->db_select_continue(p, tb, a1, &ret, &safety);
 
-    if (!DID_TRAP(p,ret) && ITERATION_SAFETY(p,tb) != ITER_SAFE) {
-	unfix_table_locked(p, tb, &kind);
+    if (!DID_TRAP(p,ret)) {
+        if (safety != ITER_SAFE) {
+            ASSERT(erts_refc_read(&tb->common.fix_count,1));
+            unfix_table_locked(p, tb, &kind);
+        }
     }
     db_unlock(tb, kind);
 
@@ -2805,8 +2801,12 @@ static BIF_RETTYPE ets_select_trap_1(BIF_ALIST_1)
 BIF_RETTYPE ets_select_1(BIF_ALIST_1)
 {
     return ets_select1(BIF_P, BIF_ets_select_1, BIF_ARG_1);
+    /* TRAP: ets_select_trap_1 */
 }
 
+/*
+ * Common impl for select/1, select_reverse/1, match/1 and match_object/1
+ */
 static BIF_RETTYPE ets_select1(Process *p, int bif_ix, Eterm arg1)
 {
     BIF_RETTYPE result;
@@ -2814,7 +2814,7 @@ static BIF_RETTYPE ets_select1(Process *p, int bif_ix, Eterm arg1)
     int cret;
     Eterm ret;
     Eterm *tptr;
-    enum DbIterSafety safety;
+    enum DbIterSafety safety, safety_copy;
 
     CHECK_TABLES();
 
@@ -2839,7 +2839,8 @@ static BIF_RETTYPE ets_select1(Process *p, int bif_ix, Eterm arg1)
 	local_fix_table(tb);
     }
 
-    cret = tb->common.meth->db_select_continue(p,tb, arg1, &ret);
+    safety_copy = safety;
+    cret = tb->common.meth->db_select_continue(p,tb, arg1, &ret, &safety_copy);
 
     if (DID_TRAP(p,ret) && safety != ITER_SAFE) {
 	fix_table_locked(p, tb);
@@ -2871,6 +2872,7 @@ BIF_RETTYPE ets_select_2(BIF_ALIST_2)
     DbTable* tb;
     DB_BIF_GET_TABLE(tb, DB_READ, LCK_READ, BIF_ets_select_2);
     return ets_select2(BIF_P, tb, BIF_ARG_1, BIF_ARG_2);
+    /* TRAP: ets_select_trap_1 */
 }
 
 static BIF_RETTYPE
@@ -2888,7 +2890,7 @@ ets_select2(Process* p, DbTable* tb, Eterm tid, Eterm ms)
 	local_fix_table(tb);
     }
 
-    cret = tb->common.meth->db_select(p, tb, tid, ms, 0, &ret);
+    cret = tb->common.meth->db_select(p, tb, tid, ms, 0, &ret, safety);
 
     if (DID_TRAP(p,ret) && safety != ITER_SAFE) {
 	fix_table_locked(p, tb);
@@ -2926,6 +2928,7 @@ static BIF_RETTYPE ets_select_count_1(BIF_ALIST_1)
     Eterm ret;
     Eterm *tptr;
     db_lock_kind_t kind = LCK_READ;
+    enum DbIterSafety safety = ITER_SAFE;
 
     CHECK_TABLES();
 
@@ -2935,9 +2938,10 @@ static BIF_RETTYPE ets_select_count_1(BIF_ALIST_1)
     DB_TRAP_GET_TABLE(tb, tptr[1], DB_READ, kind,
                       &ets_select_count_continue_exp);
 
-    cret = tb->common.meth->db_select_count_continue(p, tb, a1, &ret);
+    cret = tb->common.meth->db_select_count_continue(p, tb, a1, &ret, &safety);
 
-    if (!DID_TRAP(p,ret) && ITERATION_SAFETY(p,tb) != ITER_SAFE) {
+    if (!DID_TRAP(p,ret) && safety != ITER_SAFE) {
+        ASSERT(erts_refc_read(&tb->common.fix_count,1));
 	unfix_table_locked(p, tb, &kind);
     }
     db_unlock(tb, kind);
@@ -2975,7 +2979,8 @@ BIF_RETTYPE ets_select_count_2(BIF_ALIST_2)
     if (safety == ITER_UNSAFE) {
 	local_fix_table(tb);
     }
-    cret = tb->common.meth->db_select_count(BIF_P,tb, BIF_ARG_1, BIF_ARG_2, &ret);
+    cret = tb->common.meth->db_select_count(BIF_P,tb, BIF_ARG_1, BIF_ARG_2,
+                                            &ret, safety);
 
     if (DID_TRAP(BIF_P,ret) && safety != ITER_SAFE) {
 	fix_table_locked(BIF_P, tb);
@@ -3014,6 +3019,7 @@ static BIF_RETTYPE ets_select_replace_1(BIF_ALIST_1)
     Eterm ret;
     Eterm *tptr;
     db_lock_kind_t kind = LCK_WRITE_REC;
+    enum DbIterSafety safety = ITER_SAFE;
 
     CHECK_TABLES();
     ASSERT(is_tuple(a1));
@@ -3023,9 +3029,10 @@ static BIF_RETTYPE ets_select_replace_1(BIF_ALIST_1)
     DB_TRAP_GET_TABLE(tb, tptr[1], DB_WRITE, kind,
                       &ets_select_replace_continue_exp);
 
-    cret = tb->common.meth->db_select_replace_continue(p,tb,a1,&ret);
+    cret = tb->common.meth->db_select_replace_continue(p,tb,a1,&ret,&safety);
 
