Enforced that upb_msg lives in an Arena only, and other simplifying.

upb_msg was trying to be general enough that it could either live in
an arena or be allocated with malloc()/free().  This was too much
complexity for too little benefit.  We should commit to just saying
that upb_msg is arena-only.

I also ripped out the code to glue upb_msg to the existing
handlers-based encoder/decoder.  upb_msg has its own, small, simple
encoder/decoder.  I'm trying to whittle down upb_msg to a small
and simple core.

I updated the Lua extension for these changes.  Lua needs some more
work to properly create arenas per message.  For now I just created
a single global arena.

8 files changed, 129 insertions, 725 deletions

diff --git a/tests/bindings/lua/test_upb.pb.lua b/tests/bindings/lua/test_upb.pb.lua
index 106dc55..752a736 100644
--- a/tests/bindings/lua/test_upb.pb.lua
+++ b/tests/bindings/lua/test_upb.pb.lua
@@ -27,25 +27,13 @@ local symtab = upb.SymbolTable{
 local factory = upb.MessageFactory(symtab);
 local TestMessage = factory:get_message_class("TestMessage")
 
-function test_decodermethod()
-  local decoder = pb.MakeStringToMessageDecoder(TestMessage)
-
-  assert_error(
-    function()
-      -- Needs at least one argument to construct.
-      pb.MakeStringToMessageDecoder()
-    end)
-end
-
 function test_parse_primitive()
   local binary_pb =
          "\008\128\128\128\128\002\016\128\128\128\128\004\024\128\128"
       .. "\128\128\128\128\128\002\032\128\128\128\128\128\128\128\001\041\000"
       .. "\000\000\000\000\000\248\063\053\000\000\096\064\056\001"
-  local decoder = pb.MakeStringToMessageDecoder(TestMessage)
-  local encoder = pb.MakeMessageToStringEncoder(TestMessage)
-  collectgarbage()  -- ensure encoder/decoder GC-anchor what they need.
-  local msg = decoder(binary_pb)
+  local msg = TestMessage()
+  pb.decode(msg, binary_pb)
   assert_equal(536870912, msg.i32)
   assert_equal(1073741824, msg.u32)
   assert_equal(1125899906842624, msg.i64)
@@ -54,8 +42,9 @@ function test_parse_primitive()
   assert_equal(3.5, msg.flt)
   assert_equal(true, msg.bool)
 
-  local encoded = encoder(msg)
-  local msg2 = decoder(encoded)
+  local encoded = pb.encode(msg)
+  local msg2 = TestMessage()
+  pb.decode(msg2, encoded)
   assert_equal(536870912, msg.i32)
   assert_equal(1073741824, msg.u32)
   assert_equal(1125899906842624, msg.i64)
@@ -77,8 +66,8 @@ function test_parse_string()
   local TestMessage = factory:get_message_class("TestMessage")
 
   local binary_pb = "\010\005Hello"
-  local decoder = pb.MakeStringToMessageDecoder(TestMessage)
-  msg = decoder(binary_pb)
+  msg = TestMessage()
+  pb.decode(msg, binary_pb)
   assert_equal("Hello", msg.str)
 end
 
diff --git a/upb/bindings/lua/msg.c b/upb/bindings/lua/msg.c
index e468ace..e983f46 100644
--- a/upb/bindings/lua/msg.c
+++ b/upb/bindings/lua/msg.c
@@ -94,56 +94,6 @@ static void *lupb_newuserdata(lua_State *L, size_t size, const char *type) {
 }
 
 
-/* lupb_alloc *****************************************************************/
-
-typedef struct {
-  upb_alloc alloc;
-  lua_State *L;
-} lupb_alloc;
-
-char lupb_alloc_cache_key;
-
-static void *lupb_alloc_func(upb_alloc *alloc, void *ptr, size_t oldsize,
-                             size_t size) {
-  lupb_alloc *lalloc = (lupb_alloc*)alloc;
-  void *ud;
-
-  /* We read this every time in case the user changes the Lua alloc function. */
-  lua_Alloc func = lua_getallocf(lalloc->L, &ud);
-
-  return func(ud, ptr, oldsize, size);
-}
-
-static void lupb_alloc_pushnew(lua_State *L) {
-  lupb_alloc *lalloc = lua_newuserdata(L, sizeof(lupb_alloc));
-
-  lalloc->alloc.func = &lupb_alloc_func;
-  lalloc->L = L;
-}
-
-/* Returns the global lupb_alloc func that was created in our luaopen().
- * Callers can be guaranteed that it will be alive as long as |L| is. */
-static upb_alloc *lupb_alloc_get(lua_State *L) {
-  lupb_alloc *lalloc;
-
-  lua_pushlightuserdata(L, &lupb_alloc_cache_key);
-  lua_gettable(L, LUA_REGISTRYINDEX);
-  lalloc = lua_touserdata(L, -1);
-  UPB_ASSERT(lalloc);
-  lua_pop(L, 1);
-
-  return &lalloc->alloc;
-}
-
-static void lupb_alloc_initsingleton(lua_State *L) {
-  lua_pushlightuserdata(L, &lupb_alloc_cache_key);
-  lupb_alloc_pushnew(L);
-  lua_settable(L, LUA_REGISTRYINDEX);
-}
-
-#define ADD_BYTES(ptr, bytes) ((void*)((char*)ptr + bytes))
-
-
 /* lupb_arena *****************************************************************/
 
 /* lupb_arena only exists to wrap a upb_arena.  It is never exposed to users;
@@ -164,6 +114,30 @@ int lupb_arena_new(lua_State *L) {
   return 1;
 }
 
+char lupb_arena_cache_key;
+
+/* Returns the global lupb_arena func that was created in our luaopen().
+ * Callers can be guaranteed that it will be alive as long as |L| is.
+ * TODO(haberman): we shouldn't use a global arena!  We should have
+ * one arena for a parse, or per independently-created message. */
+static upb_arena *lupb_arena_get(lua_State *L) {
+  upb_arena *arena;
+
+  lua_pushlightuserdata(L, &lupb_arena_cache_key);
+  lua_gettable(L, LUA_REGISTRYINDEX);
+  arena = lua_touserdata(L, -1);
+  UPB_ASSERT(arena);
+  lua_pop(L, 1);
+
+  return arena;
+}
+
+static void lupb_arena_initsingleton(lua_State *L) {
+  lua_pushlightuserdata(L, &lupb_arena_cache_key);
+  lupb_arena_new(L);
+  lua_settable(L, LUA_REGISTRYINDEX);
+}
+
 static int lupb_arena_gc(lua_State *L) {
   upb_arena *a = lupb_arena_check(L, 1);
   upb_arena_uninit(a);
@@ -314,12 +288,6 @@ const upb_msglayout *lupb_msgclass_getlayout(lua_State *L, int narg) {
   return lupb_msgclass_check(L, narg)->layout;
 }
 
