Addressed PR comments.

python/descriptor.c

diff --git a/python/descriptor.c b/python/descriptor.c
index a9718da..b7e8854 100644
--- a/python/descriptor.c
+++ b/python/descriptor.c
@@ -160,6 +160,7 @@
   return str;
 
 oom:
+  upb_arena_free(arena);
   PyErr_SetNone(PyExc_MemoryError);
   return NULL;
 }

python/descriptor_pool.c

diff --git a/python/descriptor_pool.c b/python/descriptor_pool.c
index 45ea6a0..73bd067 100644
--- a/python/descriptor_pool.c
+++ b/python/descriptor_pool.c
@@ -240,6 +240,17 @@
   return PyUpb_Descriptor_Get(m);
 }
 
+// Splits a dotted symbol like foo.bar.baz on the last dot.  Returns the portion
+// after the last dot (baz) and updates `*parent_size` to the length of the
+// parent (foo.bar).  Returns NULL if no dots were present.
+static const char* PyUpb_DescriptorPool_SplitSymbolName(const char* sym,
+                                                        size_t* parent_size) {
+  const char* last_dot = strrchr(sym, '.');
+  if (!last_dot) return NULL;
+  *parent_size = last_dot - sym;
+  return last_dot + 1;
+}
+
 /*
  * PyUpb_DescriptorPool_FindFieldByName()
  *
@@ -253,14 +264,19 @@
   const char* name = PyUpb_GetStrData(arg);
   if (!name) return NULL;
 
+  // First lookup as extension.
   const upb_fielddef* f = upb_symtab_lookupext(self->symtab, name);
+
   if (!f) {
-    const char* last_dot = strrchr(name, '.');
-    if (last_dot) {
+    // Otherwise look for a normal field.
+    size_t parent_size;
+    const char* child =
+        PyUpb_DescriptorPool_SplitSymbolName(name, &parent_size);
+    if (child) {
       const upb_msgdef* parent =
-          upb_symtab_lookupmsg2(self->symtab, name, last_dot - name);
+          upb_symtab_lookupmsg2(self->symtab, name, parent_size);
       if (parent) {
-        f = upb_msgdef_ntofz(parent, last_dot + 1);
+        f = upb_msgdef_ntofz(parent, child);
       }
     }
   }
@@ -306,12 +322,14 @@
   const char* name = PyUpb_GetStrData(arg);
   if (!name) return NULL;
 
-  const char* last_dot = strrchr(name, '.');
-  if (last_dot) {
+  size_t parent_size;
+  const char* child = PyUpb_DescriptorPool_SplitSymbolName(name, &parent_size);
+
+  if (child) {
     const upb_msgdef* parent =
-        upb_symtab_lookupmsg2(self->symtab, name, last_dot - name);
+        upb_symtab_lookupmsg2(self->symtab, name, parent_size);
     if (parent) {
-      const upb_oneofdef* o = upb_msgdef_ntooz(parent, last_dot + 1);
+      const upb_oneofdef* o = upb_msgdef_ntooz(parent, child);
       return PyUpb_OneofDescriptor_Get(o);
     }
   }
@@ -376,6 +394,7 @@
   PyObject* ret = PyList_New(n);
   for (size_t i = 0; i < n; i++) {
     PyObject* field = PyUpb_FieldDescriptor_Get(ext[i]);
+    if (!field) return NULL;
     PyList_SetItem(ret, i, field);
   }
   return ret;

upb/util/def_to_proto.c

diff --git a/upb/util/def_to_proto.c b/upb/util/def_to_proto.c
index e813f7a..71b91a2 100644
--- a/upb/util/def_to_proto.c
+++ b/upb/util/def_to_proto.c
@@ -393,9 +393,8 @@
       proto, strviewdup(ctx, upb_filedef_name(f)));
 
   const char* package = upb_filedef_package(f);
-  size_t n;
   if (package) {
-    n = strlen(package);
+    size_t n = strlen(package);
     if (n) {
       google_protobuf_FileDescriptorProto_set_package(
           proto, strviewdup(ctx, upb_filedef_package(f)));
@@ -407,6 +406,7 @@
                                                    strviewdup(ctx, "proto3"));
   }
 
+  size_t n;
   n = upb_filedef_depcount(f);
   upb_strview *deps = google_protobuf_FileDescriptorProto_resize_dependency(
       proto, n, ctx->arena);