Merge branch 'john/erts/nif-binary-iovec-fixes/OTP-14845/OTP-14931/OTP-14932'

* john/erts/nif-binary-iovec-fixes/OTP-14845/OTP-14931/OTP-14932:
  Suggest using enif_make_new_binary when possible
  Make enif_make_binary return heap binaries if possible
  Expand ErlNifBinary by another word for future additions
  Unconditionally transfer ownership to the created term
  Tweak asserts and explain why the copy buf doesn't use the tmp obj list
  Free temporary iovecs through the tmp object list
  Add a helper function for attaching tmp objects to NIF environments

3 files changed, 125 insertions, 89 deletions

diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml
index 23f0c66429..9655fcd994 100644
--- a/erts/doc/src/erl_nif.xml
+++ b/erts/doc/src/erl_nif.xml
@@ -974,6 +974,10 @@ typedef struct {
           <seealso marker="#enif_make_binary"><c>enif_make_binary</c></seealso>.
           An allocated (and owned) <c>ErlNifBinary</c> can be kept between NIF
           calls.</p>
+        <p>If you do not need to reallocate or keep the data alive across NIF
+          calls, consider using <seealso marker="#enif_make_new_binary">
+          <c>enif_make_new_binary</c></seealso> instead as it will allocate
+          small binaries on the process heap when possible.</p>
         <p>Returns <c>true</c> on success, or <c>false</c> if allocation
           fails.</p>
       </desc>
diff --git a/erts/emulator/beam/erl_nif.c b/erts/emulator/beam/erl_nif.c
index 7db4af8919..d2000aa71e 100644
--- a/erts/emulator/beam/erl_nif.c
+++ b/erts/emulator/beam/erl_nif.c
@@ -237,9 +237,11 @@ static void cache_env(ErlNifEnv* env);
 static void full_flush_env(ErlNifEnv *env);
 static void flush_env(ErlNifEnv* env);
 
-/* Temporary object header, auto-deallocated when NIF returns
- * or when independent environment is cleared.
- */
+/* Temporary object header, auto-deallocated when NIF returns or when
+ * independent environment is cleared.
+ *
+ * The payload can be accessed with &tmp_obj_ptr[1] but keep in mind that its
+ * first element must not require greater alignment than `next`. */
 struct enif_tmp_obj_t {
     struct enif_tmp_obj_t* next;
     void (*dtor)(struct enif_tmp_obj_t*);
@@ -256,6 +258,46 @@ static ERTS_INLINE void free_tmp_objs(ErlNifEnv* env)
     }
 }
 
+/* Whether the given environment is bound to a process and will be cleaned up
+ * when the NIF returns. It's safe to use temp_alloc for objects in
+ * env->tmp_obj_list when this is true. */
+static ERTS_INLINE int is_proc_bound(ErlNifEnv *env)
+{
+    return env->mod_nif != NULL;
+}
+
+/* Allocates and attaches an object to the given environment, running its
+ * destructor when the environment is cleared. To avoid temporary variables the
+ * address of the allocated object is returned instead of the enif_tmp_obj_t.
+ *
+ * The destructor *must* call `erts_free(tmp_obj->allocator, tmp_obj)` to free
+ * the object. If the destructor needs to refer to the allocated object its
+ * address will be &tmp_obj[1]. */
+static ERTS_INLINE void *alloc_tmp_obj(ErlNifEnv *env, size_t size,
+                                       void (*dtor)(struct enif_tmp_obj_t*)) {
+    struct enif_tmp_obj_t *tmp_obj;
+    ErtsAlcType_t allocator;
+
+    allocator = is_proc_bound(env) ? ERTS_ALC_T_TMP : ERTS_ALC_T_NIF;
+
+    tmp_obj = erts_alloc(allocator, sizeof(struct enif_tmp_obj_t) + MAX(1, size));
+
+    tmp_obj->next = env->tmp_obj_list;
+    tmp_obj->allocator = allocator;
+    tmp_obj->dtor = dtor;
+
+    env->tmp_obj_list = tmp_obj;
+
+    return (void*)&tmp_obj[1];
+}
+
+/* Generic destructor for objects allocated through alloc_tmp_obj that don't
+ * care about their payload. */
+static void tmp_alloc_dtor(struct enif_tmp_obj_t *tmp_obj)
+{
+    erts_free(tmp_obj->allocator, tmp_obj);
+}
+
 void erts_post_nif(ErlNifEnv* env)
 {
     erts_unblock_fpe(env->fpe_was_unmasked);
@@ -446,6 +488,7 @@ static void cache_env(ErlNifEnv* env)
 	env->hp_end = env->heap_frag->mem + env->heap_frag->alloc_size;
     }
 }
+
 void* enif_priv_data(ErlNifEnv* env)
 {
     return env->mod_nif->priv_data;
@@ -1019,11 +1062,6 @@ int enif_is_number(ErlNifEnv* env, ERL_NIF_TERM term)
     return is_number(term);
 }
 