-    if(!DID_TRAP(p,ret) && ITERATION_SAFETY(p,tb) != ITER_SAFE) {
+    if(!DID_TRAP(p,ret) && safety != ITER_SAFE) {
+        ASSERT(erts_refc_read(&tb->common.fix_count,1));
         unfix_table_locked(p, tb, &kind);
     }
 
@@ -3068,7 +3075,8 @@ BIF_RETTYPE ets_select_replace_2(BIF_ALIST_2)
     if (safety == ITER_UNSAFE) {
         local_fix_table(tb);
     }
-    cret = tb->common.meth->db_select_replace(BIF_P, tb, BIF_ARG_1, BIF_ARG_2, &ret);
+    cret = tb->common.meth->db_select_replace(BIF_P, tb, BIF_ARG_1, BIF_ARG_2,
+                                              &ret, safety);
 
     if (DID_TRAP(BIF_P,ret) && safety != ITER_SAFE) {
         fix_table_locked(BIF_P,tb);
@@ -3120,7 +3128,7 @@ BIF_RETTYPE ets_select_reverse_3(BIF_ALIST_3)
     }
     cret = tb->common.meth->db_select_chunk(BIF_P,tb, BIF_ARG_1,
 					    BIF_ARG_2, chunk_size, 
-					    1 /* reversed */, &ret);
+					    1 /* reversed */, &ret, safety);
     if (DID_TRAP(BIF_P,ret) && safety != ITER_SAFE) {
 	fix_table_locked(BIF_P, tb);
     }
@@ -3165,7 +3173,7 @@ BIF_RETTYPE ets_select_reverse_2(BIF_ALIST_2)
 	local_fix_table(tb);
     }
     cret = tb->common.meth->db_select(BIF_P,tb, BIF_ARG_1, BIF_ARG_2,
-				      1 /*reversed*/, &ret);
+				      1 /*reversed*/, &ret, safety);
 
     if (DID_TRAP(BIF_P,ret) && safety != ITER_SAFE) {
 	fix_table_locked(BIF_P, tb);
diff --git a/erts/emulator/beam/erl_db_catree.c b/erts/emulator/beam/erl_db_catree.c
index 75ac1c4a93..0402c6b7b4 100644
--- a/erts/emulator/beam/erl_db_catree.c
+++ b/erts/emulator/beam/erl_db_catree.c
@@ -116,24 +116,31 @@ static int db_erase_object_catree(DbTable *tbl, Eterm object,Eterm *ret);
 static int db_slot_catree(Process *p, DbTable *tbl,
                           Eterm slot_term,  Eterm *ret);
 static int db_select_catree(Process *p, DbTable *tbl, Eterm tid,
-                            Eterm pattern, int reversed, Eterm *ret);
+                            Eterm pattern, int reversed, Eterm *ret,
+                            enum DbIterSafety);
 static int db_select_count_catree(Process *p, DbTable *tbl, Eterm tid,
-                                  Eterm pattern,  Eterm *ret);
+                                  Eterm pattern,  Eterm *ret, enum DbIterSafety);
 static int db_select_chunk_catree(Process *p, DbTable *tbl, Eterm tid,
                                   Eterm pattern, Sint chunk_size,
-                                  int reversed, Eterm *ret);
+                                  int reversed, Eterm *ret, enum DbIterSafety);
 static int db_select_continue_catree(Process *p, DbTable *tbl,
-                                     Eterm continuation, Eterm *ret);
+                                     Eterm continuation, Eterm *ret,
+                                     enum DbIterSafety*);
 static int db_select_count_continue_catree(Process *p, DbTable *tbl,
-                                           Eterm continuation, Eterm *ret);
+                                           Eterm continuation, Eterm *ret,
+                                           enum DbIterSafety*);
 static int db_select_delete_catree(Process *p, DbTable *tbl, Eterm tid,
-                                   Eterm pattern,  Eterm *ret);
+                                   Eterm pattern,  Eterm *ret,
+                                   enum DbIterSafety);
 static int db_select_delete_continue_catree(Process *p, DbTable *tbl, 
-                                            Eterm continuation, Eterm *ret);
+                                            Eterm continuation, Eterm *ret,
+                                            enum DbIterSafety*);
 static int db_select_replace_catree(Process *p, DbTable *tbl, Eterm tid,
-                                    Eterm pattern, Eterm *ret);
+                                    Eterm pattern, Eterm *ret,
+                                    enum DbIterSafety);
 static int db_select_replace_continue_catree(Process *p, DbTable *tbl,
-                                             Eterm continuation, Eterm *ret);
+                                             Eterm continuation, Eterm *ret,
+                                             enum DbIterSafety*);
 static int db_take_catree(Process *, DbTable *, Eterm, Eterm *);
 static void db_print_catree(fmtfn_t to, void *to_arg,
                             int show, DbTable *tbl);
@@ -1843,7 +1850,8 @@ static int db_slot_catree(Process *p, DbTable *tbl,
 static int db_select_continue_catree(Process *p,
                                      DbTable *tbl,
                                      Eterm continuation,
-                                     Eterm *ret)
+                                     Eterm *ret,
+                                     enum DbIterSafety* safety_p)
 {
     int result;
     CATreeRootIterator iter;
@@ -1856,7 +1864,8 @@ static int db_select_continue_catree(Process *p,
 }
 
