proto3: make strings default to "" instead of NULL

The spec talks about "empty string" and other languages like C#
return a zero length string and not null.
This is useful because when moving from proto2's "required string"
to a proto3's plain string we will be guaranteed that we
never get a null pointer.
The tradeoff is adding a special case to the library but avoiding
a lot of null checks in the calling code.
The current code is already special casing "" in pack_string.

protobuf-c/libprotobuf-c.sym

@@ -21,3 +21,8 @@ global:
 local:
         *;
 };
+
+LIBPROTOBUF_C_1.3.0 {
+global:
+        protobuf_c_empty_string;
+} LIBPROTOBUF_C_1.0.0;

protobuf-c/protobuf-c.c

@@ -84,6 +84,8 @@
 # define PROTOBUF_C_UNPACK_ERROR(...)
 #endif
 
+const char protobuf_c_empty_string[] = "";
+
 /**
  * Internal `ProtobufCMessage` manipulation macro.
  *
@@ -554,6 +556,9 @@ field_is_zeroish(const ProtobufCFieldDescriptor *field,
 		ret = (0 == *(const double *) member);
 		break;
 	case PROTOBUF_C_TYPE_STRING:
+		ret = (NULL == *(const char * const *) member) ||
+		      ('\0' == **(const char * const *) member);
+		break;
 	case PROTOBUF_C_TYPE_BYTES:
 	case PROTOBUF_C_TYPE_MESSAGE:
 		ret = (NULL == *(const void * const *) member);

protobuf-c/protobuf-c.h

@@ -237,6 +237,9 @@ PROTOBUF_C__BEGIN_DECLS
 #define PROTOBUF_C__MESSAGE_DESCRIPTOR_MAGIC    0x28aaeef9
 #define PROTOBUF_C__ENUM_DESCRIPTOR_MAGIC       0x114315af
 
+/* Empty string used for initializers */
+extern const char protobuf_c_empty_string[];
+
 /**
  * \defgroup api Public API
  *

protoc-c/c_field.cc

@@ -129,6 +129,9 @@ void FieldGenerator::GenerateDescriptorInitializerGeneric(io::Printer* printer,
     variables["default_value"] = string("&")
                                + FullNameToLower(descriptor_->full_name())
 			       + "__default_value";
+  } else if (FieldSyntax(descriptor_) == 3 &&
+    descriptor_->type() == FieldDescriptor::TYPE_STRING) {
+    variables["default_value"] = "&protobuf_c_empty_string";
   } else {
     variables["default_value"] = "NULL";
   }

protoc-c/c_string_field.cc

@@ -125,8 +125,10 @@ void StringFieldGenerator::GenerateStaticInit(io::Printer* printer) const
   std::map<string, string> vars;
   if (descriptor_->has_default_value()) {
     vars["default"] = GetDefaultValue();
-  } else {
+  } else if (FieldSyntax(descriptor_) == 2) {
     vars["default"] = "NULL";
+  } else {
+    vars["default"] = "(char *)protobuf_c_empty_string";
   }
   switch (descriptor_->label()) {
     case FieldDescriptor::LABEL_REQUIRED:

t/generated-code/test-generated-code.c

@@ -51,6 +51,11 @@ int main(void)
   person2 = foo__person__unpack (NULL, size, packed);
   assert (person2 != NULL);
   assert (person2->id == 42);
+#ifndef PROTO3
+  assert (person2->email == NULL);
+#else
+  assert (strcmp (person2->email, "") == 0);
+#endif
   assert (strcmp (person2->name, "dave b") == 0);
   assert (person2->n_phone == 1);
   assert (strcmp (person2->phone[0]->number, "1234") == 0);