-const upb_handlers *lupb_msgclass_getmergehandlers(lua_State *L, int narg) {
-  const lupb_msgclass *lmsgclass = lupb_msgclass_check(L, narg);
-  return upb_msgfactory_getmergehandlers(
-      lmsgclass->lfactory->factory, lmsgclass->msgdef);
-}
-
 const upb_msgdef *lupb_msgclass_getmsgdef(const lupb_msgclass *lmsgclass) {
   return lmsgclass->msgdef;
 }
@@ -547,12 +515,6 @@ static int lupb_array_checkindex(lua_State *L, int narg, uint32_t max) {
   return n - 1;  /* Lua uses 1-based indexing. :( */
 }
 
-static int lupb_array_gc(lua_State *L) {
-  lupb_array *larray = lupb_array_check(L, 1);
-  upb_array_uninit(larray->arr);
-  return 0;
-}
-
 /* lupb_array Public API */
 
 static int lupb_array_new(lua_State *L) {
@@ -568,11 +530,9 @@ static int lupb_array_new(lua_State *L) {
     lupb_uservalseti(L, -1, ARRAY_MSGCLASS_INDEX, 1);  /* GC-root lmsgclass. */
   }
 
-  larray =
-      lupb_newuserdata(L, sizeof(*larray) + upb_array_sizeof(type), LUPB_ARRAY);
+  larray = lupb_newuserdata(L, sizeof(*larray), LUPB_ARRAY);
   larray->lmsgclass = lmsgclass;
-  larray->arr = ADD_BYTES(larray, sizeof(*larray));
-  upb_array_init(larray->arr, type, lupb_alloc_get(L));
+  larray->arr = upb_array_new(type, lupb_arena_get(L));
 
   return 1;
 }
@@ -614,7 +574,6 @@ static int lupb_array_len(lua_State *L) {
 }
 
 static const struct luaL_Reg lupb_array_mm[] = {
-  {"__gc", lupb_array_gc},
   {"__index", lupb_array_index},
   {"__len", lupb_array_len},
   {"__newindex", lupb_array_newindex},
@@ -681,12 +640,6 @@ static upb_msgval lupb_map_typecheck(lua_State *L, int narg, int msg,
   return upb_msgval_map(map);
 }
 
-static int lupb_map_gc(lua_State *L) {
-  lupb_map *lmap = lupb_map_check(L, 1);
-  upb_map_uninit(lmap->map);
-  return 0;
-}
-
 /* lupb_map Public API */
 
 /**
@@ -707,9 +660,7 @@ static int lupb_map_new(lua_State *L) {
     value_type = UPB_TYPE_MESSAGE;
   }
 
-  lmap = lupb_newuserdata(
-      L, sizeof(*lmap) + upb_map_sizeof(key_type, value_type), LUPB_MAP);
-  lmap->map = ADD_BYTES(lmap, sizeof(*lmap));
+  lmap = lupb_newuserdata(L, sizeof(*lmap), LUPB_MAP);
 
   if (value_type == UPB_TYPE_MESSAGE) {
     value_lmsgclass = lupb_msgclass_check(L, 2);
@@ -717,7 +668,7 @@ static int lupb_map_new(lua_State *L) {
   }
 
   lmap->value_lmsgclass = value_lmsgclass;
-  upb_map_init(lmap->map, key_type, value_type, lupb_alloc_get(L));
+  lmap->map = upb_map_new(key_type, value_type, lupb_arena_get(L));
 
   return 1;
 }
@@ -858,7 +809,6 @@ static int lupb_map_pairs(lua_State *L) {
 /* upb_mapiter ]]] */
 
 static const struct luaL_Reg lupb_map_mm[] = {
-  {"__gc", lupb_map_gc},
   {"__index", lupb_map_index},
   {"__len", lupb_map_len},
   {"__newindex", lupb_map_newindex},
@@ -912,6 +862,13 @@ const upb_msg *lupb_msg_checkmsg(lua_State *L, int narg,
   return lmsg->msg;
 }
 
+upb_msg *lupb_msg_checkmsg2(lua_State *L, int narg,
+                            const upb_msglayout **layout) {
+  lupb_msg *lmsg = lupb_msg_check(L, narg);
+  *layout = lmsg->lmsgclass->layout;
+  return lmsg->msg;
+}
+
 const upb_msgdef *lupb_msg_checkdef(lua_State *L, int narg) {
   return lupb_msg_check(L, narg)->lmsgclass->msgdef;
 }
@@ -958,12 +915,6 @@ int lupb_msg_pushref(lua_State *L, int msgclass, upb_msg *msg) {
   return 1;
 }
 
-static int lupb_msg_gc(lua_State *L) {
-  lupb_msg *lmsg = lupb_msg_check(L, 1);
-  upb_msg_uninit(lmsg->msg, lmsg->lmsgclass->layout);
-  return 0;
-}
-
 /* lupb_msg Public API */
 
 /**
@@ -974,12 +925,10 @@ static int lupb_msg_gc(lua_State *L) {
  */
 static int lupb_msg_pushnew(lua_State *L, int narg) {
   const lupb_msgclass *lmsgclass = lupb_msgclass_check(L, narg);
-  size_t size = sizeof(lupb_msg) + upb_msg_sizeof(lmsgclass->layout);
-  lupb_msg *lmsg = lupb_newuserdata(L, size, LUPB_MSG);
+  lupb_msg *lmsg = lupb_newuserdata(L, sizeof(lupb_msg), LUPB_MSG);
 
   lmsg->lmsgclass = lmsgclass;
-  lmsg->msg = upb_msg_init(
-      ADD_BYTES(lmsg, sizeof(*lmsg)), lmsgclass->layout, lupb_alloc_get(L));
+  lmsg->msg = upb_msg_new(lmsgclass->layout, lupb_arena_get(L));
 
   lupb_uservalseti(L, -1, LUPB_MSG_MSGCLASSINDEX, narg);
 
@@ -1070,7 +1019,6 @@ static int lupb_msg_newindex(lua_State *L) {
 }
 
 static const struct luaL_Reg lupb_msg_mm[] = {
-  {"__gc", lupb_msg_gc},
   {"__index", lupb_msg_index},
   {"__newindex", lupb_msg_newindex},
   {NULL, NULL}
@@ -1096,5 +1044,5 @@ void lupb_msg_registertypes(lua_State *L) {
   lupb_register_type(L, LUPB_MAP,        NULL,              lupb_map_mm);
   lupb_register_type(L, LUPB_MSG,        NULL,              lupb_msg_mm);
 
-  lupb_alloc_initsingleton(L);
+  lupb_arena_initsingleton(L);
 }
diff --git a/upb/bindings/lua/upb.h b/upb/bindings/lua/upb.h
index ea2910a..52bc5a2 100644
--- a/upb/bindings/lua/upb.h
+++ b/upb/bindings/lua/upb.h
@@ -131,11 +131,12 @@ int lupb_arena_new(lua_State *L);
 int lupb_msg_pushref(lua_State *L, int msgclass, void *msg);
 const upb_msg *lupb_msg_checkmsg(lua_State *L, int narg,
                                  const lupb_msgclass *lmsgclass);
+upb_msg *lupb_msg_checkmsg2(lua_State *L, int narg,
+                            const upb_msglayout **layout);
 
 const lupb_msgclass *lupb_msgclass_check(lua_State *L, int narg);
 const upb_msglayout *lupb_msgclass_getlayout(lua_State *L, int narg);
 const upb_msgdef *lupb_msgclass_getmsgdef(const lupb_msgclass *lmsgclass);
-const upb_handlers *lupb_msgclass_getmergehandlers(lua_State *L, int narg);
 upb_msgfactory *lupb_msgclass_getfactory(const lupb_msgclass *lmsgclass);
 void lupb_msg_registertypes(lua_State *L);
 
diff --git a/upb/bindings/lua/upb/pb.c b/upb/bindings/lua/upb/pb.c
index 1d56066..466e8f7 100644
--- a/upb/bindings/lua/upb/pb.c
+++ b/upb/bindings/lua/upb/pb.c
@@ -6,135 +6,56 @@
 */
 