-static ERTS_INLINE int is_proc_bound(ErlNifEnv* env)
-{
-    return env->mod_nif != NULL;
-}
-
 static void aligned_binary_dtor(struct enif_tmp_obj_t* obj)
 {
     erts_free_aligned_binary_bytes_extra((byte*)obj, obj->allocator);
@@ -1058,22 +1096,14 @@ int enif_inspect_binary(ErlNifEnv* env, Eterm bin_term, ErlNifBinary* bin)
 	u.tmp->dtor = &aligned_binary_dtor;
 	env->tmp_obj_list = u.tmp;
     }
-    bin->bin_term = bin_term;
     bin->size = binary_size(bin_term);
     bin->ref_bin = NULL;
     ADD_READONLY_CHECK(env, bin->data, bin->size);
     return 1;
 }
 
-static void tmp_alloc_dtor(struct enif_tmp_obj_t* obj)
-{
-    erts_free(obj->allocator,  obj);
-}
-
 int enif_inspect_iolist_as_binary(ErlNifEnv* env, Eterm term, ErlNifBinary* bin)
 {
-    struct enif_tmp_obj_t* tobj;
-    ErtsAlcType_t allocator;
     ErlDrvSizeT sz;
     if (is_binary(term)) {
 	return enif_inspect_binary(env,term,bin);
@@ -1081,7 +1111,6 @@ int enif_inspect_iolist_as_binary(ErlNifEnv* env, Eterm term, ErlNifBinary* bin)
     if (is_nil(term)) {
 	bin->data = (unsigned char*) &bin->data; /* dummy non-NULL */
 	bin->size = 0;
-	bin->bin_term = THE_NON_VALUE;
 	bin->ref_bin = NULL;
 	return 1;
     }
@@ -1089,16 +1118,8 @@ int enif_inspect_iolist_as_binary(ErlNifEnv* env, Eterm term, ErlNifBinary* bin)
 	return 0;
     }
 
-    allocator = is_proc_bound(env) ? ERTS_ALC_T_TMP : ERTS_ALC_T_NIF;
-    tobj = erts_alloc(allocator, sz + sizeof(struct enif_tmp_obj_t));
-    tobj->allocator = allocator;
-    tobj->next = env->tmp_obj_list;
-    tobj->dtor = &tmp_alloc_dtor;
-    env->tmp_obj_list = tobj;
-
-    bin->data = (unsigned char*) &tobj[1]; 
+    bin->data = alloc_tmp_obj(env, sz, &tmp_alloc_dtor);
     bin->size = sz;
-    bin->bin_term = THE_NON_VALUE;
     bin->ref_bin = NULL;
     erts_iolist_to_buf(term, (char*) bin->data, sz);
     ADD_READONLY_CHECK(env, bin->data, bin->size); 
@@ -1116,7 +1137,6 @@ int enif_alloc_binary(size_t size, ErlNifBinary* bin)
 
     bin->size = size;
     bin->data = (unsigned char*) refbin->orig_bytes;
-    bin->bin_term = THE_NON_VALUE;
     bin->ref_bin = refbin;
     return 1;
 }
@@ -1150,12 +1170,10 @@ void enif_release_binary(ErlNifBinary* bin)
 {
     if (bin->ref_bin != NULL) {
 	Binary* refbin = bin->ref_bin;
-	ASSERT(bin->bin_term == THE_NON_VALUE);
         erts_bin_release(refbin);
     }
 #ifdef DEBUG
     bin->data = NULL;
-    bin->bin_term = THE_NON_VALUE;
     bin->ref_bin = NULL;
 #endif
 }
@@ -1312,29 +1330,51 @@ int enif_get_string(ErlNifEnv *env, ERL_NIF_TERM list, char* buf, unsigned len,
 
