Make Root's +extensionRegistry generation smarter.

At generation time, walk the file's dependencies to see what really contains
extensions so we can generate more minimal code that only links together the
roots that provided extensions. Gets a bunch of otherwise noop code out of
the call flow when the roots are +initialized.

Makefile.am

@@ -534,6 +534,7 @@ objectivec_EXTRA_DIST=                                                       \
   objectivec/Tests/GPBTestUtilities.h                                        \
   objectivec/Tests/GPBTestUtilities.m                                        \
   objectivec/Tests/GPBUnittestProtos.m                                       \
+  objectivec/Tests/GPBUnittestProtos2.m                                      \
   objectivec/Tests/GPBUnknownFieldSetTest.m                                  \
   objectivec/Tests/GPBUtilitiesTests.m                                       \
   objectivec/Tests/GPBWellKnownTypesTest.m                                   \
@@ -555,6 +556,13 @@ objectivec_EXTRA_DIST=                                                       \
   objectivec/Tests/text_format_map_unittest_data.txt                          \
   objectivec/Tests/text_format_unittest_data.txt                              \
   objectivec/Tests/unittest_cycle.proto                                       \
+  objectivec/Tests/unittest_extension_chain_a.proto                           \
+  objectivec/Tests/unittest_extension_chain_b.proto                           \
+  objectivec/Tests/unittest_extension_chain_c.proto                           \
+  objectivec/Tests/unittest_extension_chain_d.proto                           \
+  objectivec/Tests/unittest_extension_chain_e.proto                           \
+  objectivec/Tests/unittest_extension_chain_f.proto                           \
+  objectivec/Tests/unittest_extension_chain_g.proto                           \
   objectivec/Tests/unittest_objc.proto                                        \
   objectivec/Tests/unittest_objc_startup.proto                                \
   objectivec/Tests/unittest_runtime_proto2.proto                              \

objectivec/DevTools/compile_testing_protos.sh

-#!/bin/bash
-
+#!/bin/bash -eu
 # Invoked by the Xcode projects to build the protos needed for the unittests.
 
-set -eu
-
 readonly OUTPUT_DIR="${PROJECT_DERIVED_FILE_DIR}/protos"
 
+# -----------------------------------------------------------------------------
 # Helper for bailing.
 die() {
   echo "Error: $1"
   exit 2
 }
 
+# -----------------------------------------------------------------------------
 # What to do.
 case "${ACTION}" in
   "")
@@ -26,12 +25,19 @@ case "${ACTION}" in
     ;;
 esac
 
-# Move to the top of the protobuf directories.
-cd "${SRCROOT}/.."
+# -----------------------------------------------------------------------------
+# Ensure the output dir exists
+mkdir -p "${OUTPUT_DIR}/google/protobuf"
 
+# -----------------------------------------------------------------------------
+# Move to the top of the protobuf directories and ensure there is a protoc
+# binary to use.
+cd "${SRCROOT}/.."
 [[ -x src/protoc ]] || \
   die "Could not find the protoc binary; make sure you have built it (objectivec/DevTools/full_mac_build.sh -h)."
 
+# -----------------------------------------------------------------------------
+# See the compiler or proto files have changed.
 RUN_PROTOC=no
 if [[ ! -d "${OUTPUT_DIR}" ]] ; then
   RUN_PROTOC=yes
@@ -50,7 +56,7 @@ else
   # Find the oldest output file.
   readonly OldestOutput=$(find \
         "${OUTPUT_DIR}" \
-        -type f -print0 \
+        -type f -name "*pbobjc.[hm]" -print0 \
         | xargs -0 stat -f "%m %N" \
         | sort -n -r | tail -n1 | cut -f2- -d" ")
   # If the newest input is newer than the oldest output, regenerate.
@@ -64,8 +70,27 @@ if [[ "${RUN_PROTOC}" != "yes" ]] ; then
   exit 0
 fi
 