 static int db_select_catree(Process *p, DbTable *tbl, Eterm tid,
-                            Eterm pattern, int reverse, Eterm *ret)
+                            Eterm pattern, int reverse, Eterm *ret,
+                            enum DbIterSafety safety)
 {
     int result;
     CATreeRootIterator iter;
@@ -1871,7 +1880,8 @@ static int db_select_catree(Process *p, DbTable *tbl, Eterm tid,
 static int db_select_count_continue_catree(Process *p,
                                            DbTable *tbl,
                                            Eterm continuation,
-                                           Eterm *ret)
+                                           Eterm *ret,
+                                           enum DbIterSafety* safety_p)
 {
     int result;
     CATreeRootIterator iter;
@@ -1885,7 +1895,8 @@ static int db_select_count_continue_catree(Process *p,
 }
 
 static int db_select_count_catree(Process *p, DbTable *tbl, Eterm tid,
-                                  Eterm pattern, Eterm *ret)
+                                  Eterm pattern, Eterm *ret,
+                                  enum DbIterSafety safety)
 {
     int result;
     CATreeRootIterator iter;
@@ -1899,7 +1910,8 @@ static int db_select_count_catree(Process *p, DbTable *tbl, Eterm tid,
 
 static int db_select_chunk_catree(Process *p, DbTable *tbl, Eterm tid,
                                   Eterm pattern, Sint chunk_size,
-                                  int reversed, Eterm *ret)
+                                  int reversed, Eterm *ret,
+                                  enum DbIterSafety safety)
 {
     int result;
     CATreeRootIterator iter;
@@ -1915,7 +1927,8 @@ static int db_select_chunk_catree(Process *p, DbTable *tbl, Eterm tid,
 static int db_select_delete_continue_catree(Process *p,
                                             DbTable *tbl,
                                             Eterm continuation,
-                                            Eterm *ret)
+                                            Eterm *ret,
+                                            enum DbIterSafety* safety_p)
 {
     DbTreeStack stack;
     TreeDbTerm * stack_array[STACK_NEED];
@@ -1931,7 +1944,8 @@ static int db_select_delete_continue_catree(Process *p,
 }
 
 static int db_select_delete_catree(Process *p, DbTable *tbl, Eterm tid,
-                                   Eterm pattern, Eterm *ret)
+                                   Eterm pattern, Eterm *ret,
+                                   enum DbIterSafety safety)
 {
     DbTreeStack stack;
     TreeDbTerm * stack_array[STACK_NEED];
@@ -1948,7 +1962,8 @@ static int db_select_delete_catree(Process *p, DbTable *tbl, Eterm tid,
 }
 
 static int db_select_replace_catree(Process *p, DbTable *tbl, Eterm tid,
-                                    Eterm pattern, Eterm *ret)
+                                    Eterm pattern, Eterm *ret,
+                                    enum DbIterSafety safety_p)
 {
     int result;
     CATreeRootIterator iter;
@@ -1961,7 +1976,8 @@ static int db_select_replace_catree(Process *p, DbTable *tbl, Eterm tid,
 }
 
 static int db_select_replace_continue_catree(Process *p, DbTable *tbl,
-                                             Eterm continuation, Eterm *ret)
+                                             Eterm continuation, Eterm *ret,
+                                             enum DbIterSafety* safety_p)
 {
     int result;
     CATreeRootIterator iter;
diff --git a/erts/emulator/beam/erl_db_hash.c b/erts/emulator/beam/erl_db_hash.c
index a2690f6d6a..fac4703620 100644
--- a/erts/emulator/beam/erl_db_hash.c
+++ b/erts/emulator/beam/erl_db_hash.c
@@ -404,26 +404,31 @@ static int db_slot_hash(Process *p, DbTable *tbl,
 
 static int db_select_chunk_hash(Process *p, DbTable *tbl, Eterm tid,
 				Eterm pattern, Sint chunk_size,
-				int reverse, Eterm *ret);
+				int reverse, Eterm *ret, enum DbIterSafety);
 static int db_select_hash(Process *p, DbTable *tbl, Eterm tid,
-			  Eterm pattern, int reverse, Eterm *ret);
+			  Eterm pattern, int reverse, Eterm *ret,
+                          enum DbIterSafety);
 static int db_select_continue_hash(Process *p, DbTable *tbl,
-				   Eterm continuation, Eterm *ret);
+				   Eterm continuation, Eterm *ret,
+                                   enum DbIterSafety*);
 
 static int db_select_count_hash(Process *p, DbTable *tbl, Eterm tid,
-				Eterm pattern, Eterm *ret);
+				Eterm pattern, Eterm *ret, enum DbIterSafety);
 static int db_select_count_continue_hash(Process *p, DbTable *tbl,
-					 Eterm continuation, Eterm *ret);
-
+					 Eterm continuation, Eterm *ret,
+                                         enum DbIterSafety*);
 static int db_select_delete_hash(Process *p, DbTable *tbl, Eterm tid,
-				 Eterm pattern, Eterm *ret);
+				 Eterm pattern, Eterm *ret,
+                                 enum DbIterSafety);
 static int db_select_delete_continue_hash(Process *p, DbTable *tbl,
-					  Eterm continuation, Eterm *ret);
+					  Eterm continuation, Eterm *ret,
+                                          enum DbIterSafety*);
 
 static int db_select_replace_hash(Process *p, DbTable *tbl, Eterm tid,
-                                  Eterm pattern, Eterm *ret);
+                                  Eterm pattern, Eterm *ret, enum DbIterSafety);
 static int db_select_replace_continue_hash(Process *p, DbTable *tbl,
-                                           Eterm continuation, Eterm *ret);
+                                           Eterm continuation, Eterm *ret,
+                                           enum DbIterSafety*);
 
 static int db_take_hash(Process *, DbTable *, Eterm, Eterm *);
 static void db_print_hash(fmtfn_t to,
@@ -535,7 +540,7 @@ DbTableMethod db_hash =
     db_select_chunk_hash,
     db_select_hash,
     db_select_delete_hash,
-    db_select_continue_hash, /* hmm continue_hash? */
+    db_select_continue_hash,
     db_select_delete_continue_hash,
     db_select_count_hash,
     db_select_count_continue_hash,
@@ -1209,6 +1214,7 @@ struct traverse_context_t_
     DbTableHash* tb;
     Eterm tid;
     Eterm* prev_continuation_tptr;
+    enum DbIterSafety safety;
 };
 
