Add `prefix_to_proto_package_mappings_path` ObjC option.

objectivec/README.md

@@ -133,12 +133,12 @@ This options allow you to provide a custom prefix for all the symbols generated
 from a proto file (classes (from message), enums, the Root for extension
 support).
 
-If not set, the generation option `use_package_as_prefix` (documented below)
+If not set, the generation options `prefix_to_proto_package_mappings_path` and
-controls what is used instead. Since Objective C uses a global namespace for all
+`use_package_as_prefix` (documented below) controls what is used instead. Since
-of its classes, there can be collisions. `use_package_as_prefix=yes` should
+Objective C uses a global namespace for all of its classes, there can be collisions.
-avoid collisions since proto package are used to scope/name things in other
+`use_package_as_prefix=yes` should avoid collisions since proto package are used to
-languages, but this option can be used to get shorter names instead. Convention
+scope/name things in other languages, but this option can be used to get shorter
-is to base the explicit prefix on the proto package.
+names instead. Convention is to base the explicit prefix on the proto package.
 
 Objective C Generator `protoc` Options
 --------------------------------------
@@ -182,6 +182,24 @@ supported keys are:
     having to add the runtime directory to the header search path since the
     generate `#import` will be more complete.
 
+  * `prefix_to_proto_package_mappings_path`: The `value` used for
+    this key is a path to a file containing a list of prefixes and proto packages.
+    The generator will use this to locate which ObjC class prefix to use when
+    generating sources _unless_ the `objc_class_prefix` file option is set.
+    This option can be useful if multiple apps consume a common set of
+    proto files but wish to use a different prefix for the generated sources
+    between them. This option takes precedent over the `use_package_as_prefix`
+    option.
+
+    The format of the file is:
+      * An entry is a line of "package=prefix".
+      * Comments start with `#`.
+      * A comment can go on a line after a expected package/prefix pair.
+        (i.e. - "package=prefix # comment")
+      * For files that do NOT have a proto package (not recommended), an
+        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

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

@@ -190,6 +190,21 @@ bool ObjectiveCGenerator::GenerateAll(
       // header search path since the generate #import will be more complete.
       generation_options.runtime_import_prefix =
           StripSuffixString(options[i].second, "/");
+    } else if (options[i].first == "prefix_to_proto_package_mappings_path") {
+      // Path to use for when loading the objc class prefix mappings to use.
+      // The `objc_class_prefix` file option is always honored first if one is present.
+      // This option also has precedent over the use_package_as_prefix option.
+      //
+      // The format of the file is:
+      //   - An entry is a line of "package=prefix".
+      //   - Comments start with "#".
+      //   - A comment can go on a line after a expected package/prefix pair.
+      //     (i.e. - "package=prefix # comment")
+      //   - For files that do NOT have a proto package (not recommended), an
+      //     entry can be made as "no_package:PATH=prefix", where PATH is the
+      //     path for the .proto file.
+      //
+      SetPrefixToProtoPackageMappingsPath(options[i].second);
     } else if (options[i].first == "use_package_as_prefix") {
       // Controls how the symbols should be prefixed to avoid symbols
       // collisions. The objc_class_prefix file option is always honored, this

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

@@ -95,10 +95,29 @@ class SimpleLineCollector : public LineConsumer {
   std::unordered_set<std::string>* set_;
 };
 
+class ExpectedPrefixesCollector : public LineConsumer {
+ public:
+  ExpectedPrefixesCollector(std::map<std::string, std::string>* inout_package_to_prefix_map)
+      : prefix_map_(inout_package_to_prefix_map) {}
+
+  virtual bool ConsumeLine(const StringPiece& line, std::string* out_error) override;
+
+ private:
+  std::map<std::string, std::string>* prefix_map_;
+};
+
 class PrefixModeStorage {
  public:
   PrefixModeStorage();
 
+  const std::string prefix_to_proto_package_mappings_path() const { return prefix_to_proto_package_mappings_path_; }
+  void set_prefix_to_proto_package_mappings_path(const std::string& path) {
+    prefix_to_proto_package_mappings_path_ = path;
+    prefix_to_proto_package_map_.clear();
+  }
+
+  std::string prefix_from_proto_package_mappings(const FileDescriptor* file);
+
   bool use_package_name() const { return use_package_name_; }
   void set_use_package_name(bool on_or_off) { use_package_name_ = on_or_off; }
 