-# Ensure the output dir exists
-mkdir -p "${OUTPUT_DIR}/google/protobuf"
+# -----------------------------------------------------------------------------
+# Prune out all the files from previous generations to ensure we only have
+# current ones.
+find "${OUTPUT_DIR}" \
+    -type f -name "*pbobjc.[hm]" -print0 \
+    | xargs -0 rm -rf
+
+# -----------------------------------------------------------------------------
+# Helper to invoke protoc
+compile_protos() {
+  src/protoc                                   \
+    --objc_out="${OUTPUT_DIR}/google/protobuf" \
+    --proto_path=src/google/protobuf/          \
+    --proto_path=src                           \
+    "$@"
+}
+
+# -----------------------------------------------------------------------------
+# Generate most of the proto files that exist in the C++ src tree.  Several
+# are used in the tests, but the extra don't hurt in that they ensure ObjC
+# sources can be generated from them.
 
 CORE_PROTO_FILES=(
   src/google/protobuf/unittest_arena.proto
@@ -90,23 +115,12 @@ CORE_PROTO_FILES=(
   src/google/protobuf/map_lite_unittest.proto
   src/google/protobuf/map_proto2_unittest.proto
   src/google/protobuf/map_unittest.proto
-)
-
-# The unittest_custom_options.proto extends the messages in descriptor.proto
-# so we build it in to test extending in general. The library doesn't provide
-# a descriptor as it doesn't use the classes/enums.
-CORE_PROTO_FILES+=(
+  # The unittest_custom_options.proto extends the messages in descriptor.proto
+  # so we build it in to test extending in general. The library doesn't provide
+  # a descriptor as it doesn't use the classes/enums.
   src/google/protobuf/descriptor.proto
 )
 
-compile_protos() {
-  src/protoc                                   \
-    --objc_out="${OUTPUT_DIR}/google/protobuf" \
-    --proto_path=src/google/protobuf/          \
-    --proto_path=src                           \
-    "$@"
-}
-
 # Note: there is overlap in package.Message names between some of the test
 # files, so they can't be generated all at once. This works because the overlap
 # isn't linked into a single binary.
@@ -114,10 +128,18 @@ for a_proto in "${CORE_PROTO_FILES[@]}" ; do
   compile_protos "${a_proto}"
 done
 
-# Objective C specific testing protos.
+# -----------------------------------------------------------------------------
+# Generate the Objective C specific testing protos.
 compile_protos \
   --proto_path="objectivec/Tests" \
   objectivec/Tests/unittest_cycle.proto \
+  objectivec/Tests/unittest_extension_chain_a.proto \
+  objectivec/Tests/unittest_extension_chain_b.proto \
+  objectivec/Tests/unittest_extension_chain_c.proto \
+  objectivec/Tests/unittest_extension_chain_d.proto \
+  objectivec/Tests/unittest_extension_chain_e.proto \
+  objectivec/Tests/unittest_extension_chain_f.proto \
+  objectivec/Tests/unittest_extension_chain_g.proto \
   objectivec/Tests/unittest_runtime_proto2.proto \
   objectivec/Tests/unittest_runtime_proto3.proto \
   objectivec/Tests/unittest_objc.proto \