 #include "upb/bindings/lua/upb.h"
-#include "upb/pb/decoder.h"
-#include "upb/pb/encoder.h"
+#include "upb/decode.h"
+#include "upb/encode.h"
 
 #define LUPB_PBDECODERMETHOD "lupb.pb.decodermethod"
 
-static int lupb_pb_strtomessage(lua_State *L) {
+static int lupb_pb_decode(lua_State *L) {
   size_t len;
   upb_status status = UPB_STATUS_INIT;
-  const char *pb = lua_tolstring(L, 1, &len);
-  const upb_msglayout *layout = lua_touserdata(L, lua_upvalueindex(1));
-  const upb_pbdecodermethod *method = lua_touserdata(L, lua_upvalueindex(2));
-  const upb_handlers *handlers = upb_pbdecodermethod_desthandlers(method);
-
-  upb_arena *msg_arena;
+  const upb_msglayout *layout;
+  upb_msg *msg = lupb_msg_checkmsg2(L, 1, &layout);
+  const char *pb = lua_tolstring(L, 2, &len);
+  upb_stringview buf = upb_stringview_make(pb, len);
   upb_env env;
-  upb_sink sink;
-  upb_pbdecoder *decoder;
-  void *msg;
-
-  lupb_arena_new(L);
-  msg_arena = lupb_arena_check(L, -1);
 
-  msg = upb_msg_new(layout, upb_arena_alloc(msg_arena));
   upb_env_init(&env);
   upb_env_reporterrorsto(&env, &status);
-  upb_sink_reset(&sink, handlers, msg);
-  decoder = upb_pbdecoder_create(&env, method, &sink);
 
-  upb_bufsrc_putbuf(pb, len, upb_pbdecoder_input(decoder));
+  upb_decode(buf, msg, (const void*)layout, &env);
 
   /* Free resources before we potentially bail on error. */
   upb_env_uninit(&env);
   lupb_checkstatus(L, &status);
 
-  /* References the arena at the top of the stack. */
-  lupb_msg_pushref(L, lua_upvalueindex(3), msg);
-  return 1;
+  return 0;
 }
 
-static int lupb_pb_messagetostr(lua_State *L) {
-  const lupb_msgclass *lmsgclass = lua_touserdata(L, lua_upvalueindex(1));
-  const upb_msg *msg = lupb_msg_checkmsg(L, 1, lmsgclass);
-  const upb_visitorplan *vp = lua_touserdata(L, lua_upvalueindex(2));
-  const upb_handlers *encode_handlers = lua_touserdata(L, lua_upvalueindex(3));
-
+static int lupb_pb_encode(lua_State *L) {
+  const upb_msglayout *layout;
+  const upb_msg *msg = lupb_msg_checkmsg2(L, 1, &layout);
   upb_env env;
-  upb_bufsink *bufsink;
-  upb_bytessink *bytessink;
-  upb_pb_encoder *encoder;
-  upb_visitor *visitor;
-  const char *buf;
-  size_t len;
+  size_t size;
   upb_status status = UPB_STATUS_INIT;
+  char *result;
 
   upb_env_init(&env);
   upb_env_reporterrorsto(&env, &status);
-  bufsink = upb_bufsink_new(&env);
-  bytessink = upb_bufsink_sink(bufsink);
-  encoder = upb_pb_encoder_create(&env, encode_handlers, bytessink);
-  visitor = upb_visitor_create(&env, vp, upb_pb_encoder_input(encoder));
 
-  upb_visitor_visitmsg(visitor, msg);
-  buf = upb_bufsink_getdata(bufsink, &len);
-  lua_pushlstring(L, buf, len);
+  result = upb_encode(msg, (const void*)layout, &env, &size);
 
   /* Free resources before we potentially bail on error. */
   upb_env_uninit(&env);
   lupb_checkstatus(L, &status);
 
+  lua_pushlstring(L, result, size);
   return 1;
 }
 
-static int lupb_pb_makestrtomsgdecoder(lua_State *L) {
-  const upb_msglayout *layout = lupb_msgclass_getlayout(L, 1);
-  const upb_handlers *handlers = lupb_msgclass_getmergehandlers(L, 1);
-  const upb_pbdecodermethod *m;
-
-  upb_pbdecodermethodopts opts;
-  upb_pbdecodermethodopts_init(&opts, handlers);
-
-  m = upb_pbdecodermethod_new(&opts, &m);
-
-  /* Push upvalues for the closure. */
-  lua_pushlightuserdata(L, (void*)layout);
-  lua_pushlightuserdata(L, (void*)m);
-  lua_pushvalue(L, 1);
-
-  /* Upvalue for the closure, only to keep the decodermethod alive. */
-  lupb_refcounted_pushnewrapper(
-      L, upb_pbdecodermethod_upcast(m), LUPB_PBDECODERMETHOD, &m);
-
-  lua_pushcclosure(L, &lupb_pb_strtomessage, 4);
-
-  return 1;  /* The decoder closure. */
-}
-
-static int lupb_pb_makemsgtostrencoder(lua_State *L) {
-  const lupb_msgclass *lmsgclass = lupb_msgclass_check(L, 1);
-  const upb_msgdef *md = lupb_msgclass_getmsgdef(lmsgclass);
-  upb_msgfactory *factory = lupb_msgclass_getfactory(lmsgclass);
-  const upb_handlers *encode_handlers;
-  const upb_visitorplan *vp;
-
-  encode_handlers = upb_pb_encoder_newhandlers(md, &encode_handlers);
-  vp = upb_msgfactory_getvisitorplan(factory, encode_handlers);
-
-  /* Push upvalues for the closure. */
-  lua_pushlightuserdata(L, (void*)lmsgclass);
-  lua_pushlightuserdata(L, (void*)vp);
-  lua_pushlightuserdata(L, (void*)encode_handlers);
-
-  /* Upvalues for the closure, only to keep the other upvalues alive. */
-  lua_pushvalue(L, 1);
-  lupb_refcounted_pushnewrapper(
-      L, upb_handlers_upcast(encode_handlers), LUPB_PBDECODERMETHOD, &encode_handlers);
-
-  lua_pushcclosure(L, &lupb_pb_messagetostr, 5);
-
-  return 1;  /* The decoder closure. */
-}
-
-static const struct luaL_Reg decodermethod_mm[] = {
-  {"__gc", lupb_refcounted_gc},
-  {NULL, NULL}
-};
-
 static const struct luaL_Reg toplevel_m[] = {
-  {"MakeStringToMessageDecoder", lupb_pb_makestrtomsgdecoder},
-  {"MakeMessageToStringEncoder", lupb_pb_makemsgtostrencoder},
+  {"decode", lupb_pb_decode},
+  {"encode", lupb_pb_encode},
   {NULL, NULL}
 };
 