 Eterm enif_make_binary(ErlNifEnv* env, ErlNifBinary* bin)
 {
-    if (bin->bin_term != THE_NON_VALUE) {
-	return bin->bin_term;
-    }
-    else if (bin->ref_bin != NULL) {
-	Binary* bptr = bin->ref_bin;
-	Eterm bin_term;
-	
-        bin_term = erts_build_proc_bin(&MSO(env->proc),
-                                       alloc_heap(env, PROC_BIN_SIZE),
-                                       bptr);
-	if (erts_refc_read(&bptr->intern.refc, 1) == 1) {
-	    /* Total ownership transfer */
-	    bin->ref_bin = NULL;
-	    bin->bin_term = bin_term;
-	}
-	return bin_term;
-    }
-    else {
-	flush_env(env);
-	bin->bin_term = new_binary(env->proc, bin->data, bin->size);
-	cache_env(env);
-	return bin->bin_term;
+    Eterm bin_term;
+
+    if (bin->ref_bin != NULL) {
+        Binary* binary = bin->ref_bin;
+
+        /* If the binary is smaller than the heap binary limit we'll return a
+         * heap binary to reduce the number of small refc binaries in the
+         * system. We can't simply release the refc binary right away however;
+         * the documentation states that the binary should be considered
+         * read-only from this point on, which implies that it should still be
+         * readable.
+         *
+         * We could keep it alive until we return by adding it to the temporary
+         * object list, but that requires an off-heap allocation which is
+         * potentially quite slow, so we create a dummy ProcBin instead and
+         * rely on the next minor GC to get rid of it. */
+        if (bin->size <= ERL_ONHEAP_BIN_LIMIT) {
+            ErlHeapBin* hb;
+
+            hb = (ErlHeapBin*)alloc_heap(env, heap_bin_size(bin->size));
+            hb->thing_word = header_heap_bin(bin->size);
+            hb->size = bin->size;
+
+            sys_memcpy(hb->data, bin->data, bin->size);
+
+            erts_build_proc_bin(&MSO(env->proc),
+                                alloc_heap(env, PROC_BIN_SIZE),
+                                binary);
+
+            bin_term = make_binary(hb);
+        } else {
+            bin_term = erts_build_proc_bin(&MSO(env->proc),
+                                           alloc_heap(env, PROC_BIN_SIZE),
+                                           binary);
+        }
+
+        /* Our (possibly shared) ownership has been transferred to the term. */
+        bin->ref_bin = NULL;
+    } else {
+        flush_env(env);
+        bin_term = new_binary(env->proc, bin->data, bin->size);
+        cache_env(env);
     }
+
+    return bin_term;
 }
 
 Eterm enif_make_sub_binary(ErlNifEnv* env, ERL_NIF_TERM bin_term,
@@ -3380,7 +3420,6 @@ static void marshal_iovec_binary(Eterm binary, ErlNifBinary *copy_buffer,
     parent_header = binary_val(parent_binary);
 
     result->size = binary_size(binary);
-    result->bin_term = binary;
 
     if (thing_subtag(*parent_header) == REFC_BINARY_SUBTAG) {
         ProcBin *pb = (ProcBin*)parent_header;
@@ -3408,6 +3447,7 @@ static void marshal_iovec_binary(Eterm binary, ErlNifBinary *copy_buffer,
      * and reference that instead. */
 
     if (result->ref_bin == NULL || bit_offset != 0) {
+        ASSERT(copy_buffer->ref_bin != NULL && copy_buffer->data != NULL);
         ASSERT(result->size <= (copy_buffer->size - *copy_offset));
 
         if (bit_offset == 0) {
@@ -3429,8 +3469,8 @@ static void marshal_iovec_binary(Eterm binary, ErlNifBinary *copy_buffer,
 static int fill_iovec_with_slice(ErlNifEnv *env,
                                  iovec_slice_t *slice,
                                  ErlNifIOVec *iovec) {
+    ErlNifBinary copy_buffer = {0};
     UWord copy_offset, iovec_idx;
-    ErlNifBinary copy_buffer;
     Eterm sublist_iterator;
 
     /* Set up a common refc binary for all on-heap and unaligned binaries. */
@@ -3438,11 +3478,8 @@ static int fill_iovec_with_slice(ErlNifEnv *env,
         if (!enif_alloc_binary(slice->copied_size, &copy_buffer)) {
             return 0;
         }
-    } else {
-#ifdef DEBUG
-        copy_buffer.data = NULL;
-        copy_buffer.size = 0;
-#endif
+
+        ASSERT(copy_buffer.ref_bin != NULL);
     }
 
     sublist_iterator = slice->sublist_start;
@@ -3492,9 +3529,11 @@ static int fill_iovec_with_slice(ErlNifEnv *env,
         }
     } else {
         if (slice->copied_size > 0) {
-            /* Attach the binary to our environment and let the GC take care of
-             * it after returning. */
-            enif_make_binary(env, &copy_buffer);
+            /* Attach the binary to our environment and let the next minor GC
+             * get rid of it. This is slightly faster than using the tmp object
+             * list since it avoids off-heap allocations. */
+            erts_build_proc_bin(&MSO(env->proc),
+                alloc_heap(env, PROC_BIN_SIZE), copy_buffer.ref_bin);
         }
     }
 