objectivec/ProtocolBuffers_OSX.xcodeproj/project.pbxproj

@@ -65,6 +65,7 @@
 		F4E675A11B21D0000054530B /* Struct.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = F4E675911B21D0000054530B /* Struct.pbobjc.m */; };
 		F4E675A31B21D0000054530B /* Type.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = F4E675931B21D0000054530B /* Type.pbobjc.m */; };
 		F4E675A51B21D0000054530B /* Wrappers.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = F4E675951B21D0000054530B /* Wrappers.pbobjc.m */; };
+		F4F8D8831D789FD9002CE128 /* GPBUnittestProtos2.m in Sources */ = {isa = PBXBuildFile; fileRef = F4F8D8811D789FCE002CE128 /* GPBUnittestProtos2.m */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -211,6 +212,7 @@
 		F4E675AB1B21D05C0054530B /* struct.proto */ = {isa = PBXFileReference; lastKnownFileType = text; name = struct.proto; path = ../src/google/protobuf/struct.proto; sourceTree = "<group>"; };
 		F4E675AC1B21D05C0054530B /* type.proto */ = {isa = PBXFileReference; lastKnownFileType = text; name = type.proto; path = ../src/google/protobuf/type.proto; sourceTree = "<group>"; };
 		F4E675AD1B21D05C0054530B /* wrappers.proto */ = {isa = PBXFileReference; lastKnownFileType = text; name = wrappers.proto; path = ../src/google/protobuf/wrappers.proto; sourceTree = "<group>"; };
+		F4F8D8811D789FCE002CE128 /* GPBUnittestProtos2.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBUnittestProtos2.m; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -408,6 +410,7 @@
 				7461B6AB0F94FDF800A0C422 /* GPBTestUtilities.h */,
 				7461B6AC0F94FDF800A0C422 /* GPBTestUtilities.m */,
 				8BD3981E14BE59D70081D629 /* GPBUnittestProtos.m */,
+				F4F8D8811D789FCE002CE128 /* GPBUnittestProtos2.m */,
 				7461B6B80F94FDF900A0C422 /* GPBUnknownFieldSetTest.m */,
 				7461B6BA0F94FDF900A0C422 /* GPBUtilitiesTests.m */,
 				8B4248DB1A92933A00BC1EC6 /* GPBWellKnownTypesTest.m */,
@@ -659,6 +662,7 @@
 				F4B51B1E1BBC610700744318 /* GPBObjectiveCPlusPlusTest.mm in Sources */,
 				F4487C7F1AAF62CD00531423 /* GPBMessageTests+Serialization.m in Sources */,
 				8B4248DC1A92933A00BC1EC6 /* GPBWellKnownTypesTest.m in Sources */,
+				F4F8D8831D789FD9002CE128 /* GPBUnittestProtos2.m in Sources */,
 				F4353D1D1AB8822D005A6198 /* GPBDescriptorTests.m in Sources */,
 				8B4248BB1A8C256A00BC1EC6 /* GPBSwiftTests.swift in Sources */,
 				5102DABC1891A073002037B6 /* GPBConcurrencyTests.m in Sources */,

objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj

@@ -73,6 +73,7 @@
 		F4E675D51B21D1620054530B /* Struct.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = F4E675C21B21D1440054530B /* Struct.pbobjc.m */; };
 		F4E675D61B21D1620054530B /* Type.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = F4E675C51B21D1440054530B /* Type.pbobjc.m */; };
 		F4E675D71B21D1620054530B /* Wrappers.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = F4E675C71B21D1440054530B /* Wrappers.pbobjc.m */; };
+		F4F8D8861D78A193002CE128 /* GPBUnittestProtos2.m in Sources */ = {isa = PBXBuildFile; fileRef = F4F8D8841D78A186002CE128 /* GPBUnittestProtos2.m */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -234,6 +235,7 @@
 		F4E675DD1B21D1DE0054530B /* struct.proto */ = {isa = PBXFileReference; lastKnownFileType = text; name = struct.proto; path = ../src/google/protobuf/struct.proto; sourceTree = "<group>"; };
 		F4E675DE1B21D1DE0054530B /* type.proto */ = {isa = PBXFileReference; lastKnownFileType = text; name = type.proto; path = ../src/google/protobuf/type.proto; sourceTree = "<group>"; };
 		F4E675DF1B21D1DE0054530B /* wrappers.proto */ = {isa = PBXFileReference; lastKnownFileType = text; name = wrappers.proto; path = ../src/google/protobuf/wrappers.proto; sourceTree = "<group>"; };
+		F4F8D8841D78A186002CE128 /* GPBUnittestProtos2.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBUnittestProtos2.m; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -446,6 +448,7 @@
 				7461B6AB0F94FDF800A0C422 /* GPBTestUtilities.h */,
 				7461B6AC0F94FDF800A0C422 /* GPBTestUtilities.m */,
 				8BD3981E14BE59D70081D629 /* GPBUnittestProtos.m */,
+				F4F8D8841D78A186002CE128 /* GPBUnittestProtos2.m */,
 				7461B6B80F94FDF900A0C422 /* GPBUnknownFieldSetTest.m */,
 				7461B6BA0F94FDF900A0C422 /* GPBUtilitiesTests.m */,
 				8B4248E51A929C9900BC1EC6 /* GPBWellKnownTypesTest.m */,
@@ -755,6 +758,7 @@
 				F4487C811AAF62FC00531423 /* GPBMessageTests+Serialization.m in Sources */,
 				8B4248E61A929C9900BC1EC6 /* GPBWellKnownTypesTest.m in Sources */,
 				F4353D1F1AB88243005A6198 /* GPBDescriptorTests.m in Sources */,
+				F4F8D8861D78A193002CE128 /* GPBUnittestProtos2.m in Sources */,
 				F4B51B1C1BBC5C7100744318 /* GPBObjectiveCPlusPlusTest.mm in Sources */,
 				8B4248B41A8BD96E00BC1EC6 /* GPBSwiftTests.swift in Sources */,
 				5102DABC1891A073002037B6 /* GPBConcurrencyTests.m in Sources */,

