Added upb::FileDef, which represents the file defs are declared in.

It is entirely optional: MessageDef/EnumDef can still exist
on their own.  But this can represent a def's file when it is
desirable to do so (eg. for code generators).

This approach will require that we change the way we handle
extensions.  But I think it will be a good change overall.

Specifically, we previously handled extensions by duplicating
the extended message and then adding the extension as a regular
field to the duplicated message.  This required also duplicating
any messages that could reach the extended message.

In the new world we will need a way of declaring and looking up
extensions separately from the message being extended.

This change also involves some notable changes to the generated
code:

- files are now called foo.upbdefs.h instead of foo.upb.h.
  This reflects the fact that we might possibly generate several
  different output files for a .proto file, and this one is just
  for defs.

- we no longer generate selectors in the .h file.

- the upbdefs.c no longer vends a SymbolTable.  Now it vends the
  individual messages (and possibly a FileDef later).  I think this
  will compose better once we can generate files where one
  generated files imports another.

We also make the descriptor reader vend a list of FileDefs now.
This is the best conceptual match for parsing a FileDescriptorSet.

1 files changed, 22 insertions, 2 deletions

diff --git a/tests/pb/test_encoder.cc b/tests/pb/test_encoder.cc
index 8a222f4..6dd10ba 100644
--- a/tests/pb/test_encoder.cc
+++ b/tests/pb/test_encoder.cc
@@ -1,11 +1,31 @@
 
 #include "tests/upb_test.h"
 #include "upb/bindings/stdc++/string.h"
-#include "upb/descriptor/descriptor.upb.h"
+#include "upb/descriptor/descriptor.upbdefs.h"
 #include "upb/pb/decoder.h"
 #include "upb/pb/encoder.h"
 #include "upb/pb/glue.h"
 
+static char *upb_readfile(const char *filename, size_t *len) {
+  long size;
+  char *buf;
+  FILE *f = fopen(filename, "rb");
+  if(!f) return NULL;
+  if(fseek(f, 0, SEEK_END) != 0) goto error;
+  size = ftell(f);
+  if(size < 0) goto error;
+  if(fseek(f, 0, SEEK_SET) != 0) goto error;
+  buf = (char*)malloc(size + 1);
+  if(size && fread(buf, size, 1, f) != 1) goto error;
+  fclose(f);
+  if (len) *len = size;
+  return buf;
+
+error:
+  fclose(f);
+  return NULL;
+}
+
 std::string read_string(const char *filename) {
   size_t len;
   char *str = upb_readfile(filename, &len);
@@ -18,7 +38,7 @@ std::string read_string(const char *filename) {
 
 void test_pb_roundtrip() {
   upb::reffed_ptr<const upb::MessageDef> md(
-      upbdefs::google::protobuf::FileDescriptorSet::MessageDef());
+      upbdefs::google::protobuf::FileDescriptorSet::get());
   upb::reffed_ptr<const upb::Handlers> encoder_handlers(
       upb::pb::Encoder::NewHandlers(md.get()));
   upb::reffed_ptr<const upb::pb::DecoderMethod> method(