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.

11 files changed, 378 insertions, 7 deletions

diff --git a/objectivec/Tests/GPBMessageTests+Runtime.m b/objectivec/Tests/GPBMessageTests+Runtime.m
index 2cf9ccef..0058311b 100644
--- a/objectivec/Tests/GPBMessageTests+Runtime.m
+++ b/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 {
diff --git a/objectivec/Tests/GPBUnittestProtos.m b/objectivec/Tests/GPBUnittestProtos.m
index d19beee9..7b938e72 100644
--- a/objectivec/Tests/GPBUnittestProtos.m
+++ b/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"
diff --git a/objectivec/Tests/GPBUnittestProtos2.m b/objectivec/Tests/GPBUnittestProtos2.m
new file mode 100644
index 00000000..ef9f0702
--- /dev/null
+++ b/objectivec/Tests/GPBUnittestProtos2.m
@@ -0,0 +1,34 @@
+// 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"
diff --git a/objectivec/Tests/unittest_cycle.proto b/objectivec/Tests/unittest_cycle.proto
index 5f6f56a1..afc1b0fe 100644
--- a/objectivec/Tests/unittest_cycle.proto
+++ b/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.
diff --git a/objectivec/Tests/unittest_extension_chain_a.proto b/objectivec/Tests/unittest_extension_chain_a.proto
new file mode 100644
index 00000000..6a227eb9
--- /dev/null
+++ b/objectivec/Tests/unittest_extension_chain_a.proto
@@ -0,0 +1,51 @@
+// 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;
+}
diff --git a/objectivec/Tests/unittest_extension_chain_b.proto b/objectivec/Tests/unittest_extension_chain_b.proto
new file mode 100644
index 00000000..0da7ed3e
--- /dev/null
+++ b/objectivec/Tests/unittest_extension_chain_b.proto
@@ -0,0 +1,47 @@
+// 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;
+}
diff --git a/objectivec/Tests/unittest_extension_chain_c.proto b/objectivec/Tests/unittest_extension_chain_c.proto
new file mode 100644
index 00000000..c702900a
--- /dev/null
+++ b/objectivec/Tests/unittest_extension_chain_c.proto
@@ -0,0 +1,45 @@
+// 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;
+}
diff --git a/objectivec/Tests/unittest_extension_chain_d.proto b/objectivec/Tests/unittest_extension_chain_d.proto
new file mode 100644
index 00000000..f9abe3ba
--- /dev/null
+++ b/objectivec/Tests/unittest_extension_chain_d.proto
@@ -0,0 +1,49 @@
+// 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;
+}
diff --git a/objectivec/Tests/unittest_extension_chain_e.proto b/objectivec/Tests/unittest_extension_chain_e.proto
new file mode 100644
index 00000000..fe116631
--- /dev/null
+++ b/objectivec/Tests/unittest_extension_chain_e.proto
@@ -0,0 +1,40 @@
+// 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;
+}
diff --git a/objectivec/Tests/unittest_extension_chain_f.proto b/objectivec/Tests/unittest_extension_chain_f.proto
new file mode 100644
index 00000000..b9bed723
--- /dev/null
+++ b/objectivec/Tests/unittest_extension_chain_f.proto
@@ -0,0 +1,44 @@
+// 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;
+}
diff --git a/objectivec/Tests/unittest_extension_chain_g.proto b/objectivec/Tests/unittest_extension_chain_g.proto
new file mode 100644
index 00000000..aee827b1
--- /dev/null
+++ b/objectivec/Tests/unittest_extension_chain_g.proto
@@ -0,0 +1,41 @@
+// 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;
+}