objectivec/Tests/GPBMessageTests+Runtime.m

@@ -36,6 +36,7 @@
 
 #import "google/protobuf/MapUnittest.pbobjc.h"
 #import "google/protobuf/Unittest.pbobjc.h"
+#import "google/protobuf/UnittestCycle.pbobjc.h"
 #import "google/protobuf/UnittestObjcStartup.pbobjc.h"
 #import "google/protobuf/UnittestRuntimeProto2.pbobjc.h"
 #import "google/protobuf/UnittestRuntimeProto3.pbobjc.h"
@@ -49,11 +50,24 @@
 // specific.
 
 - (void)testStartupOrdering {
-  // Just have to create a message.  Nothing else uses the classes from
-  // this file, so the first selector invoked on the class will initialize
-  // it, which also initializes the root.
+  // Message class/Root class initialization is a little tricky, so these just
+  // create some possible patterns that can be a problem. The messages don't
+  // have to be exercised, just creating them is enough to test.  If there
+  // is a problem, the runtime should assert or hang.
+  //
+  // Note: the messages from these proto files should not be used in any other
+  // tests, that way when they are referenced here it will be the first use and
+  // initialization will take place now.
+
   TestObjCStartupMessage *message = [TestObjCStartupMessage message];
   XCTAssertNotNil(message);
+
+  CycleBaz *baz = [CycleBaz message];
+  CycleBar *bar = [CycleBar message];
+  CycleFoo *foo = [CycleFoo message];
+  XCTAssertNotNil(baz);
+  XCTAssertNotNil(bar);
+  XCTAssertNotNil(foo);
 }
 
 - (void)testProto2HasMethodSupport {

objectivec/Tests/GPBUnittestProtos.m

@@ -62,3 +62,11 @@
 #import "google/protobuf/UnittestPreserveUnknownEnum.pbobjc.m"
 #import "google/protobuf/UnittestRuntimeProto2.pbobjc.m"
 #import "google/protobuf/UnittestRuntimeProto3.pbobjc.m"
+
+#import "google/protobuf/UnittestExtensionChainA.pbobjc.m"
+#import "google/protobuf/UnittestExtensionChainB.pbobjc.m"
+#import "google/protobuf/UnittestExtensionChainC.pbobjc.m"
+#import "google/protobuf/UnittestExtensionChainD.pbobjc.m"
+#import "google/protobuf/UnittestExtensionChainE.pbobjc.m"
+// See GPBUnittestProtos2.m for for "UnittestExtensionChainF.pbobjc.m"
+#import "google/protobuf/UnittestExtensionChainG.pbobjc.m"

objectivec/Tests/GPBUnittestProtos2.m

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// This one file in the chain tests is compiled by itself to ensure if was
+// generated with the extra #imports needed to pull in the indirect Root class
+// used in its Root registry.
+#import "google/protobuf/UnittestExtensionChainF.pbobjc.m"

objectivec/Tests/unittest_cycle.proto

@@ -31,10 +31,8 @@ syntax = "proto2";
 
 package protobuf_unittest;
 
-// Cycles in the Message graph can cause problems for the mutable classes
-// since the properties on the mutable class change types. This file just
-// needs to generate source, and that source must compile, to ensure the
-// generated source works for this sort of case.
+// Cycles in the Message graph can cause problems for message class
+// initialization order.
 