 
@@ -1476,11 +1482,11 @@ static ERTS_INLINE int on_simple_trap(Export* trap_function,
 
     BUMP_ALL_REDS(ctx->p);
     if (IS_USMALL(0, got)) {
-	hp = HAllocX(ctx->p,  base_halloc_sz + 5, ERTS_MAGIC_REF_THING_SIZE);
+	hp = HAllocX(ctx->p,  base_halloc_sz + 6, ERTS_MAGIC_REF_THING_SIZE);
 	egot = make_small(got);
     }
     else {
-	hp = HAllocX(ctx->p, base_halloc_sz + BIG_UINT_HEAP_SIZE + 5,
+	hp = HAllocX(ctx->p, base_halloc_sz + BIG_UINT_HEAP_SIZE + 6,
                      ERTS_MAGIC_REF_THING_SIZE);
 	egot = uint_to_big(got, hp);
 	hp += BIG_UINT_HEAP_SIZE;
@@ -1497,12 +1503,13 @@ static ERTS_INLINE int on_simple_trap(Export* trap_function,
         mpb = ctx->prev_continuation_tptr[3];
     }
 
-    continuation = TUPLE4(
+    continuation = TUPLE5(
             hp,
             ctx->tid,
             make_small(slot_ix),
             mpb,
-            egot);
+            egot,
+            make_small(ctx->safety));
     ERTS_BIF_PREP_TRAP1(*ret, trap_function, ctx->p, continuation);
     return DB_ERROR_NONE;
 }
@@ -1512,17 +1519,18 @@ static ERTS_INLINE int unpack_simple_continuation(Eterm continuation,
                                                   Eterm* tid_ptr,
                                                   Sint* slot_ix_p,
                                                   Binary** mpp,
-                                                  Sint* got_p)
+                                                  Sint* got_p,
+                                                  enum DbIterSafety* safety_p)
 {
     Eterm* tptr;
     ASSERT(is_tuple(continuation));
     tptr = tuple_val(continuation);
-    if (arityval(*tptr) != 4)
+    if (*tptr != make_arityval(5))
         return 1;
 
-    if (! is_small(tptr[2]) || !(is_big(tptr[4]) || is_small(tptr[4]))) {
+    if (!is_small(tptr[2]) || !(is_big(tptr[4]) || is_small(tptr[4]))
+        || !is_small(tptr[5]))
         return 1;
-    }
 
     *tptr_ptr = tptr;
     *tid_ptr = tptr[1];
@@ -1534,6 +1542,7 @@ static ERTS_INLINE int unpack_simple_continuation(Eterm continuation,
     else {
         *got_p = unsigned_val(tptr[4]);
     }
+    *safety_p = signed_val(tptr[5]);
     return 0;
 }
 
@@ -1657,32 +1666,34 @@ static int select_chunk_on_trap(traverse_context_t* ctx_base,
     if (ctx->base.prev_continuation_tptr == NULL) {
         Eterm tid = ctx->base.tid;
         /* First time we're trapping */
-        hp = HAllocX(ctx->base.p, 7 + ERTS_MAGIC_REF_THING_SIZE,
+        hp = HAllocX(ctx->base.p, 8 + ERTS_MAGIC_REF_THING_SIZE,
                      ERTS_MAGIC_REF_THING_SIZE);
         if (is_atom(tid))
             tid = erts_db_make_tid(ctx->base.p, &ctx->base.tb->common);
         mpb = erts_db_make_match_prog_ref(ctx->base.p, *mpp, &hp);
-        continuation = TUPLE6(
+        continuation = TUPLE7(
                 hp,
                 tid,
                 make_small(slot_ix),
                 make_small(ctx->chunk_size),
                 mpb,
                 ctx->match_list,
-                make_small(got));
+                make_small(got),
+                make_small(ctx->base.safety));
         *mpp = NULL; /* otherwise the caller will destroy it */
     }
     else {
         /* Not the first time we're trapping; reuse continuation terms */
-        hp = HAlloc(ctx->base.p, 7);
-        continuation = TUPLE6(
+        hp = HAlloc(ctx->base.p, 8);
+        continuation = TUPLE7(
                 hp,
                 ctx->base.prev_continuation_tptr[1],
                 make_small(slot_ix),
                 ctx->base.prev_continuation_tptr[3],
                 ctx->base.prev_continuation_tptr[4],
                 ctx->match_list,
-                make_small(got));
+                make_small(got),
+                make_small(ctx->base.safety));
     }
     ERTS_BIF_PREP_TRAP1(*ret, &ets_select_continue_exp, ctx->base.p,
                         continuation);
@@ -1690,14 +1701,14 @@ static int select_chunk_on_trap(traverse_context_t* ctx_base,
 }
 
 static int db_select_hash(Process *p, DbTable *tbl, Eterm tid, Eterm pattern,
-                          int reverse, Eterm *ret)
+                          int reverse, Eterm *ret, enum DbIterSafety safety)
 {
-    return db_select_chunk_hash(p, tbl, tid, pattern, 0, reverse, ret);
+    return db_select_chunk_hash(p, tbl, tid, pattern, 0, reverse, ret, safety);
 }
 
 static int db_select_chunk_hash(Process *p, DbTable *tbl, Eterm tid,
                                 Eterm pattern, Sint chunk_size,
-                                int reverse, Eterm *ret)
+                                int reverse, Eterm *ret, enum DbIterSafety safety)
 {
     select_chunk_context_t ctx;
 