@@ -3520,19 +3559,14 @@ static int create_iovec_from_slice(ErlNifEnv *env,
         alloc_size = binv_offset;
         alloc_size += slice->iovec_len * sizeof(Binary*);
 
-        /* If we have an environment we'll attach the allocated data to it. The
-         * GC will take care of releasing it later on. */
+        /* When the user passes an environment, we attach the iovec to it so
+         * the user won't have to bother managing it (similar to
+         * enif_inspect_binary). It'll disappear once the environment is
+         * cleaned up. */
         if (env != NULL) {
-            ErlNifBinary gc_bin;
-
-            if (!enif_alloc_binary(alloc_size, &gc_bin)) {
-                return 0;
-            }
-
-            alloc_base = (char*)gc_bin.data;
-            enif_make_binary(env, &gc_bin);
+            alloc_base = alloc_tmp_obj(env, alloc_size, &tmp_alloc_dtor);
         } else {
-            alloc_base = enif_alloc(alloc_size);
+            alloc_base = erts_alloc(ERTS_ALC_T_NIF, alloc_size);
         }
 
         iovec = (ErlNifIOVec*)alloc_base;
@@ -3546,7 +3580,7 @@ static int create_iovec_from_slice(ErlNifEnv *env,
 
     if(!fill_iovec_with_slice(env, slice, iovec)) {
         if (env == NULL && !(iovec->flags & ERL_NIF_IOVEC_FLAGS_PREALLOC)) {
-            enif_free(iovec);
+            erts_free(ERTS_ALC_T_NIF, iovec);
         }
 
         return 0;
@@ -4236,34 +4270,31 @@ static unsigned calc_checksum(unsigned char* ptr, unsigned size);
 
 struct readonly_check_t
 {
-    struct enif_tmp_obj_t hdr;
     unsigned char* ptr;
     unsigned size;
     unsigned checksum;
 };
 static void add_readonly_check(ErlNifEnv* env, unsigned char* ptr, unsigned sz)
 {
-    ErtsAlcType_t allocator = is_proc_bound(env) ? ERTS_ALC_T_TMP : ERTS_ALC_T_NIF;
-    struct readonly_check_t* obj = erts_alloc(allocator, 
-					      sizeof(struct readonly_check_t));
-    obj->hdr.allocator = allocator;
-    obj->hdr.next = env->tmp_obj_list;
-    env->tmp_obj_list = &obj->hdr;
-    obj->hdr.dtor = &readonly_check_dtor;
+    struct readonly_check_t* obj;
+
+    obj = alloc_tmp_obj(env, sizeof(struct readonly_check_t),
+        &readonly_check_dtor);
+
     obj->ptr = ptr;
     obj->size = sz;
-    obj->checksum = calc_checksum(ptr, sz);    
+    obj->checksum = calc_checksum(ptr, sz);
 }
-static void readonly_check_dtor(struct enif_tmp_obj_t* o)
+static void readonly_check_dtor(struct enif_tmp_obj_t* tmp_obj)
 {
-    struct readonly_check_t* obj = (struct readonly_check_t*) o;
-    unsigned chksum = calc_checksum(obj->ptr, obj->size);
-    if (chksum != obj->checksum) { 
+    struct readonly_check_t* ro_check = (struct readonly_check_t*)&tmp_obj[1];
+    unsigned chksum = calc_checksum(ro_check->ptr, ro_check->size);
+    if (chksum != ro_check->checksum) { 
 	fprintf(stderr, "\r\nReadonly data written by NIF, checksums differ"
-		" %x != %x\r\nABORTING\r\n", chksum, obj->checksum);
+		" %x != %x\r\nABORTING\r\n", chksum, ro_check->checksum);
 	abort();
     }
-    erts_free(obj->hdr.allocator,  obj);
+    erts_free(tmp_obj->allocator, tmp_obj);
 }
 static unsigned calc_checksum(unsigned char* ptr, unsigned size)
 {
diff --git a/erts/emulator/beam/erl_nif.h b/erts/emulator/beam/erl_nif.h
index 053f7673c4..a99b4db705 100644
--- a/erts/emulator/beam/erl_nif.h
+++ b/erts/emulator/beam/erl_nif.h
@@ -137,8 +137,9 @@ typedef struct
     unsigned char* data;
 
     /* Internals (avert your eyes) */
-    ERL_NIF_TERM bin_term;
     void* ref_bin;
+    /* for future additions to be ABI compatible (same struct size) */
+    void* __spare__[2];
 }ErlNifBinary;
 
 #if (defined(__WIN32__) || defined(_WIN32) || defined(_WIN32_))