@@ -144,7 +65,5 @@ int luaopen_upb_pb_c(lua_State *L) {
     return 1;
   }
 
-  lupb_register_type(L, LUPB_PBDECODERMETHOD, NULL, decodermethod_mm);
-
   return 1;
 }
diff --git a/upb/decode.c b/upb/decode.c
index b5033f0..ca18e95 100644
--- a/upb/decode.c
+++ b/upb/decode.c
@@ -172,6 +172,7 @@ static bool upb_array_grow(upb_array *arr, size_t elements) {
   size_t new_bytes;
   size_t old_bytes;
   void *new_data;
+  upb_alloc *alloc = upb_arena_alloc(arr->arena);
 
   while (new_size < needed) {
     new_size *= 2;
@@ -179,7 +180,7 @@ static bool upb_array_grow(upb_array *arr, size_t elements) {
 
   old_bytes = arr->len * arr->element_size;
   new_bytes = new_size * arr->element_size;
-  new_data = upb_realloc(arr->alloc, arr->data, old_bytes, new_bytes);
+  new_data = upb_realloc(alloc, arr->data, old_bytes, new_bytes);
   CHK(new_data);
 
   arr->data = new_data;
@@ -212,12 +213,11 @@ static upb_array *upb_getorcreatearr(upb_decstate *d,
   upb_array *arr = upb_getarr(frame, field);
 
   if (!arr) {
-    arr = upb_env_malloc(d->env, sizeof(*arr));
+    upb_fieldtype_t type = upb_desctype_to_fieldtype[field->descriptortype];
+    arr = upb_array_new(type, upb_env_arena(d->env));
     if (!arr) {
       return NULL;
     }
-    upb_array_init(arr, upb_desctype_to_fieldtype[field->descriptortype],
-                   upb_arena_alloc(upb_env_arena(d->env)));
     *(upb_array**)&frame->msg[field->offset] = arr;
   }
 
@@ -278,10 +278,8 @@ static bool upb_decode_submsg(upb_decstate *d,
   UPB_ASSERT(subm);
 
   if (!submsg) {
-    submsg = upb_env_malloc(d->env, upb_msg_sizeof((upb_msglayout *)subm));
+    submsg = upb_msg_new((upb_msglayout *)subm, upb_env_arena(d->env));
     CHK(submsg);
-    submsg = upb_msg_init(
-        submsg, (upb_msglayout*)subm, upb_arena_alloc(upb_env_arena(d->env)));
     *(void**)submsg_slot = submsg;
   }
 
@@ -460,10 +458,8 @@ static bool upb_decode_toarray(upb_decstate *d, upb_decframe *frame,
       subm = frame->m->submsgs[field->submsg_index];
       UPB_ASSERT(subm);
 
-      submsg = upb_env_malloc(d->env, upb_msg_sizeof((upb_msglayout *)subm));
+      submsg = upb_msg_new((upb_msglayout *)subm, upb_env_arena(d->env));
       CHK(submsg);
-      submsg = upb_msg_init(submsg, (upb_msglayout*)subm,
-                            upb_arena_alloc(upb_env_arena(d->env)));
 
       field_mem = upb_array_add(arr, 1);
       CHK(field_mem);
diff --git a/upb/msg.c b/upb/msg.c
index 527587f..501e4c3 100644
--- a/upb/msg.c
+++ b/upb/msg.c
@@ -22,9 +22,6 @@ bool upb_fieldtype_mapkeyok(upb_fieldtype_t type) {
          type == UPB_TYPE_UINT64 || type == UPB_TYPE_STRING;
 }
 
-void *upb_array_pack(const upb_array *arr, void *p, size_t *ofs, size_t size);
-void *upb_map_pack(const upb_map *map, void *p, size_t *ofs, size_t size);
-
 #define PTR_AT(msg, ofs, type) (type*)((char*)msg + ofs)
 #define VOIDPTR_AT(msg, ofs) PTR_AT(msg, ofs, void)
 #define ENCODE_MAX_NESTING 64
@@ -176,16 +173,6 @@ static size_t upb_msglayout_place(upb_msglayout *l, size_t size) {
   return ret;
 }
 
-static uint32_t upb_msglayout_offset(const upb_msglayout *l,
-                                     const upb_fielddef *f) {
-  return l->data.fields[upb_fielddef_index(f)].offset;
-}
-
-static uint32_t upb_msglayout_hasbit(const upb_msglayout *l,
-                                     const upb_fielddef *f) {
-  return l->data.fields[upb_fielddef_index(f)].hasbit;
-}
-
 static bool upb_msglayout_initdefault(upb_msglayout *l, const upb_msgdef *m) {
   upb_msg_field_iter it;
 
@@ -420,229 +407,6 @@ const upb_msglayout *upb_msgfactory_getlayout(upb_msgfactory *f,
   }
 }
 