@@ -1709,6 +1720,7 @@ static int db_select_chunk_hash(Process *p, DbTable *tbl, Eterm tid,
     ctx.base.tb = &tbl->hash;
     ctx.base.tid = tid;
     ctx.base.prev_continuation_tptr = NULL;
+    ctx.base.safety = safety;
     ctx.hp = NULL;
     ctx.chunk_size = chunk_size;
     ctx.match_list = NIL;
@@ -1784,10 +1796,11 @@ int select_chunk_continue_on_loop_ended(traverse_context_t* ctx_base,
 }
 
 /*
- * This is called when select traps
+ * This is called when ets:select/1/2/3 traps
+ * and for ets:select/1 with user continuation term.
  */
 static int db_select_continue_hash(Process* p, DbTable* tbl, Eterm continuation,
-                                   Eterm* ret)
+                                   Eterm* ret, enum DbIterSafety* safety_p)
 {
     select_chunk_context_t ctx;
     Eterm* tptr;
@@ -1803,7 +1816,13 @@ static int db_select_continue_hash(Process* p, DbTable* tbl, Eterm continuation,
     ASSERT(is_tuple(continuation));
     tptr = tuple_val(continuation);
 
-    if (arityval(*tptr) != 6)
+    /*
+     * 6-tuple is select/1 user continuation term
+     * 7-tuple is select trap continuation
+     */
+    if (*tptr == make_arityval(7) && is_small(tptr[7]))
+        *safety_p = signed_val(tptr[7]);
+    else if (*tptr != make_arityval(6))
         goto badparam;
 
     if (!is_small(tptr[2]) || !is_small(tptr[3]) ||
@@ -1831,6 +1850,7 @@ static int db_select_continue_hash(Process* p, DbTable* tbl, Eterm continuation,
     ctx.base.tb = &tbl->hash;
     ctx.base.tid = tid;
     ctx.base.prev_continuation_tptr = tptr;
+    ctx.base.safety = *safety_p;
     ctx.hp = NULL;
     ctx.chunk_size = chunk_size;
     ctx.match_list = match_list;
@@ -1891,7 +1911,8 @@ static int select_count_on_trap(traverse_context_t* ctx,
 }
 
 static int db_select_count_hash(Process *p, DbTable *tbl, Eterm tid,
-                                Eterm pattern, Eterm *ret)
+                                Eterm pattern, Eterm *ret,
+                                enum DbIterSafety safety)
 {
     traverse_context_t ctx;
     Sint iterations_left = MAX_SELECT_COUNT_ITERATIONS;
@@ -1905,6 +1926,7 @@ static int db_select_count_hash(Process *p, DbTable *tbl, Eterm tid,
     ctx.tb = &tbl->hash;
     ctx.tid = tid;
     ctx.prev_continuation_tptr = NULL;
+    ctx.safety = safety;
 
     return match_traverse(
             &ctx,
@@ -1917,7 +1939,8 @@ static int db_select_count_hash(Process *p, DbTable *tbl, Eterm tid,
  * This is called when select_count traps
  */
 static int db_select_count_continue_hash(Process* p, DbTable* tbl,
-                                         Eterm continuation, Eterm* ret)
+                                         Eterm continuation, Eterm* ret,
+                                         enum DbIterSafety* safety_p)
 {
     traverse_context_t ctx;
     Eterm* tptr;
@@ -1928,7 +1951,8 @@ static int db_select_count_continue_hash(Process* p, DbTable* tbl,
     Sint chunk_size = 0;
     *ret = NIL;
 
-    if (unpack_simple_continuation(continuation, &tptr, &tid, &slot_ix, &mp, &got)) {
+    if (unpack_simple_continuation(continuation, &tptr, &tid, &slot_ix, &mp,
+                                   &got, safety_p)) {
         *ret = NIL;
         return DB_ERROR_BADPARAM;
     }
@@ -1940,6 +1964,7 @@ static int db_select_count_continue_hash(Process* p, DbTable* tbl,
     ctx.tb = &tbl->hash;
     ctx.tid = tid;
     ctx.prev_continuation_tptr = tptr;
+    ctx.safety = *safety_p;
 
     return match_traverse_continue(
             &ctx, chunk_size,
@@ -2033,7 +2058,8 @@ static int select_delete_on_trap(traverse_context_t* ctx_base,
 }
 
 static int db_select_delete_hash(Process *p, DbTable *tbl, Eterm tid,
-                                 Eterm pattern, Eterm *ret)
+                                 Eterm pattern, Eterm *ret,
+                                 enum DbIterSafety safety)
 {
     select_delete_context_t ctx;
     Sint chunk_size = 0;
@@ -2046,6 +2072,7 @@ static int db_select_delete_hash(Process *p, DbTable *tbl, Eterm tid,
     ctx.base.tb = &tbl->hash;
     ctx.base.tid = tid;
     ctx.base.prev_continuation_tptr = NULL;
+    ctx.base.safety = safety;
     ctx.fixated_by_me = ctx.base.tb->common.is_thread_safe ? 0 : 1;
     ctx.last_pseudo_delete = (Uint) -1;
     ctx.free_us = NULL;
@@ -2062,7 +2089,8 @@ static int db_select_delete_hash(Process *p, DbTable *tbl, Eterm tid,
  * This is called when select_delete traps
  */
 static int db_select_delete_continue_hash(Process* p, DbTable* tbl,
-                                          Eterm continuation, Eterm* ret)
+                                          Eterm continuation, Eterm* ret,
+                                          enum DbIterSafety* safety_p)
 {
     select_delete_context_t ctx;
     Eterm* tptr;
@@ -2072,7 +2100,8 @@ static int db_select_delete_continue_hash(Process* p, DbTable* tbl,
     Sint slot_ix;
     Sint chunk_size = 0;
 