@@ -116,6 +135,8 @@ class PrefixModeStorage {
 
  private:
   bool use_package_name_;
+  std::map<std::string, std::string> prefix_to_proto_package_map_;
+  std::string prefix_to_proto_package_mappings_path_;
   std::string exception_path_;
   std::string forced_prefix_;
   std::unordered_set<std::string> exceptions_;
@@ -140,6 +161,44 @@ PrefixModeStorage::PrefixModeStorage() {
   }
 }
 
+std::string PrefixModeStorage::prefix_from_proto_package_mappings(const FileDescriptor* file) {
+  if (!file) {
+    return "";
+  }
+
+  if (prefix_to_proto_package_map_.empty() && !prefix_to_proto_package_mappings_path_.empty()) {
+    std::string error_str;
+    // Re use the same collector as we use for expected_prefixes_path since the file
+    // format is the same.
+    ExpectedPrefixesCollector collector(&prefix_to_proto_package_map_);
+    if (!ParseSimpleFile(prefix_to_proto_package_mappings_path_, &collector, &error_str)) {
+      if (error_str.empty()) {
+        error_str = std::string("protoc:0: warning: Failed to parse")
+           + std::string(" prefix to proto package mappings file: ")
+           + prefix_to_proto_package_mappings_path_;
+      }
+      std::cerr << error_str << std::endl;
+      std::cerr.flush();
+      prefix_to_proto_package_map_.clear();      
+    }
+  }
+
+  const std::string package = file->package();
+  // For files without packages, the can be registered as "no_package:PATH",
+  // allowing the expected prefixes file.
+  static const std::string no_package_prefix("no_package:");
+  const std::string lookup_key = package.empty() ? no_package_prefix + file->name() : package;
+
+  std::map<std::string, std::string>::const_iterator prefix_lookup =
+      prefix_to_proto_package_map_.find(lookup_key);
+
+  if (prefix_lookup != prefix_to_proto_package_map_.end()) {
+    return prefix_lookup->second;
+  }  
+
+  return "";
+}
+
 bool PrefixModeStorage::is_package_exempted(const std::string& package) {
   if (exceptions_.empty() && !exception_path_.empty()) {
     std::string error_str;
@@ -169,6 +228,14 @@ PrefixModeStorage g_prefix_mode;
 
 }  // namespace
 
+std::string GetPrefixToProtoPackageMappingsPath() {
+  return g_prefix_mode.prefix_to_proto_package_mappings_path();
+}
+
+void SetPrefixToProtoPackageMappingsPath(const std::string& file_path) {
+  g_prefix_mode.set_prefix_to_proto_package_mappings_path(file_path);
+}
+
 bool UseProtoPackageAsDefaultPrefix() {
   return g_prefix_mode.use_package_name();
 }
@@ -531,6 +598,12 @@ std::string FileClassPrefix(const FileDescriptor* file) {
     return file->options().objc_class_prefix();
   }
 
+  // If package prefix is specified in an prefix to proto mappings file then use that.
+  std::string objc_class_prefix = g_prefix_mode.prefix_from_proto_package_mappings(file);
+  if (!objc_class_prefix.empty()) {
+    return objc_class_prefix;
+  }
+
   // If package prefix isn't enabled, done.
   if (!g_prefix_mode.use_package_name()) {
     return "";
@@ -1205,17 +1278,6 @@ void RemoveComment(StringPiece* input) {
 
 namespace {
 
-class ExpectedPrefixesCollector : public LineConsumer {
- public:
-  ExpectedPrefixesCollector(std::map<std::string, std::string>* inout_package_to_prefix_map)
-      : prefix_map_(inout_package_to_prefix_map) {}
-
-  virtual bool ConsumeLine(const StringPiece& line, std::string* out_error) override;
-
- private:
-  std::map<std::string, std::string>* prefix_map_;
-};
-
 bool ExpectedPrefixesCollector::ConsumeLine(
     const StringPiece& line, std::string* out_error) {
   int offset = line.find('=');

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

@@ -47,6 +47,10 @@ namespace protobuf {
 namespace compiler {
 namespace objectivec {
 
+// Get/Set the path to a file to load for objc class prefix lookups.
+std::string PROTOC_EXPORT GetPrefixToProtoPackageMappingsPath();
+void PROTOC_EXPORT SetPrefixToProtoPackageMappingsPath(
+    const std::string& file_path);
 // Get/Set if the proto package should be used to make the default prefix for
 // symbols. This will then impact most of the type naming apis below. It is done
 // as a global to not break any other generator reusing the methods since they