-/* Our handlers that we don't expose externally. */
-
-void *upb_msg_startstr(void *msg, const void *hd, size_t size_hint) {
-  uint32_t ofs = (uintptr_t)hd;
-  upb_alloc *alloc = upb_msg_alloc(msg);
-  upb_msgval val;
-  UPB_UNUSED(size_hint);
-
-  val = upb_msgval_read(msg, ofs, upb_msgval_sizeof(UPB_TYPE_STRING));
-
-  upb_free(alloc, (void*)val.str.data);
-  val.str.data = NULL;
-  val.str.size = 0;
-
-  upb_msgval_write(msg, ofs, val, upb_msgval_sizeof(UPB_TYPE_STRING));
-  return msg;
-}
-
-size_t upb_msg_str(void *msg, const void *hd, const char *ptr, size_t size,
-                   const upb_bufhandle *handle) {
-  uint32_t ofs = (uintptr_t)hd;
-  upb_alloc *alloc = upb_msg_alloc(msg);
-  upb_msgval val;
-  size_t newsize;
-  UPB_UNUSED(handle);
-
-  val = upb_msgval_read(msg, ofs, upb_msgval_sizeof(UPB_TYPE_STRING));
-
-  newsize = val.str.size + size;
-  val.str.data = upb_realloc(alloc, (void*)val.str.data, val.str.size, newsize);
-
-  if (!val.str.data) {
-    return false;
-  }
-
-  memcpy((char*)val.str.data + val.str.size, ptr, size);
-  val.str.size = newsize;
-  upb_msgval_write(msg, ofs, val, upb_msgval_sizeof(UPB_TYPE_STRING));
-  return size;
-}
-
-static void callback(const void *closure, upb_handlers *h) {
-  upb_msgfactory *factory = (upb_msgfactory*)closure;
-  const upb_msgdef *md = upb_handlers_msgdef(h);
-  const upb_msglayout* layout = upb_msgfactory_getlayout(factory, md);
-  upb_msg_field_iter i;
-  UPB_UNUSED(factory);
-
-  for(upb_msg_field_begin(&i, md);
-      !upb_msg_field_done(&i);
-      upb_msg_field_next(&i)) {
-    const upb_fielddef *f = upb_msg_iter_field(&i);
-    size_t offset = upb_msglayout_offset(layout, f);
-    upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
-    upb_handlerattr_sethandlerdata(&attr, (void*)offset);
-
-    if (upb_fielddef_isseq(f)) {
-    } else if (upb_fielddef_isstring(f)) {
-      upb_handlers_setstartstr(h, f, upb_msg_startstr, &attr);
-      upb_handlers_setstring(h, f, upb_msg_str, &attr);
-    } else {
-      upb_msg_setscalarhandler(
-          h, f, offset, upb_msglayout_hasbit(layout, f));
-    }
-  }
-}
-
-const upb_handlers *upb_msgfactory_getmergehandlers(upb_msgfactory *f,
-                                                    const upb_msgdef *m) {
-  upb_msgfactory *mutable_f = (void*)f;
-
-  /* TODO(haberman): properly cache these. */
-  const upb_handlers *ret = upb_handlers_newfrozen(m, f, callback, f);
-  upb_inttable_push(&mutable_f->mergehandlers, upb_value_constptr(ret));
-
-  return ret;
-}
-
-const upb_visitorplan *upb_msgfactory_getvisitorplan(upb_msgfactory *f,
-                                                     const upb_handlers *h) {
-  const upb_msgdef *md = upb_handlers_msgdef(h);
-  return (const upb_visitorplan*)upb_msgfactory_getlayout(f, md);
-}
-
-
-/** upb_visitor ***************************************************************/
-
-struct upb_visitor {
-  const upb_msglayout *layout;
-  upb_sink *sink;
-};
-
-static upb_selector_t getsel2(const upb_fielddef *f, upb_handlertype_t type) {
-  upb_selector_t ret;
-  bool ok = upb_handlers_getselector(f, type, &ret);
-  UPB_ASSERT(ok);
-  return ret;
-}
-
-static bool upb_visitor_hasfield(const upb_msg *msg, const upb_fielddef *f,
-                                 const upb_msglayout *layout) {
-  int field_index = upb_fielddef_index(f);
-  if (upb_fielddef_isseq(f)) {
-    return upb_msgval_getarr(upb_msg_get(msg, field_index, layout)) != NULL;
-  } else if (upb_msgdef_syntax(upb_fielddef_containingtype(f)) ==
-             UPB_SYNTAX_PROTO2) {
-    return upb_msg_has(msg, field_index, layout);
-  } else {
-    upb_msgval val = upb_msg_get(msg, field_index, layout);
-    switch (upb_fielddef_type(f)) {
-      case UPB_TYPE_FLOAT:
-        return upb_msgval_getfloat(val) != 0;
-      case UPB_TYPE_DOUBLE:
-        return upb_msgval_getdouble(val) != 0;
-      case UPB_TYPE_BOOL:
-        return upb_msgval_getbool(val);
-      case UPB_TYPE_ENUM:
-      case UPB_TYPE_INT32:
-        return upb_msgval_getint32(val) != 0;
-      case UPB_TYPE_UINT32:
-        return upb_msgval_getuint32(val) != 0;
-      case UPB_TYPE_INT64:
-        return upb_msgval_getint64(val) != 0;
-      case UPB_TYPE_UINT64:
-        return upb_msgval_getuint64(val) != 0;
-      case UPB_TYPE_STRING:
-      case UPB_TYPE_BYTES:
-        return upb_msgval_getstr(val).size > 0;
-      case UPB_TYPE_MESSAGE:
-        return upb_msgval_getmsg(val) != NULL;
-    }
-    UPB_UNREACHABLE();
-  }
-}
-
-static bool upb_visitor_visitmsg2(const upb_msg *msg,
-                                  const upb_msglayout *layout, upb_sink *sink,
-                                  int depth) {
-  const upb_msgdef *md = upb_handlers_msgdef(sink->handlers);
-  upb_msg_field_iter i;
-  upb_status status;
-
-  upb_sink_startmsg(sink);
-
-  /* Protect against cycles (possible because users may freely reassign message
-   * and repeated fields) by imposing a maximum recursion depth. */
-  if (depth > ENCODE_MAX_NESTING) {
-    return false;
-  }
-
-  for (upb_msg_field_begin(&i, md);
-       !upb_msg_field_done(&i);
-       upb_msg_field_next(&i)) {
-    upb_fielddef *f = upb_msg_iter_field(&i);
-    upb_msgval val;
-
-    if (!upb_visitor_hasfield(msg, f, layout)) {
-      continue;
-    }
-
-    val = upb_msg_get(msg, upb_fielddef_index(f), layout);
-
-    if (upb_fielddef_isseq(f)) {
-      const upb_array *arr = upb_msgval_getarr(val);
-      UPB_ASSERT(arr);
-      /* TODO: putary(ary, f, sink, depth);*/
-    } else if (upb_fielddef_issubmsg(f)) {
-      const upb_map *map = upb_msgval_getmap(val);
-      UPB_ASSERT(map);
-      /* TODO: putmap(map, f, sink, depth);*/
-    } else if (upb_fielddef_isstring(f)) {
-      /* TODO putstr(); */
-    } else {
-      upb_selector_t sel = getsel2(f, upb_handlers_getprimitivehandlertype(f));
-      UPB_ASSERT(upb_fielddef_isprimitive(f));
-
-      switch (upb_fielddef_type(f)) {
-        case UPB_TYPE_FLOAT:
-          CHECK_TRUE(upb_sink_putfloat(sink, sel, upb_msgval_getfloat(val)));
-          break;
-        case UPB_TYPE_DOUBLE:
-          CHECK_TRUE(upb_sink_putdouble(sink, sel, upb_msgval_getdouble(val)));
-          break;
-        case UPB_TYPE_BOOL:
-          CHECK_TRUE(upb_sink_putbool(sink, sel, upb_msgval_getbool(val)));
-          break;
-        case UPB_TYPE_ENUM:
-        case UPB_TYPE_INT32:
-          CHECK_TRUE(upb_sink_putint32(sink, sel, upb_msgval_getint32(val)));
-          break;
-        case UPB_TYPE_UINT32:
-          CHECK_TRUE(upb_sink_putuint32(sink, sel, upb_msgval_getuint32(val)));
-          break;
-        case UPB_TYPE_INT64:
-          CHECK_TRUE(upb_sink_putint64(sink, sel, upb_msgval_getint64(val)));
-          break;
-        case UPB_TYPE_UINT64:
-          CHECK_TRUE(upb_sink_putuint64(sink, sel, upb_msgval_getuint64(val)));
-          break;
-        case UPB_TYPE_STRING:
-        case UPB_TYPE_BYTES:
-        case UPB_TYPE_MESSAGE:
-          UPB_UNREACHABLE();
-      }
-    }
-  }
-
-  upb_sink_endmsg(sink, &status);
-  return true;
-}
-
-upb_visitor *upb_visitor_create(upb_env *e, const upb_visitorplan *vp,
-                                upb_sink *output) {
-  upb_visitor *visitor = upb_env_malloc(e, sizeof(*visitor));
-  visitor->layout = (const upb_msglayout*)vp;
-  visitor->sink = output;
-  return visitor;
-}
-
-bool upb_visitor_visitmsg(upb_visitor *visitor, const upb_msg *msg) {
-  return upb_visitor_visitmsg2(msg, visitor->layout, visitor->sink, 0);
-}
-
 