-    if (unpack_simple_continuation(continuation, &tptr, &tid, &slot_ix, &mp, &got)) {
+    if (unpack_simple_continuation(continuation, &tptr, &tid, &slot_ix, &mp,
+                                   &got, safety_p)) {
         *ret = NIL;
         return DB_ERROR_BADPARAM;
     }
@@ -2084,6 +2113,7 @@ static int db_select_delete_continue_hash(Process* p, DbTable* tbl,
     ctx.base.tb = &tbl->hash;
     ctx.base.tid = tid;
     ctx.base.prev_continuation_tptr = tptr;
+    ctx.base.safety = *safety_p;
     ctx.fixated_by_me = ONLY_WRITER(p, ctx.base.tb) ? 0 : 1;
     ctx.last_pseudo_delete = (Uint) -1;
     ctx.free_us = NULL;
@@ -2164,7 +2194,9 @@ static int select_replace_on_trap(traverse_context_t* ctx,
             slot_ix, got, mpp, ret);
 }
 
-static int db_select_replace_hash(Process *p, DbTable *tbl, Eterm tid, Eterm pattern, Eterm *ret)
+static int db_select_replace_hash(Process *p, DbTable *tbl, Eterm tid,
+                                  Eterm pattern, Eterm *ret,
+                                  enum DbIterSafety safety)
 {
     traverse_context_t ctx;
     Sint chunk_size = 0;
@@ -2194,7 +2226,9 @@ static int db_select_replace_hash(Process *p, DbTable *tbl, Eterm tid, Eterm pat
 /*
  * This is called when select_replace traps
  */
-static int db_select_replace_continue_hash(Process* p, DbTable* tbl, Eterm continuation, Eterm* ret)
+static int db_select_replace_continue_hash(Process* p, DbTable* tbl,
+                                           Eterm continuation, Eterm* ret,
+                                           enum DbIterSafety* safety_p)
 {
     traverse_context_t ctx;
     Eterm* tptr;
@@ -2205,7 +2239,8 @@ static int db_select_replace_continue_hash(Process* p, DbTable* tbl, Eterm conti
     Sint chunk_size = 0;
     *ret = NIL;
 
-    if (unpack_simple_continuation(continuation, &tptr, &tid, &slot_ix, &mp, &got)) {
+    if (unpack_simple_continuation(continuation, &tptr, &tid, &slot_ix, &mp,
+                                   &got, safety_p)) {
         *ret = NIL;
         return DB_ERROR_BADPARAM;
     }
@@ -2218,6 +2253,7 @@ static int db_select_replace_continue_hash(Process* p, DbTable* tbl, Eterm conti
     ctx.tb = &tbl->hash;
     ctx.tid = tid;
     ctx.prev_continuation_tptr = tptr;
+    ctx.safety = *safety_p;
 
     return match_traverse_continue(
             &ctx, chunk_size,
diff --git a/erts/emulator/beam/erl_db_tree.c b/erts/emulator/beam/erl_db_tree.c
index fe57348700..f9ba04f399 100644
--- a/erts/emulator/beam/erl_db_tree.c
+++ b/erts/emulator/beam/erl_db_tree.c
@@ -397,24 +397,31 @@ static int db_erase_object_tree(DbTable *tbl, Eterm object,Eterm *ret);
 static int db_slot_tree(Process *p, DbTable *tbl, 
 			Eterm slot_term,  Eterm *ret);
 static int db_select_tree(Process *p, DbTable *tbl, Eterm tid,
-			  Eterm pattern, int reversed, Eterm *ret);
+			  Eterm pattern, int reversed, Eterm *ret,
+                          enum DbIterSafety);
 static int db_select_count_tree(Process *p, DbTable *tbl, Eterm tid,
-				Eterm pattern,  Eterm *ret);
+				Eterm pattern,  Eterm *ret, enum DbIterSafety);
 static int db_select_chunk_tree(Process *p, DbTable *tbl, Eterm tid,
 				Eterm pattern, Sint chunk_size,
-				int reversed, Eterm *ret);
+				int reversed, Eterm *ret, enum DbIterSafety);
 static int db_select_continue_tree(Process *p, DbTable *tbl,
-				   Eterm continuation, Eterm *ret);
+				   Eterm continuation, Eterm *ret,
+                                   enum DbIterSafety*);
 static int db_select_count_continue_tree(Process *p, DbTable *tbl,
-					 Eterm continuation, Eterm *ret);
+					 Eterm continuation, Eterm *ret,
+                                         enum DbIterSafety*);
 static int db_select_delete_tree(Process *p, DbTable *tbl, Eterm tid,
-				 Eterm pattern,  Eterm *ret);
+				 Eterm pattern,  Eterm *ret,
+                                 enum DbIterSafety);
 static int db_select_delete_continue_tree(Process *p, DbTable *tbl, 
-					  Eterm continuation, Eterm *ret);
+					  Eterm continuation, Eterm *ret,
+                                          enum DbIterSafety*);
 static int db_select_replace_tree(Process *p, DbTable *tbl, Eterm tid,
-                                  Eterm pattern, Eterm *ret);
+                                  Eterm pattern, Eterm *ret,
+                                  enum DbIterSafety);
 static int db_select_replace_continue_tree(Process *p, DbTable *tbl,
-                                           Eterm continuation, Eterm *ret);
+                                           Eterm continuation, Eterm *ret,
+                                           enum DbIterSafety*);
 static int db_take_tree(Process *, DbTable *, Eterm, Eterm *);
 static void db_print_tree(fmtfn_t to, void *to_arg,
 			  int show, DbTable *tbl);
@@ -1160,7 +1167,8 @@ int db_select_continue_tree_common(Process *p,
 static int db_select_continue_tree(Process *p, 
 				   DbTable *tbl,
 				   Eterm continuation,
-				   Eterm *ret)
+				   Eterm *ret,
+                                   enum DbIterSafety* safety_p)
 {
     DbTableTree *tb = &tbl->tree;
     return db_select_continue_tree_common(p, &tb->common,
@@ -1297,7 +1305,8 @@ int db_select_tree_common(Process *p, DbTable *tb,
 }
 
