Add two new generation options around objc prefix usage.

- `require_prefixes`: Fail generation if a file doesn't have the option.
- `prefixes_must_be_registered`: If a prefix is in a file, it must be registered
  in the expected file. There is a grey space here for files without packages as
  there is no way to register it, so these case will still warn at the moment.

src/google/protobuf/compiler/objectivec/objectivec_generator.cc

@@ -101,6 +101,41 @@ bool ObjectiveCGenerator::GenerateAll(
         generation_options.expected_prefixes_suppressions.push_back(
             std::string(split_piece));
       }
+    } else if (options[i].first == "prefixes_must_be_registered") {
+      // If objc prefix file option value must be registered to be used. This
+      // option has no meaning if an "expected_prefixes_path" isn't set. The
+      // available options are:
+      //   "no": They don't have to be registered.
+      //   "yes": They must be registered and an error will be raised if a files
+      //     tried to use a prefix that isn't registered.
+      // Default is "no".
+      std::string upper_value(options[i].second);
+      UpperString(&upper_value);
+      if (upper_value == "NO") {
+        generation_options.prefixes_must_be_registered = false;
+      } else if (upper_value == "YES") {
+        generation_options.prefixes_must_be_registered = true;
+      } else {
+        *error = "error: Unknown value for prefixes_must_be_registered: " + options[i].second;
+        return false;
+      }
+    } else if (options[i].first == "require_prefixes") {
+      // If every file must have an objc prefix file option to be used. The
+      // available options are:
+      //   "no": Files can be generated without the prefix option.
+      //   "yes": Files must have the objc prefix option, and an error will be
+      //     raised if a files doesn't have one.
+      // Default is "no".
+      std::string upper_value(options[i].second);
+      UpperString(&upper_value);
+      if (upper_value == "NO") {
+        generation_options.require_prefixes = false;
+      } else if (upper_value == "YES") {
+        generation_options.require_prefixes = true;
+      } else {
+        *error = "error: Unknown value for require_prefixes: " + options[i].second;
+        return false;
+      }
     } else if (options[i].first == "generate_for_named_framework") {
       // The name of the framework that protos are being generated for. This
       // will cause the #import statements to be framework based using this

src/google/protobuf/compiler/objectivec/objectivec_helpers.cc

@@ -178,6 +178,8 @@ Options::Options() {
     expected_prefixes_suppressions =
         Split(suppressions, ";", true);
   }
+  prefixes_must_be_registered = false;
+  require_prefixes = false;
 }
 
 namespace {
@@ -1227,6 +1229,7 @@ bool LoadExpectedPackagePrefixes(const Options& generation_options,
 bool ValidateObjCClassPrefix(
     const FileDescriptor* file, const std::string& expected_prefixes_path,
     const std::map<std::string, std::string>& expected_package_prefixes,
+    bool prefixes_must_be_registered, bool require_prefixes,
     std::string* out_error) {
   // Reminder: An explicit prefix option of "" is valid in case the default
   // prefixing is set to use the proto package and a file needs to be generated
@@ -1265,6 +1268,12 @@ bool ValidateObjCClassPrefix(
 
   // If there was no prefix option, we're done at this point.
   if (!has_prefix) {
+    if (require_prefixes) {
+      *out_error =
+        "error: '" + file->name() + "' does not have a required 'option" +
+        " objc_class_prefix'.";
+      return false;
+    }
     return true;
   }
 
@@ -1344,9 +1353,18 @@ bool ValidateObjCClassPrefix(
     std::cerr.flush();
   }
 
-  // Check: Warning - If the given package/prefix pair wasn't expected, issue a
-  // warning suggesting it gets added to the file.
+  // Check: Error/Warning - If the given package/prefix pair wasn't expected,
+  // issue a error/warning to added to the file.
   if (have_expected_prefix_file) {
+    if (prefixes_must_be_registered) {
+      *out_error =
+        "error: '" + file->name() + "' has 'option objc_class_prefix = \"" +
+        prefix + "\";', but it is not registered; add it to the expected" +
+        " prefixes file (" + expected_prefixes_path + ") for the package" +
+        "'" + package + "'.";
+      return false;
+    }
+
     std::cerr
          << "protoc:0: warning: Found unexpected 'option objc_class_prefix = \""
          << prefix << "\";' in '" << file->name() << "';"
@@ -1391,6 +1409,8 @@ bool ValidateObjCClassPrefixes(const std::vector<const FileDescriptor*>& files,
         ValidateObjCClassPrefix(files[i],
                                 generation_options.expected_prefixes_path,
                                 expected_package_prefixes,
+                                generation_options.prefixes_must_be_registered,
+                                generation_options.require_prefixes,
                                 out_error);
     if (!is_valid) {
       return false;

src/google/protobuf/compiler/objectivec/objectivec_helpers.h

@@ -67,6 +67,8 @@ struct Options {
   std::string generate_for_named_framework;
   std::string named_framework_to_proto_path_mappings_path;
   std::string runtime_import_prefix;
+  bool prefixes_must_be_registered;
+  bool require_prefixes;
 };
 
 // Escape C++ trigraphs by escaping question marks to "\?".