Enforce that filenames are unique when loaded into symtab.

This brings upb into line with C++. PHP already checks this
internally, so this should not be an issue there. Ruby on the
other hand does not currently check this, so this change will
cause our Ruby implementation to reject some programs that
would otherwise have been accepted.

tests/bindings/lua/test_upb.lua

@@ -717,6 +717,15 @@ function test_descriptor_error()
   assert_nil(symtab:lookup_msg("ABC"))
 end
 
+function test_duplicate_filename_error()
+  local symtab = upb.SymbolTable()
+  local file = descriptor.FileDescriptorProto()
+  file.name = "test.proto"
+  symtab:add_file(upb.encode(file))
+  -- Second add with the same filename fails.
+  assert_error(function () symtab:add_file(upb.encode(file)) end)
+end
+
 function test_encode_skipunknown()
   -- Test that upb.ENCODE_SKIPUNKNOWN does not encode unknown fields.
   local msg = test_messages_proto3.TestAllTypesProto3{

upb/def.c

@@ -1907,13 +1907,18 @@ static void build_filedef(
   const upb_strview* strs;
   size_t i, n;
 
-  count_types_in_file(file_proto, file);
+  file->symtab = ctx->symtab;
 
+  /* One pass to count and allocate. */
+  file->msg_count = 0;
+  file->enum_count = 0;
+  file->ext_count = 0;
+  count_types_in_file(file_proto, file);
   file->msgs = symtab_alloc(ctx, sizeof(*file->msgs) * file->msg_count);
   file->enums = symtab_alloc(ctx, sizeof(*file->enums) * file->enum_count);
   file->exts = symtab_alloc(ctx, sizeof(*file->exts) * file->ext_count);
 
-  /* We increment these as defs are added. */
+  /* In the second pass we increment these as defs are added. */
   file->msg_count = 0;
   file->enum_count = 0;
   file->ext_count = 0;
@@ -2042,41 +2047,43 @@ static void remove_filedef(upb_symtab *s, upb_filedef *file) {
 static const upb_filedef *_upb_symtab_addfile(
     upb_symtab *s, const google_protobuf_FileDescriptorProto *file_proto,
     const upb_msglayout **layouts, upb_status *status) {
-  upb_arena *file_arena = upb_arena_new();
-  upb_filedef *file;
   symtab_addctx ctx;
+  upb_strview name = google_protobuf_FileDescriptorProto_name(file_proto);
 
-  if (!file_arena) return NULL;
-
-  file = upb_arena_malloc(file_arena, sizeof(*file));
-  if (!file) goto done;
+  if (upb_strtable_lookup2(&s->files, name.data, name.size, NULL)) {
+    upb_status_seterrf(status, "duplicate file name (%.*s)",
+                       UPB_STRVIEW_ARGS(name));
+    return NULL;
+  }
 
-  ctx.file = file;
   ctx.symtab = s;
-  ctx.arena = file_arena;
   ctx.layouts = layouts;
   ctx.status = status;
+  ctx.file = NULL;
+  ctx.arena = upb_arena_new();
 
-  file->msg_count = 0;
-  file->enum_count = 0;
-  file->ext_count = 0;
-  file->symtab = s;
+  if (!ctx.arena) {
+    upb_status_setoom(status);
+    return NULL;
+  }
 
   if (UPB_UNLIKELY(UPB_SETJMP(ctx.err))) {
     UPB_ASSERT(!upb_ok(status));
-    remove_filedef(s, file);
-    file = NULL;
+    if (ctx.file) {
+      remove_filedef(s, ctx.file);
+      ctx.file = NULL;
+    }
   } else {
-    build_filedef(&ctx, file, file_proto);
-    upb_strtable_insert(&s->files, file->name, strlen(file->name),
-                        upb_value_constptr(file), ctx.arena);
+    ctx.file = symtab_alloc(&ctx, sizeof(*ctx.file));
+    build_filedef(&ctx, ctx.file, file_proto);
+    upb_strtable_insert(&s->files, name.data, name.size,
+                        upb_value_constptr(ctx.file), ctx.arena);
     UPB_ASSERT(upb_ok(status));
-    upb_arena_fuse(s->arena, file_arena);
+    upb_arena_fuse(s->arena, ctx.arena);
   }
 
-done:
-  upb_arena_free(file_arena);
-  return file;
+  upb_arena_free(ctx.arena);
+  return ctx.file;
 }
 
 const upb_filedef *upb_symtab_addfile(