 static int db_select_tree(Process *p, DbTable *tbl, Eterm tid,
-			  Eterm pattern, int reverse, Eterm *ret)
+			  Eterm pattern, int reverse, Eterm *ret,
+                          enum DbIterSafety safety)
 {
     return db_select_tree_common(p, tbl, tid,
                                  pattern, reverse, ret, &tbl->tree, NULL);
@@ -1408,7 +1417,8 @@ int db_select_count_continue_tree_common(Process *p,
 static int db_select_count_continue_tree(Process *p, 
                                          DbTable *tbl,
                                          Eterm continuation,
-                                         Eterm *ret)
+                                         Eterm *ret,
+                                         enum DbIterSafety* safety_p)
 {
     DbTableTree *tb = &tbl->tree;
     return db_select_count_continue_tree_common(p, tbl,
@@ -1527,7 +1537,8 @@ int db_select_count_tree_common(Process *p, DbTable *tb,
 }
 
 static int db_select_count_tree(Process *p, DbTable *tbl, Eterm tid,
-                                Eterm pattern, Eterm *ret)
+                                Eterm pattern, Eterm *ret,
+                                enum DbIterSafety safety)
 {
     DbTableTree *tb = &tbl->tree;
     return db_select_count_tree_common(p, tbl,
@@ -1704,7 +1715,7 @@ int db_select_chunk_tree_common(Process *p, DbTable *tb,
 static int db_select_chunk_tree(Process *p, DbTable *tbl, Eterm tid,
                                 Eterm pattern, Sint chunk_size,
                                 int reverse,
-                                Eterm *ret)
+                                Eterm *ret, enum DbIterSafety safety)
 {
     DbTableTree *tb = &tbl->tree;
     return db_select_chunk_tree_common(p, tbl,
@@ -1813,7 +1824,8 @@ int db_select_delete_continue_tree_common(Process *p,
 static int db_select_delete_continue_tree(Process *p, 
 					  DbTable *tbl,
 					  Eterm continuation,
-					  Eterm *ret)
+					  Eterm *ret,
+                                          enum DbIterSafety* safety_p)
 {
     DbTableTree *tb = &tbl->tree;
     ASSERT(!erts_atomic_read_nob(&tb->is_stack_busy));
@@ -1942,7 +1954,8 @@ int db_select_delete_tree_common(Process *p, DbTable *tbl,
 }
 
 static int db_select_delete_tree(Process *p, DbTable *tbl, Eterm tid,
-				 Eterm pattern, Eterm *ret)
+				 Eterm pattern, Eterm *ret,
+                                 enum DbIterSafety safety)
 {
     DbTableTree *tb = &tbl->tree;
     return db_select_delete_tree_common(p, tbl, tid, pattern, ret,
@@ -2052,7 +2065,8 @@ int db_select_replace_continue_tree_common(Process *p,
 static int db_select_replace_continue_tree(Process *p,
                                            DbTable *tbl,
                                            Eterm continuation,
-                                           Eterm *ret)
+                                           Eterm *ret,
+                                           enum DbIterSafety* safety_p)
 {
     return db_select_replace_continue_tree_common(p, tbl, continuation, ret,
                                                   &tbl->tree, NULL);
@@ -2177,7 +2191,8 @@ int db_select_replace_tree_common(Process *p, DbTable *tbl,
 }
 
 static int db_select_replace_tree(Process *p, DbTable *tbl, Eterm tid,
-                                  Eterm pattern, Eterm *ret)
+                                  Eterm pattern, Eterm *ret,
+                                  enum DbIterSafety safety)
 {
     return db_select_replace_tree_common(p, tbl, tid, pattern, ret,
                                          &tbl->tree, NULL);
diff --git a/erts/emulator/beam/erl_db_util.h b/erts/emulator/beam/erl_db_util.h
index e1af9210ea..e3d3c0e804 100644
--- a/erts/emulator/beam/erl_db_util.h
+++ b/erts/emulator/beam/erl_db_util.h
@@ -101,6 +101,14 @@ typedef struct {
     } u;
 } DbUpdateHandle;
 
