Extend previous change for flat arrays as well.

c++/src/capnp/serialize-test.c++

@@ -74,6 +74,14 @@ TEST(Serialize, FlatArray) {
     EXPECT_EQ(serialized.end(), reader.getEnd());
   }
 
+  {
+    MallocMessageBuilder builder2;
+    auto remaining = initMessageBuilderFromFlatArrayCopy(serialized, builder2);
+    checkTestMessage(builder2.getRoot<TestAllTypes>());
+    EXPECT_EQ(serialized.end(), remaining.begin());
+    EXPECT_EQ(serialized.end(), remaining.end());
+  }
+
   kj::Array<word> serializedWithSuffix = kj::heapArray<word>(serialized.size() + 5);
   memcpy(serializedWithSuffix.begin(), serialized.begin(), serialized.size() * sizeof(word));
 
@@ -82,6 +90,14 @@ TEST(Serialize, FlatArray) {
     checkTestMessage(reader.getRoot<TestAllTypes>());
     EXPECT_EQ(serializedWithSuffix.end() - 5, reader.getEnd());
   }
+
+  {
+    MallocMessageBuilder builder2;
+    auto remaining = initMessageBuilderFromFlatArrayCopy(serializedWithSuffix, builder2);
+    checkTestMessage(builder2.getRoot<TestAllTypes>());
+    EXPECT_EQ(serializedWithSuffix.end() - 5, remaining.begin());
+    EXPECT_EQ(serializedWithSuffix.end(), remaining.end());
+  }
 }
 
 TEST(Serialize, FlatArrayOddSegmentCount) {

c++/src/capnp/serialize.c++

@@ -90,6 +90,13 @@ kj::ArrayPtr<const word> FlatArrayMessageReader::getSegment(uint id) {
   }
 }
 
+kj::ArrayPtr<const word> initMessageBuilderFromFlatArrayCopy(
+    kj::ArrayPtr<const word> array, MessageBuilder& target, ReaderOptions options) {
+  FlatArrayMessageReader reader(array, options);
+  target.setRoot(reader.getRoot<AnyPointer>());
+  return kj::arrayPtr(reader.getEnd(), array.end());
+}
+
 kj::Array<word> messageToFlatArray(kj::ArrayPtr<const kj::ArrayPtr<const word>> segments) {
   kj::Array<word> result = kj::heapArray<word>(computeSerializedSizeInWords(segments));
 

c++/src/capnp/serialize.h

@@ -73,6 +73,21 @@ private:
   const word* end;
 };
 
+kj::ArrayPtr<const word> initMessageBuilderFromFlatArrayCopy(
+    kj::ArrayPtr<const word> array, MessageBuilder& target,
+    ReaderOptions options = ReaderOptions());
+// Convenience function which reads a message using `FlatArrayMessageReader` then copies the
+// content into the target `MessageBuilder`, verifying that the message structure is valid
+// (although not necessarily that it matches the desired schema).
+//
+// Returns an ArrayPtr containing any words left over in the array after consuming the whole
+// message. This is useful when reading multiple messages that have been concatenated. See also
+// FlatArrayMessageReader::getEnd().
+//
+// (Note that it's also possible to initialize a `MessageBuilder` directly without a copy using one
+// of `MessageBuilder`'s constructors. However, this approach skips the validation step and is not
+// safe to use on untrusted input. Therefore, we do not provide a convenience method for it.)
+
 kj::Array<word> messageToFlatArray(MessageBuilder& builder);
 // Constructs a flat array containing the entire content of the given message.
 //