Add @dwrensha's canonicalization AFL test case.

I changed it to use TestAllTypes instead of a trivial struct, which will probably get more coverage quicker.

c++/src/capnp/afl-testcase.c++

@@ -40,6 +40,8 @@ public:
         "message of type TestAllTypes.")
         .addOption({"lists"}, KJ_BIND_METHOD(*this, runLists),
             "Expect a message of type TestLists instead of TestAllTypes.")
+        .addOption({"canonicalize"}, KJ_BIND_METHOD(*this, canonicalize),
+            "Test canonicalization code.")
         .callAfterParsing(KJ_BIND_METHOD(*this, run))
         .build();
   }
@@ -74,6 +76,62 @@ public:
     context.exit();
   }
 
+  kj::MainBuilder::Validity canonicalize() {
+    // (Test case contributed by David Renshaw.)
+
+    kj::Array<capnp::word> canonical;
+    bool equal = false;
+    KJ_IF_MAYBE(e, kj::runCatchingExceptions([&]() {
+      capnp::ReaderOptions options;
+
+      // The default traversal limit of 8 * 1024 * 1024 causes
+      // AFL to think that it has found "hang" bugs.
+      options.traversalLimitInWords = 8 * 1024;
+
+      capnp::StreamFdMessageReader message(0, options); // read from stdin
+      TestAllTypes::Reader myStruct = message.getRoot<TestAllTypes>();
+      canonical = capnp::canonicalize(myStruct);
+
+      kj::ArrayPtr<const capnp::word> segments[1] = {canonical.asPtr()};
+      capnp::SegmentArrayMessageReader reader(kj::arrayPtr(segments, 1));
+
+      auto originalAny = message.getRoot<capnp::AnyPointer>();
+
+      // Discard cases where the original message is null.
+      KJ_ASSERT(!originalAny.isNull());
+
+      equal = originalAny == reader.getRoot<capnp::AnyPointer>();
+    })) {
+      // Probably some kind of decoding exception.
+      KJ_LOG(ERROR, "threw");
+      context.exit();
+    }
+
+    KJ_ASSERT(equal);
+
+    kj::ArrayPtr<const capnp::word> segments[1] = {canonical.asPtr()};
+    capnp::SegmentArrayMessageReader reader(kj::arrayPtr(segments, 1));
+    KJ_ASSERT(reader.isCanonical());
+
+    kj::Array<capnp::word> canonical2;
+    {
+      capnp::ReaderOptions options;
+      options.traversalLimitInWords = 8 * 1024;
+
+      TestAllTypes::Reader myStruct = reader.getRoot<TestAllTypes>();
+      canonical2 = capnp::canonicalize(myStruct);
+    }
+
+    KJ_ASSERT(canonical.size() == canonical2.size());
+    auto b1 = canonical.asBytes();
+    auto b2 = canonical2.asBytes();
+    for (int idx = 0; idx < b1.size(); ++idx) {
+      KJ_ASSERT(b1[idx] == b2[idx], idx, b1.size());
+    }
+
+    context.exit();
+  }
+
 private:
   kj::ProcessContext& context;
 };