+/* How safe are we from double-hits or missed objects
+ * when iterating without fixation?
+ */
+enum DbIterSafety {
+    ITER_UNSAFE,      /* Must fixate to be safe */
+    ITER_SAFE_LOCKED, /* Safe while table is locked, not between trap calls */
+    ITER_SAFE         /* No need to fixate at all */
+};
 
 typedef struct db_table_method
 {
@@ -150,44 +158,53 @@ typedef struct db_table_method
 			   Eterm pattern,
 			   Sint chunk_size,
 			   int reverse,
-			   Eterm* ret);
+			   Eterm* ret,
+                           enum DbIterSafety);
     int (*db_select)(Process* p, 
 		     DbTable* tb, /* [in out] */
                      Eterm tid,
 		     Eterm pattern,
 		     int reverse,
-		     Eterm* ret);
+		     Eterm* ret,
+                     enum DbIterSafety);
     int (*db_select_delete)(Process* p, 
 			    DbTable* tb, /* [in out] */
                             Eterm tid,
 			    Eterm pattern,
-			    Eterm* ret);
+			    Eterm* ret,
+                            enum DbIterSafety);
     int (*db_select_continue)(Process* p, 
 			      DbTable* tb, /* [in out] */
 			      Eterm continuation,
-			      Eterm* ret);
+			      Eterm* ret,
+                              enum DbIterSafety*);
     int (*db_select_delete_continue)(Process* p, 
 				     DbTable* tb, /* [in out] */
 				     Eterm continuation,
-				     Eterm* ret);
+				     Eterm* ret,
+                                     enum DbIterSafety*);
     int (*db_select_count)(Process* p, 
 			   DbTable* tb, /* [in out] */
                            Eterm tid,
 			   Eterm pattern, 
-			   Eterm* ret);
+			   Eterm* ret,
+                           enum DbIterSafety);
     int (*db_select_count_continue)(Process* p, 
 				    DbTable* tb, /* [in out] */ 
 				    Eterm continuation, 
-				    Eterm* ret);
+				    Eterm* ret,
+                                    enum DbIterSafety*);
     int (*db_select_replace)(Process* p,
             DbTable* tb, /* [in out] */
             Eterm tid,
             Eterm pattern,
-            Eterm* ret);
+            Eterm* ret,
+            enum DbIterSafety);
     int (*db_select_replace_continue)(Process* p,
             DbTable* tb, /* [in out] */
             Eterm continuation,
-            Eterm* ret);
+            Eterm* ret,
+            enum DbIterSafety*);
     int (*db_take)(Process *, DbTable *, Eterm, Eterm *);
 
     SWord (*db_delete_all_objects)(Process* p, DbTable* db, SWord reds);
diff --git a/lib/stdlib/test/ets_SUITE.erl b/lib/stdlib/test/ets_SUITE.erl
index 7703198c4c..8561491d50 100644
--- a/lib/stdlib/test/ets_SUITE.erl
+++ b/lib/stdlib/test/ets_SUITE.erl
@@ -56,6 +56,7 @@
 -export([t_match_spec_run/1]).
 -export([t_bucket_disappears/1]).
 -export([t_named_select/1]).
+-export([select_fixtab_owner_change/1]).
 -export([otp_5340/1]).
 -export([otp_6338/1]).
 -export([otp_6842_select_1000/1]).
@@ -130,7 +131,7 @@ all() ->
      t_insert_list, t_test_ms, t_select_delete, t_select_replace,
      t_select_replace_next_bug,
      t_ets_dets, memory, t_select_reverse, t_bucket_disappears,
-     t_named_select,
+     t_named_select, select_fixtab_owner_change,
      select_fail, t_insert_new, t_repair_continuation,
      otp_5340, otp_6338, otp_6842_select_1000, otp_7665,
      select_mbuf_trapping,
@@ -249,7 +250,64 @@ t_named_select_do(Opts) ->
     verify_etsmem(EtsMem).
 
 
+%% Verify select and friends release fixtab as they should
+%% even when owneship is changed between traps.
+select_fixtab_owner_change(_Config) ->
+    T = ets:new(xxx, [protected]),
+    NKeys = 2000,
+    [ets:insert(T,{K,K band 7}) || K <- lists:seq(1,NKeys)],
 
+    %% Buddy and Papa will ping-pong table ownership between them
+    %% and the aim is to give Buddy the table when he is
+    %% in the middle of a yielding select* call.
+    {Buddy,_} = spawn_opt(fun() -> sfoc_buddy_loop(T, 1, undefined) end,
+                          [link,monitor]),
+
+    sfoc_papa_loop(T, Buddy),
+
+    receive {'DOWN', _, process, Buddy, _} -> ok end,
+    ets:delete(T),
+    ok.
+
+sfoc_buddy_loop(T, I, State0) ->
+    receive
+        {'ETS-TRANSFER', T, Papa, _} ->
+            ets:give_away(T, Papa, State0),
+            case State0 of
+                done ->
+                    ok;
+                _ ->
+                    State1 = sfoc_traverse(T, I, State0),
+                    %% Verify no fixation left
+                    {I, false} = {I, ets:info(T, safe_fixed_monotonic_time)},
+                    sfoc_buddy_loop(T, I+1, State1)
+            end
+    end.
+
+sfoc_papa_loop(T, Buddy) ->
+    ets:give_away(T, Buddy, "Catch!"),
+    receive
+        {'ETS-TRANSFER', T, Buddy, State} ->
+            case State of
+                done ->
+                    ok;
+                _ ->
+                    sfoc_papa_loop(T, Buddy)
+            end
+    end.
+
+sfoc_traverse(T, 1, S) ->
+    ets:select(T, [{{'$1',7}, [], ['$1']}]), S;
+sfoc_traverse(T, 2, S) ->
+    0 = ets:select_count(T, [{{'$1',7}, [], [false]}]), S;
+sfoc_traverse(T, 3, _) ->
+    Limit = ets:info(T, size) div 2,
+    {_, Continuation} = ets:select(T, [{{'$1',7}, [], ['$1']}],
+                                   Limit),
+    Continuation;
+sfoc_traverse(_T, 4, Continuation) ->
+    _ = ets:select(Continuation),
+    done.
 
 %% Check ets:match_spec_run/2.
 t_match_spec_run(Config) when is_list(Config) ->