 // You can't make a object graph that spans files, so this can only be done
 // within a single proto file.

objectivec/Tests/unittest_extension_chain_a.proto

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package protobuf_unittest;
+
+import "google/protobuf/unittest.proto";
+
+import "unittest_extension_chain_b.proto";
+import "unittest_extension_chain_c.proto";
+import "unittest_extension_chain_d.proto";
+
+// The Root for this file should end up adding the local extension and merging
+// in the extensions from D's Root (unittest and C will come via D's).
+
+message ChainAMessage {
+  optional ChainBMessage b = 1;
+  optional ChainCMessage c = 2;
+  optional ChainDMessage d = 3;
+}
+
+extend TestAllExtensions {
+  optional int32 chain_a_extension = 10001;
+}

objectivec/Tests/unittest_extension_chain_b.proto

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package protobuf_unittest;
+
+import "google/protobuf/unittest.proto";
+
+import "unittest_extension_chain_c.proto";
+
+// The Root for this file should end up adding the local extension and merging
+// in the extensions from C's Root (unittest will come via C's).
+
+message ChainBMessage {
+  optional ChainCMessage c = 1;
+}
+
+extend TestAllExtensions {
+  optional int32 chain_b_extension = 10002;
+}

objectivec/Tests/unittest_extension_chain_c.proto

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package protobuf_unittest;
+
+import "google/protobuf/unittest.proto";
+
+// The Root for this file should end up adding the local extension and merging
+// in the extensions from unittest.proto's Root.
+
+message ChainCMessage {
+  optional int32 my_field = 1;
+}
+
+extend TestAllExtensions {
+  optional int32 chain_c_extension = 10003;
+}

objectivec/Tests/unittest_extension_chain_d.proto

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package protobuf_unittest;
+
+import "google/protobuf/unittest.proto";
+
+import "unittest_extension_chain_b.proto";
+import "unittest_extension_chain_c.proto";
+
+// The root should end up needing to merge B (C will be merged into B, so it
+// doesn't need to be directly merged).
+
+message ChainDMessage {
+  optional ChainBMessage b = 1;
+  optional ChainCMessage c = 2;
+}
+
+extend TestAllExtensions {
+  optional int32 chain_d_extension = 10004;
+}

objectivec/Tests/unittest_extension_chain_e.proto

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package protobuf_unittest;
+
+import "google/protobuf/unittest.proto";
+
+// The Root for this file should end up just merging in unittest's Root.
+
+message ChainEMessage {
+  optional TestAllTypes my_field = 1;
+}

objectivec/Tests/unittest_extension_chain_f.proto

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package protobuf_unittest;
+
+import "unittest_extension_chain_g.proto";
+
+// The Root for this file should just be merging in the extensions from C's
+// Root (because G doens't define anything itself).
+
+// The generated source will also have to directly import C's .h file so it can
+// compile the reference to C's Root class.
+
+message ChainFMessage {
+  optional ChainGMessage g = 1;
+}

objectivec/Tests/unittest_extension_chain_g.proto

+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 Google Inc.  All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package protobuf_unittest;
+
+import "unittest_extension_chain_c.proto";
+
+// The Root for this file should just be merging in the extensions from C's
+// Root.
+
+message ChainGMessage {
+  optional ChainCMessage c = 1;
+}

objectivec/google/protobuf/Any.pbobjc.m

@@ -27,6 +27,9 @@
 
 @implementation GPBAnyRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBAnyRoot_FileDescriptor

objectivec/google/protobuf/Api.pbobjc.m

@@ -31,18 +31,8 @@
 
 @implementation GPBApiRoot
 
-+ (GPBExtensionRegistry*)extensionRegistry {
-  // This is called by +initialize so there is no need to worry
-  // about thread safety and initialization of registry.
-  static GPBExtensionRegistry* registry = nil;
-  if (!registry) {
-    GPBDebugCheckRuntimeVersion();
-    registry = [[GPBExtensionRegistry alloc] init];
-    [registry addExtensions:[GPBSourceContextRoot extensionRegistry]];
-    [registry addExtensions:[GPBTypeRoot extensionRegistry]];
-  }
-  return registry;
-}
+// No extensions in the file and none of the imports (direct or indirect)
+// defined extensions, so no need to generate +extensionRegistry.
 