 /** upb_msg *******************************************************************/
 
@@ -658,7 +422,7 @@ bool upb_visitor_visitmsg(upb_visitor *visitor, const upb_msg *msg) {
 /* Used when a message is not extendable. */
 typedef struct {
   /* TODO(haberman): add unknown fields. */
-  upb_alloc *alloc;
+  upb_arena *arena;
 } upb_msg_internal;
 
 /* Used when a message is extendable. */
@@ -702,12 +466,20 @@ static uint32_t *upb_msg_oneofcase(const upb_msg *msg, int field_index,
   return PTR_AT(msg, l->data.oneofs[field->oneof_index].case_offset, uint32_t);
 }
 
-size_t upb_msg_sizeof(const upb_msglayout *l) {
+static size_t upb_msg_sizeof(const upb_msglayout *l) {
   return l->data.size + upb_msg_internalsize(l);
 }
 
-upb_msg *upb_msg_init(void *mem, const upb_msglayout *l, upb_alloc *a) {
-  upb_msg *msg = VOIDPTR_AT(mem, upb_msg_internalsize(l));
+upb_msg *upb_msg_new(const upb_msglayout *l, upb_arena *a) {
+  upb_alloc *alloc = upb_arena_alloc(a);
+  void *mem = upb_malloc(alloc, upb_msg_sizeof(l));
+  upb_msg *msg;
+
+  if (!mem) {
+    return NULL;
+  }
+
+  msg = VOIDPTR_AT(mem, upb_msg_internalsize(l));
 
   /* Initialize normal members. */
   if (l->data.default_msg) {
@@ -717,7 +489,7 @@ upb_msg *upb_msg_init(void *mem, const upb_msglayout *l, upb_alloc *a) {
   }
 
   /* Initialize internal members. */
-  upb_msg_getinternal(msg)->alloc = a;
+  upb_msg_getinternal(msg)->arena = a;
 
   if (l->data.extendable) {
     upb_msg_getinternalwithext(msg, l)->extdict = NULL;
@@ -726,29 +498,8 @@ upb_msg *upb_msg_init(void *mem, const upb_msglayout *l, upb_alloc *a) {
   return msg;
 }
 
-void *upb_msg_uninit(upb_msg *msg, const upb_msglayout *l) {
-  if (l->data.extendable) {
-    upb_inttable *ext_dict = upb_msg_getinternalwithext(msg, l)->extdict;
-    if (ext_dict) {
-      upb_inttable_uninit2(ext_dict, upb_msg_alloc(msg));
-      upb_free(upb_msg_alloc(msg), ext_dict);
-    }
-  }
-
-  return VOIDPTR_AT(msg, -upb_msg_internalsize(l));
-}
-
-upb_msg *upb_msg_new(const upb_msglayout *l, upb_alloc *a) {
-  void *mem = upb_malloc(a, upb_msg_sizeof(l));
-  return mem ? upb_msg_init(mem, l, a) : NULL;
-}
-
-void upb_msg_free(upb_msg *msg, const upb_msglayout *l) {
-  upb_free(upb_msg_alloc(msg), upb_msg_uninit(msg, l));
-}
-
-upb_alloc *upb_msg_alloc(const upb_msg *msg) {
-  return upb_msg_getinternal_const(msg)->alloc;
+upb_arena *upb_msg_arena(const upb_msg *msg) {
+  return upb_msg_getinternal_const(msg)->arena;
 }
 
 bool upb_msg_has(const upb_msg *msg,
@@ -805,37 +556,22 @@ void upb_msg_set(upb_msg *msg, int field_index, upb_msgval val,
 
 #define DEREF_ARR(arr, i, type) ((type*)arr->data)[i]
 
-size_t upb_array_sizeof(upb_fieldtype_t type) {
-  UPB_UNUSED(type);
-  return sizeof(upb_array);
-}
-
-void upb_array_init(upb_array *arr, upb_fieldtype_t type, upb_alloc *alloc) {
-  arr->type = type;
-  arr->data = NULL;
-  arr->len = 0;
-  arr->size = 0;
-  arr->element_size = upb_msgval_sizeof(type);
-  arr->alloc = alloc;
-}
-
-void upb_array_uninit(upb_array *arr) {
-  upb_free(arr->alloc, arr->data);
-}
-
-upb_array *upb_array_new(upb_fieldtype_t type, upb_alloc *a) {
-  upb_array *ret = upb_malloc(a, upb_array_sizeof(type));
+upb_array *upb_array_new(upb_fieldtype_t type, upb_arena *a) {
+  upb_alloc *alloc = upb_arena_alloc(a);
+  upb_array *ret = upb_malloc(alloc, sizeof(upb_array));
 
-  if (ret) {
-    upb_array_init(ret, type, a);
+  if (!ret) {
+    return NULL;
   }
 
-  return ret;
-}
+  ret->type = type;
+  ret->data = NULL;
+  ret->len = 0;
+  ret->size = 0;
+  ret->element_size = upb_msgval_sizeof(type);
+  ret->arena = a;
 
-void upb_array_free(upb_array *arr) {
-  upb_array_uninit(arr);
-  upb_free(arr->alloc, arr);
+  return ret;
 }
 
 size_t upb_array_size(const upb_array *arr) {
@@ -862,8 +598,9 @@ bool upb_array_set(upb_array *arr, size_t i, upb_msgval val) {
       size_t new_size = UPB_MAX(arr->size * 2, 8);
       size_t new_bytes = new_size * arr->element_size;
       size_t old_bytes = arr->size * arr->element_size;
+      upb_alloc *alloc = upb_arena_alloc(arr->arena);
       upb_msgval *new_data =
-          upb_realloc(arr->alloc, arr->data, old_bytes, new_bytes);
+          upb_realloc(alloc, arr->data, old_bytes, new_bytes);
 
       if (!new_data) {
         return false;
@@ -889,7 +626,7 @@ struct upb_map {
   /* We may want to optimize this to use inttable where possible, for greater
    * efficiency and lower memory footprint. */
   upb_strtable strtab;
-  upb_alloc *alloc;
+  upb_arena *arena;
 };
 
 static void upb_map_tokey(upb_fieldtype_t type, upb_msgval *key,
@@ -940,52 +677,28 @@ static upb_msgval upb_map_fromkey(upb_fieldtype_t type, const char *key,
   UPB_UNREACHABLE();
 }
 
