`IonValue.writeTo(IonHashWriter)` fails for IonStruct

[mcliedtke]

Hello,
Just was experimenting with the IonHashWriter and encountered an error when writing an IonStruct DOM object.

IonSystem ion = IonSystemBuilder.standard().build();
IonHasherProvider hasherProvider = new MessageDigestIonHasherProvider("SHA-256");

// write a simple Ion struct and compute the hash
ByteArrayOutputStream baos = new ByteArrayOutputStream();
IonWriter writer = ion.newTextWriter(baos);
IonHashWriter hashWriter = IonHashWriterBuilder.standard()
        .withHasherProvider(hasherProvider)
	.withWriter(writer)
	.build();
		
IonValue ionValue = ion.singleValue("{first_name:\"Amanda\",middle_name:\"Amanda\",last_name:\"Smith\"}");

System.out.println("writer");
ionValue.writeTo(hashWriter);

Stack trace (fails here)

com.amazon.ionhash.IonHashWriterImpl cannot be cast to com.amazon.ion.impl._Private_IonWriter
	at com.amazon.ion.impl.lite.IonValueLite.writeTo(IonValueLite.java:993)
	at com.amazon.ion.impl.lite.IonValueLite.writeChildren(IonValueLite.java:986)
	at com.amazon.ion.impl.lite.IonStructLite.writeBodyTo(IonStructLite.java:809)
	at com.amazon.ion.impl.lite.IonValueLite.writeTo(IonValueLite.java:1010)
	at com.amazon.ion.impl.lite.IonValueLite.writeTo(IonValueLite.java:965)
	at com.amazon.ionhash.IonHashWriterImpl.writeValue(IonHashWriterImpl.java:324)

This seems to only fail on Structs but I haven't tried too many other values. Is the IonHashWriter intended to support writes like this?

[pbcornell]

This is a bug, although perhaps not too surprising as the focus has been solely on non-DOM usage. Wrapping the IonReader returned by IonSystem.newReader(IonValue) with an IonHashReader allows a hash to be computed from a DOM, but clearly doesn't address the "serialize an IonValue and compute its hash" use case suggested by the code above.

[pbcornell]

We could address this with changes to ion-hash-java; alternatively, fixing amazon-ion/ion-java#11 would enable other use cases as well (e.g., a schema-validating reader/writer).

[othonrsanchez]

any update?