 @end
 

objectivec/google/protobuf/Duration.pbobjc.m

@@ -27,6 +27,9 @@
 
 @implementation GPBDurationRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBDurationRoot_FileDescriptor

objectivec/google/protobuf/Empty.pbobjc.m

@@ -27,6 +27,9 @@
 
 @implementation GPBEmptyRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBEmptyRoot_FileDescriptor

objectivec/google/protobuf/FieldMask.pbobjc.m

@@ -27,6 +27,9 @@
 
 @implementation GPBFieldMaskRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBFieldMaskRoot_FileDescriptor

objectivec/google/protobuf/SourceContext.pbobjc.m

@@ -27,6 +27,9 @@
 
 @implementation GPBSourceContextRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBSourceContextRoot_FileDescriptor

objectivec/google/protobuf/Struct.pbobjc.m

@@ -28,6 +28,9 @@
 
 @implementation GPBStructRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBStructRoot_FileDescriptor

objectivec/google/protobuf/Timestamp.pbobjc.m

@@ -27,6 +27,9 @@
 
 @implementation GPBTimestampRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBTimestampRoot_FileDescriptor

objectivec/google/protobuf/Type.pbobjc.m

@@ -31,18 +31,8 @@
 
 @implementation GPBTypeRoot
 
-+ (GPBExtensionRegistry*)extensionRegistry {
-  // This is called by +initialize so there is no need to worry
-  // about thread safety and initialization of registry.
-  static GPBExtensionRegistry* registry = nil;
-  if (!registry) {
-    GPBDebugCheckRuntimeVersion();
-    registry = [[GPBExtensionRegistry alloc] init];
-    [registry addExtensions:[GPBAnyRoot extensionRegistry]];
-    [registry addExtensions:[GPBSourceContextRoot extensionRegistry]];
-  }
-  return registry;
-}
+// No extensions in the file and none of the imports (direct or indirect)
+// defined extensions, so no need to generate +extensionRegistry.
 
 @end
 

objectivec/google/protobuf/Wrappers.pbobjc.m

@@ -27,6 +27,9 @@
 
 @implementation GPBWrappersRoot
 
+// No extensions in the file and no imports, so no need to generate
+// +extensionRegistry.
+
 @end
 
 #pragma mark - GPBWrappersRoot_FileDescriptor

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

@@ -45,6 +45,10 @@
 
 namespace google {
 namespace protobuf {
+namespace compiler {
+namespace objectivec {
+
+namespace {
 
 // This is also found in GPBBootstrap.h, and needs to be kept in sync.  It
 // is the version check done to ensure generated code works with the current
@@ -53,8 +57,106 @@ const int32 GOOGLE_PROTOBUF_OBJC_GEN_VERSION = 30001;
 
 const char* kHeaderExtension = ".pbobjc.h";
 
