Add package_as_prefix_forced_prefix generation option.

This is a generation option that provides serves the same purpose as the
existing GPB_OBJC_USE_PACKAGE_AS_PREFIX_PREFIX environment variable; just
providing a different way to set/enable it.

objectivec/README.md

@@ -200,12 +200,13 @@ supported keys are:
         entry can be made as "no_package:PATH=prefix", where PATH is the
       path for the .proto file.
 
-  * `use_package_as_prefix` and `proto_package_prefix_exceptions_path`: The
-    `value` for `use_package_as_prefix` can be `yes` or `no`, and indicates
-    if a prefix should be derived from the proto package for all the symbols
-    for files that don't have the `objc_class_prefix` file option (mentioned
-    above). This helps ensure the symbols are more unique and means there is
-    less chance of ObjC class name collisions.
+  * `use_package_as_prefix`, `package_as_prefix_forced_prefix` and
+    `proto_package_prefix_exceptions_path`: The `value` for
+    `use_package_as_prefix` can be `yes` or `no`, and indicates if a prefix
+    should be derived from the proto package for all the symbols for files that
+    don't have the `objc_class_prefix` file option (mentioned above). This helps
+    ensure the symbols are more unique and means there is less chance of ObjC
+    class name collisions.
 
     To help in migrating code to using this support,
     `proto_package_prefix_exceptions_path` can be used to provide the path
@@ -213,10 +214,16 @@ supported keys are:
     if prefixed with `#`). These package won't get the derived prefix, allowing
     migrations to the behavior one proto package at a time across a code base.
 
+    `package_as_prefix_forced_prefix` can be used to provide a value that will
+    be used before all prefixes derived from the packages to help group all of
+    these types with a common prefix. Thus it only makes sense to use it when
+    `use_package_as_prefix` is also enabled. For example, setting this to
+    "XYZ\_" and generating a file with the package "something" defining
+    "MyMessage", would have Objective-C class be `XYZ_Something_MyMessage`.
+
     `use_package_as_prefix` currently defaults to `no` (existing behavior), but
-    in the future (as a breaking change), that is likely to change since it
-    helps prepare folks before they end up using a lot of protos and getting a
-    lot of collisions.
+    that could change in the future as it helps avoid collisions when more
+    protos get added to the build. Note that this would be a breaking change.
 
   * `headers_use_forward_declarations`: The `value` for this can be `yes` or
     `no`, and indicates if the generated headers use forward declarations for

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

@@ -230,6 +230,10 @@ bool ObjectiveCGenerator::GenerateAll(
       //   - A comment can go on a line after a expected package/prefix pair.
       //     (i.e. - "some.proto.package # comment")
       SetProtoPackagePrefixExceptionList(options[i].second);
+    } else if (options[i].first == "package_as_prefix_forced_prefix") {
+      // String to use as the prefix when deriving a prefix from the package
+      // name. So this only applies when use_package_as_prefix is also used.
+      SetForcedPackagePrefix(options[i].second);
     } else if (options[i].first == "headers_use_forward_declarations") {
       if (!StringToBool(options[i].second,
                         &generation_options.headers_use_forward_declarations)) {

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

@@ -134,6 +134,7 @@ class PrefixModeStorage {
   // When using a proto package as the prefix, this should be added as the
   // prefix in front of it.
   const std::string& forced_package_prefix() const { return forced_prefix_; }
+  void set_forced_package_prefix(const std::string& prefix) { forced_prefix_ = prefix; }
 
  private:
   bool use_package_name_;
@@ -155,8 +156,6 @@ PrefixModeStorage::PrefixModeStorage() {
     exception_path_ = exception_path;
   }
 
-  // This one is a not expected to be common, so it doesn't get a generation
-  // option, just the env var.
   const char* prefix = getenv("GPB_OBJC_USE_PACKAGE_AS_PREFIX_PREFIX");
   if (prefix) {
     forced_prefix_ = prefix;
@@ -254,6 +253,14 @@ void SetProtoPackagePrefixExceptionList(const std::string& file_path) {
   g_prefix_mode.set_exception_path(file_path);
 }
 
+std::string GetForcedPackagePrefix() {
+  return g_prefix_mode.forced_package_prefix();
+}
+
+void SetForcedPackagePrefix(const std::string& prefix) {
+  g_prefix_mode.set_forced_package_prefix(prefix);
+}
+
 Options::Options() {
   // While there are generator options, also support env variables to help with
   // build systems where it isn't as easy to hook in for add the generation

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

@@ -63,6 +63,10 @@ void PROTOC_EXPORT SetUseProtoPackageAsDefaultPrefix(bool on_or_off);
 std::string PROTOC_EXPORT GetProtoPackagePrefixExceptionList();
 void PROTOC_EXPORT SetProtoPackagePrefixExceptionList(
     const std::string& file_path);
+// Get/Set a prefix to add before the prefix generated from the package name.
+// This is only used when UseProtoPackageAsDefaultPrefix() is True.
+std::string PROTOC_EXPORT GetForcedPackagePrefix();
+void PROTOC_EXPORT SetForcedPackagePrefix(const std::string& prefix);
 
 // Generator Prefix Validation Options (see objectivec_generator.cc for a
 // description of each):