-size_t upb_map_sizeof(upb_fieldtype_t ktype, upb_fieldtype_t vtype) {
-  /* Size does not currently depend on key/value type. */
-  UPB_UNUSED(ktype);
-  UPB_UNUSED(vtype);
-  return sizeof(upb_map);
-}
-
-bool upb_map_init(upb_map *map, upb_fieldtype_t ktype, upb_fieldtype_t vtype,
-                  upb_alloc *a) {
-  upb_ctype_t vtabtype = upb_fieldtotabtype(vtype);
-  UPB_ASSERT(upb_fieldtype_mapkeyok(ktype));
-  map->key_type = ktype;
-  map->val_type = vtype;
-  map->alloc = a;
-
-  if (!upb_strtable_init2(&map->strtab, vtabtype, a)) {
-    return false;
-  }
-
-  return true;
-}
-
-void upb_map_uninit(upb_map *map) {
-  upb_strtable_uninit2(&map->strtab, map->alloc);
-}
-
 upb_map *upb_map_new(upb_fieldtype_t ktype, upb_fieldtype_t vtype,
-                     upb_alloc *a) {
-  upb_map *map = upb_malloc(a, upb_map_sizeof(ktype, vtype));
+                     upb_arena *a) {
+  upb_ctype_t vtabtype = upb_fieldtotabtype(vtype);
+  upb_alloc *alloc = upb_arena_alloc(a);
+  upb_map *map = upb_malloc(alloc, sizeof(upb_map));
 
   if (!map) {
     return NULL;
   }
 
-  if (!upb_map_init(map, ktype, vtype, a)) {
+  UPB_ASSERT(upb_fieldtype_mapkeyok(ktype));
+  map->key_type = ktype;
+  map->val_type = vtype;
+  map->arena = a;
+
+  if (!upb_strtable_init2(&map->strtab, vtabtype, alloc)) {
     return NULL;
   }
 
   return map;
 }
 
-void upb_map_free(upb_map *map) {
-  upb_map_uninit(map);
-  upb_free(map->alloc, map);
-}
-
 size_t upb_map_size(const upb_map *map) {
   return upb_strtable_count(&map->strtab);
 }
@@ -1019,7 +732,7 @@ bool upb_map_set(upb_map *map, upb_msgval key, upb_msgval val,
   size_t key_len;
   upb_value tabval = upb_toval(val);
   upb_value removedtabval;
-  upb_alloc *a = map->alloc;
+  upb_alloc *a = upb_arena_alloc(map->arena);
 
   upb_map_tokey(map->key_type, &key, &key_str, &key_len);
 
@@ -1035,7 +748,7 @@ bool upb_map_set(upb_map *map, upb_msgval key, upb_msgval val,
 bool upb_map_del(upb_map *map, upb_msgval key) {
   const char *key_str;
   size_t key_len;
-  upb_alloc *a = map->alloc;
+  upb_alloc *a = upb_arena_alloc(map->arena);
 
   upb_map_tokey(map->key_type, &key, &key_str, &key_len);
   return upb_strtable_remove3(&map->strtab, key_str, key_len, NULL, a);
@@ -1097,99 +810,3 @@ void upb_mapiter_setdone(upb_mapiter *i) {
 bool upb_mapiter_isequal(const upb_mapiter *i1, const upb_mapiter *i2) {
   return upb_strtable_iter_isequal(&i1->iter, &i2->iter);
 }
-
-
-/** Handlers for upb_msg ******************************************************/
-
-typedef struct {
-  size_t offset;
-  int32_t hasbit;
-} upb_msg_handlerdata;
-
-/* Fallback implementation if the handler is not specialized by the producer. */
-#define MSG_WRITER(type, ctype)                                               \
-  bool upb_msg_set ## type (void *c, const void *hd, ctype val) {             \
-    uint8_t *m = c;                                                           \
-    const upb_msg_handlerdata *d = hd;                                        \
-    if (d->hasbit > 0)                                                        \
-      *(uint8_t*)&m[d->hasbit / 8] |= 1 << (d->hasbit % 8);                   \
-    *(ctype*)&m[d->offset] = val;                                             \
-    return true;                                                              \
-  }                                                                           \
-
-MSG_WRITER(double, double)
-MSG_WRITER(float,  float)
-MSG_WRITER(int32,  int32_t)
-MSG_WRITER(int64,  int64_t)
-MSG_WRITER(uint32, uint32_t)
-MSG_WRITER(uint64, uint64_t)
-MSG_WRITER(bool,   bool)
-
-bool upb_msg_setscalarhandler(upb_handlers *h, const upb_fielddef *f,
-                              size_t offset, int32_t hasbit) {
-  upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
-  bool ok;
-
-  upb_msg_handlerdata *d = upb_gmalloc(sizeof(*d));
-  if (!d) return false;
-  d->offset = offset;
-  d->hasbit = hasbit;
-
-  upb_handlerattr_sethandlerdata(&attr, d);
-  upb_handlerattr_setalwaysok(&attr, true);
-  upb_handlers_addcleanup(h, d, upb_gfree);
-
-#define TYPE(u, l) \
-  case UPB_TYPE_##u: \
-    ok = upb_handlers_set##l(h, f, upb_msg_set##l, &attr); break;
-
-  ok = false;
-
-  switch (upb_fielddef_type(f)) {
-    TYPE(INT64,  int64);
-    TYPE(INT32,  int32);
-    TYPE(ENUM,   int32);
-    TYPE(UINT64, uint64);
-    TYPE(UINT32, uint32);
-    TYPE(DOUBLE, double);
-    TYPE(FLOAT,  float);
-    TYPE(BOOL,   bool);
-    default: UPB_ASSERT(false); break;
-  }
-#undef TYPE
-
-  upb_handlerattr_uninit(&attr);
-  return ok;
-}
-
-bool upb_msg_getscalarhandlerdata(const upb_handlers *h,
-                                  upb_selector_t s,
-                                  upb_fieldtype_t *type,
-                                  size_t *offset,
-                                  int32_t *hasbit) {
-  const upb_msg_handlerdata *d;
-  upb_func *f = upb_handlers_gethandler(h, s);
-
-  if ((upb_int64_handlerfunc*)f == upb_msg_setint64) {
-    *type = UPB_TYPE_INT64;
-  } else if ((upb_int32_handlerfunc*)f == upb_msg_setint32) {
-    *type = UPB_TYPE_INT32;
-  } else if ((upb_uint64_handlerfunc*)f == upb_msg_setuint64) {
-    *type = UPB_TYPE_UINT64;
-  } else if ((upb_uint32_handlerfunc*)f == upb_msg_setuint32) {
-    *type = UPB_TYPE_UINT32;
-  } else if ((upb_double_handlerfunc*)f == upb_msg_setdouble) {
-    *type = UPB_TYPE_DOUBLE;
-  } else if ((upb_float_handlerfunc*)f == upb_msg_setfloat) {
-    *type = UPB_TYPE_FLOAT;
-  } else if ((upb_bool_handlerfunc*)f == upb_msg_setbool) {
-    *type = UPB_TYPE_BOOL;
-  } else {
-    return false;
-  }
-
-  d = upb_handlers_gethandlerdata(h, s);
-  *offset = d->offset;
-  *hasbit = d->hasbit;
-  return true;
-}
diff --git a/upb/msg.h b/upb/msg.h
index e875f6e..17c3e65 100644
--- a/upb/msg.h
+++ b/upb/msg.h
@@ -4,22 +4,17 @@
 ** However it differs from other common representations like
 ** google::protobuf::Message in one key way: it does not prescribe any
 ** ownership between messages and submessages, and it relies on the
-** client to delete each message/submessage/array/map at the appropriate
-** time.
+** client to ensure that each submessage/array/map outlives its parent.
+**
+** All messages, arrays, and maps live in an Arena.  If the entire message
+** tree is in the same arena, ensuring proper lifetimes is simple.  However
+** the client can mix arenas as long as they ensure that there are no
+** dangling pointers.
 **
 ** A client can access a upb::Message without knowing anything about
 ** ownership semantics, but to create or mutate a message a user needs
 ** to implement the memory management themselves.
 **
-** Currently all messages, arrays, and maps store a upb_alloc* internally.
-** Mutating operations use this when they require dynamically-allocated
-** memory.  We could potentially eliminate this size overhead later by
-** letting the user flip a bit on the factory that prevents this from
-** being stored.  The user would then need to use separate functions where
-** the upb_alloc* is passed explicitly.  However for handlers to populate
-** such structures, they would need a place to store this upb_alloc* during
-** parsing; upb_handlers don't currently have a good way to accommodate this.
-**
 ** TODO: UTF-8 checking?
 **/
 