-namespace compiler {
-namespace objectivec {
+// Checks if a message contains any extension definitions (on the message or
+// a nested message under it).
+bool MessageContainsExtensions(const Descriptor* message) {
+  if (message->extension_count() > 0) {
+    return true;
+  }
+  for (int i = 0; i < message->nested_type_count(); i++) {
+    if (MessageContainsExtensions(message->nested_type(i))) {
+      return true;
+    }
+  }
+  return false;
+}
+
+// Checks if the file contains any extensions definitions (at the root or
+// nested under a message).
+bool FileContainsExtensions(const FileDescriptor* file) {
+  if (file->extension_count() > 0) {
+    return true;
+  }
+  for (int i = 0; i < file->message_type_count(); i++) {
+    if (MessageContainsExtensions(file->message_type(i))) {
+      return true;
+    }
+  }
+  return false;
+}
+
+// Helper for CollectMinimalFileDepsContainingExtensionsWorker that marks all
+// deps as visited and prunes them from the needed files list.
+void PruneFileAndDepsMarkingAsVisited(
+    const FileDescriptor* file,
+    vector<const FileDescriptor*>* files,
+    set<const FileDescriptor*>* files_visited) {
+  vector<const FileDescriptor*>::iterator iter =
+      std::find(files->begin(), files->end(), file);
+  if (iter != files->end()) {
+    files->erase(iter);
+  }
+  files_visited->insert(file);
+  for (int i = 0; i < file->dependency_count(); i++) {
+    PruneFileAndDepsMarkingAsVisited(file->dependency(i), files, files_visited);
+  }
+}
+
+// Helper for CollectMinimalFileDepsContainingExtensions.
+void CollectMinimalFileDepsContainingExtensionsWorker(
+    const FileDescriptor* file,
+    vector<const FileDescriptor*>* files,
+    set<const FileDescriptor*>* files_visited) {
+  if (files_visited->find(file) != files_visited->end()) {
+    return;
+  }
+  files_visited->insert(file);
+
+  if (FileContainsExtensions(file)) {
+    files->push_back(file);
+    for (int i = 0; i < file->dependency_count(); i++) {
+      const FileDescriptor* dep = file->dependency(i);
+      PruneFileAndDepsMarkingAsVisited(dep, files, files_visited);
+    }
+  } else {
+    for (int i = 0; i < file->dependency_count(); i++) {
+      const FileDescriptor* dep = file->dependency(i);
+      CollectMinimalFileDepsContainingExtensionsWorker(dep, files,
+                                                       files_visited);
+    }
+  }
+}
+
+// Collect the deps of the given file that contain extensions. This can be used to
+// create the chain of roots that need to be wired together.
+//
+// NOTE: If any changes are made to this and the supporting functions, you will
+// need to manually validate what the generated code is for the test files:
+//   objectivec/Tests/unittest_extension_chain_*.proto
+// There are comments about what the expected code should be line and limited
+// testing objectivec/Tests/GPBUnittestProtos2.m around compilation (#imports
+// specifically).
+void CollectMinimalFileDepsContainingExtensions(
+    const FileDescriptor* file,
+    vector<const FileDescriptor*>* files) {
+  set<const FileDescriptor*> files_visited;
+  for (int i = 0; i < file->dependency_count(); i++) {
+    const FileDescriptor* dep = file->dependency(i);
+    CollectMinimalFileDepsContainingExtensionsWorker(dep, files,
+                                                     &files_visited);
+  }
+}
+
+bool IsDirectDependency(const FileDescriptor* dep, const FileDescriptor* file) {
+  for (int i = 0; i < file->dependency_count(); i++) {
+    if (dep == file->dependency(i)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+}  // namespace
 
 FileGenerator::FileGenerator(const FileDescriptor *file, const Options& options)
     : file_(file),
@@ -204,6 +306,9 @@ void FileGenerator::GenerateSource(io::Printer *printer) {
   // #import the runtime support.
   PrintFileRuntimePreamble(printer, "GPBProtocolBuffers_RuntimeSupport.h");
 
+  vector<const FileDescriptor*> deps_with_extensions;
+  CollectMinimalFileDepsContainingExtensions(file_, &deps_with_extensions);
+
   {
     ImportWriter import_writer(
         options_.generate_for_named_framework,
@@ -227,6 +332,18 @@ void FileGenerator::GenerateSource(io::Printer *printer) {
       }
     }
 
+    // If any indirect dependency provided extensions, it needs to be directly
+    // imported so it can get merged into the root's extensions registry.
+    // See the Note by CollectMinimalFileDepsContainingExtensions before
+    // changing this.
+    for (vector<const FileDescriptor *>::iterator iter =
+             deps_with_extensions.begin();
+         iter != deps_with_extensions.end(); ++iter) {
+      if (!IsDirectDependency(*iter, file_)) {
+        import_writer.AddFile(*iter, header_extension);
+      }
+    }
+
     import_writer.Print(printer);
   }
 
@@ -263,29 +380,11 @@ void FileGenerator::GenerateSource(io::Printer *printer) {
       "@implementation $root_class_name$\n\n",
       "root_class_name", root_class_name_);
 
-  // Generate the extension initialization structures for the top level and
-  // any nested messages.
-  ostringstream extensions_stringstream;
-  if (file_->extension_count() + file_->message_type_count() > 0) {
-    io::OstreamOutputStream extensions_outputstream(&extensions_stringstream);
-    io::Printer extensions_printer(&extensions_outputstream, '$');
-    for (vector<ExtensionGenerator *>::iterator iter =
-             extension_generators_.begin();
-         iter != extension_generators_.end(); ++iter) {
-      (*iter)->GenerateStaticVariablesInitialization(&extensions_printer);
-    }
-    for (vector<MessageGenerator *>::iterator iter =
-             message_generators_.begin();
-         iter != message_generators_.end(); ++iter) {
-      (*iter)->GenerateStaticVariablesInitialization(&extensions_printer);
-    }
-    extensions_stringstream.flush();
-  }
+  const bool file_contains_extensions = FileContainsExtensions(file_);
 
   // If there were any extensions or this file has any dependencies, output
   // a registry to override to create the file specific registry.
-  const string& extensions_str = extensions_stringstream.str();
-  if (extensions_str.length() > 0 || file_->dependency_count() > 0) {
+  if (file_contains_extensions || !deps_with_extensions.empty()) {
     printer->Print(
         "+ (GPBExtensionRegistry*)extensionRegistry {\n"
         "  // This is called by +initialize so there is no need to worry\n"
@@ -298,11 +397,20 @@ void FileGenerator::GenerateSource(io::Printer *printer) {
     printer->Indent();
     printer->Indent();
 
-    if (extensions_str.length() > 0) {
+    if (file_contains_extensions) {
       printer->Print(
           "static GPBExtensionDescription descriptions[] = {\n");
       printer->Indent();
-      printer->Print(extensions_str.c_str());
+      for (vector<ExtensionGenerator *>::iterator iter =
+               extension_generators_.begin();
+           iter != extension_generators_.end(); ++iter) {
+        (*iter)->GenerateStaticVariablesInitialization(printer);
+      }
+      for (vector<MessageGenerator *>::iterator iter =
+               message_generators_.begin();
+           iter != message_generators_.end(); ++iter) {
+        (*iter)->GenerateStaticVariablesInitialization(printer);
+      }
       printer->Outdent();
       printer->Print(
           "};\n"
@@ -315,11 +423,21 @@ void FileGenerator::GenerateSource(io::Printer *printer) {
           "}\n");
     }
 
-    for (int i = 0; i < file_->dependency_count(); i++) {
-      const string root_class_name(FileClassName(file_->dependency(i)));
+    if (deps_with_extensions.empty()) {
       printer->Print(
-          "[registry addExtensions:[$dependency$ extensionRegistry]];\n",
-          "dependency", root_class_name);
+          "// None of the imports (direct or indirect) defined extensions, so no need to add\n"
+          "// them to this registry.\n");
+    } else {
+      printer->Print(
+          "// Merge in the imports (direct or indirect) that defined extensions.\n");
+      for (vector<const FileDescriptor *>::iterator iter =
+               deps_with_extensions.begin();
+           iter != deps_with_extensions.end(); ++iter) {
+        const string root_class_name(FileClassName((*iter)));
+        printer->Print(
+            "[registry addExtensions:[$dependency$ extensionRegistry]];\n",
+            "dependency", root_class_name);
+      }
     }
 
     printer->Outdent();
@@ -328,11 +446,20 @@ void FileGenerator::GenerateSource(io::Printer *printer) {
     printer->Print(
         "  }\n"
         "  return registry;\n"
-        "}\n"
-        "\n");
+        "}\n");
+  } else {
+    if (file_->dependency_count() > 0) {
+      printer->Print(
+          "// No extensions in the file and none of the imports (direct or indirect)\n"
+          "// defined extensions, so no need to generate +extensionRegistry.\n");
+    } else {
+      printer->Print(
+          "// No extensions in the file and no imports, so no need to generate\n"
+          "// +extensionRegistry.\n");
+    }
   }
 
-  printer->Print("@end\n\n");
+  printer->Print("\n@end\n\n");
 
   // File descriptor only needed if there are messages to use it.
   if (message_generators_.size() > 0) {