@@ -66,16 +61,6 @@ typedef void upb_msg;
  * msglayout. */
 
 
-/** upb_visitor ***************************************************************/
-
-/* upb_visitor will visit all the fields of a message and its submessages.  It
- * uses a upb_visitorplan which you can obtain from a upb_msgfactory. */
-
-upb_visitor *upb_visitor_create(upb_env *e, const upb_visitorplan *vp,
-                                upb_sink *output);
-bool upb_visitor_visitmsg(upb_visitor *v, const upb_msg *msg);
-
-
 /** upb_msgfactory ************************************************************/
 
 /* A upb_msgfactory contains a cache of upb_msglayout, upb_handlers, and
@@ -104,10 +89,6 @@ const upb_symtab *upb_msgfactory_symtab(const upb_msgfactory *f);
  * upb_msgfactory. */
 const upb_msglayout *upb_msgfactory_getlayout(upb_msgfactory *f,
                                               const upb_msgdef *m);
-const upb_handlers *upb_msgfactory_getmergehandlers(upb_msgfactory *f,
-                                                    const upb_msgdef *m);
-const upb_visitorplan *upb_msgfactory_getvisitorplan(upb_msgfactory *f,
-                                                     const upb_handlers *h);
 
 
 /** upb_stringview ************************************************************/
@@ -183,52 +164,13 @@ UPB_INLINE upb_msgval upb_msgval_makestr(const char *data, size_t size) {
 /** upb_msg *******************************************************************/
 
 /* A upb_msg represents a protobuf message.  It always corresponds to a specific
- * upb_msglayout, which describes how it is laid out in memory.
- *
- * The message will have a fixed size, as returned by upb_msg_sizeof(), which
- * will be used to store fixed-length fields.  The upb_msg may also allocate
- * dynamic memory internally to store data such as:
- *
- * - extensions
- * - unknown fields
- */
+ * upb_msglayout, which describes how it is laid out in memory.  */
 
-/* Returns the size of a message given this layout. */
-size_t upb_msg_sizeof(const upb_msglayout *l);
+/* Creates a new message of the given type/layout in this arena. */
+upb_msg *upb_msg_new(const upb_msglayout *l, upb_arena *a);
 
-/* upb_msg_init() / upb_msg_uninit() allow the user to use a pre-allocated
- * block of memory as a message.  The block's size should be upb_msg_sizeof().
- * upb_msg_uninit() must be called to release internally-allocated memory
- * unless the allocator is an arena that does not require freeing.
- *
- * Please note that upb_msg_init() may return a value that is different than
- * |msg|, so you must assign the return value and not cast your memory block
- * to upb_msg* directly!
- *
- * Please note that upb_msg_uninit() does *not* free any submessages, maps,
- * or arrays referred to by this message's fields.  You must free them manually
- * yourself.
- *
- * upb_msg_uninit returns the original memory block, which may be useful if
- * you dynamically allocated it (though upb_msg_new() would normally be more
- * appropriate in this case). */
-upb_msg *upb_msg_init(void *msg, const upb_msglayout *l, upb_alloc *a);
-void *upb_msg_uninit(upb_msg *msg, const upb_msglayout *l);
-
-/* Like upb_msg_init() / upb_msg_uninit(), except the message's memory is
- * allocated / freed from the given upb_alloc. */
-upb_msg *upb_msg_new(const upb_msglayout *l, upb_alloc *a);
-void upb_msg_free(upb_msg *msg, const upb_msglayout *l);
-
-/* Returns the upb_alloc for the given message.
- * TODO(haberman): get rid of this?  Not sure we want to be storing this
- * for every message. */
-upb_alloc *upb_msg_alloc(const upb_msg *msg);
-
-/* Packs the tree of messages rooted at "msg" into a single hunk of memory,
- * allocated from the given allocator. */
-void *upb_msg_pack(const upb_msg *msg, const upb_msglayout *l,
-                   void *p, size_t *ofs, size_t size);
+/* Returns the arena for the given message. */
+upb_arena *upb_msg_arena(const upb_msg *msg);
 
 /* Read-only message API.  Can be safely called by anyone. */
 
@@ -282,16 +224,12 @@ bool upb_msg_clearfield(upb_msg *msg,
  * semantics are the same as upb_msg.  A upb_array allocates dynamic
  * memory internally for the array elements. */
 
-size_t upb_array_sizeof(upb_fieldtype_t type);
-void upb_array_init(upb_array *arr, upb_fieldtype_t type, upb_alloc *a);
-void upb_array_uninit(upb_array *arr);
-upb_array *upb_array_new(upb_fieldtype_t type, upb_alloc *a);
-void upb_array_free(upb_array *arr);
+upb_array *upb_array_new(upb_fieldtype_t type, upb_arena *a);
+upb_fieldtype_t upb_array_type(const upb_array *arr);
 
 /* Read-only interface.  Safe for anyone to call. */
 
 size_t upb_array_size(const upb_array *arr);
-upb_fieldtype_t upb_array_type(const upb_array *arr);
 upb_msgval upb_array_get(const upb_array *arr, size_t i);
 
 /* Write interface.  May only be called by the message's owner who can enforce
@@ -308,12 +246,8 @@ bool upb_array_set(upb_array *arr, size_t i, upb_msgval val);
  * So you must ensure that any string or message values outlive the map, and you
  * must delete them manually when they are no longer required. */
 
-size_t upb_map_sizeof(upb_fieldtype_t ktype, upb_fieldtype_t vtype);
-bool upb_map_init(upb_map *map, upb_fieldtype_t ktype, upb_fieldtype_t vtype,
-                  upb_alloc *a);
-void upb_map_uninit(upb_map *map);
-upb_map *upb_map_new(upb_fieldtype_t ktype, upb_fieldtype_t vtype, upb_alloc *a);
-void upb_map_free(upb_map *map);
+upb_map *upb_map_new(upb_fieldtype_t ktype, upb_fieldtype_t vtype,
+                     upb_arena *a);
 
 /* Read-only interface.  Safe for anyone to call. */
 
diff --git a/upb/structs.int.h b/upb/structs.int.h
index 242155b..6bc502e 100644
--- a/upb/structs.int.h
+++ b/upb/structs.int.h
@@ -11,7 +11,7 @@ struct upb_array {
   void *data;   /* Each element is element_size. */
   size_t len;   /* Measured in elements. */
   size_t size;  /* Measured in elements. */
-  upb_alloc *alloc;
+  upb_arena *arena;
 };
 
 #endif  /* UPB_STRUCTS_H_ */