[GitHub] [beam] damccorm opened a new issue, #19787: PubsubMessageWithAttributesCoder should not produce null attributes map

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damccorm opened a new issue, #19787:
URL: https://github.com/apache/beam/issues/19787

   Hi, I just got caught by an issue where PubsubMessage.getAttributeMap() returned null, because the message was created by PubsubMessageWithAttributesCoder which uses a NullableCoder for attributes.
   
   Here are the relevant code snippets:
   
   ```
   
   public class PubsubMessageWithAttributesCoder extends CustomCoder<PubsubMessage> {
     // A message's
   payload can not be null
     private static final Coder<byte[]> PAYLOAD_CODER = ByteArrayCoder.of();
   
    // A message's attributes can be null.
     private static final Coder<Map<String, String>> ATTRIBUTES_CODER
   =
         NullableCoder.of(MapCoder.of(StringUtf8Coder.of(), StringUtf8Coder.of()));
   
     @Override
   
    public PubsubMessage decode(InputStream inStream) throws IOException {
       return decode(inStream,
   Context.NESTED);
     }
   
     @Override
     public PubsubMessage decode(InputStream inStream, Context context)
   throws IOException {
       byte[] payload = PAYLOAD_CODER.decode(inStream);
       Map<String, String>
   attributes = ATTRIBUTES_CODER.decode(inStream, context);
       return new PubsubMessage(payload, attributes);
   
    }
   }
   
   
   ```
   
    
   ```
   
   public class PubsubMessage {
   
     private byte[] message;
     private Map<String, String> attributes;
   
   
    public PubsubMessage(byte[] payload, Map<String, String> attributes) {
       this.message = payload;
   
      this.attributes = attributes;
     }
   
     /** Returns the main PubSub message. */
     public byte[]
   getPayload() {
       return message;
     }
   
     /** Returns the given attribute value. If not such attribute
   exists, returns null. */
     @Nullable
     public String getAttribute(String attribute) {
       checkNotNull(attribute,
   "attribute");
       return attributes.get(attribute);
     }
   
     /** Returns the full map of attributes.
   This is an unmodifiable map. */
     public Map<String, String> getAttributeMap() {
       return attributes;
   
    }
   }
   
   ```
   
   
   There are a handful of potential solutions:
   
   - Remove the NullableCoder
   - In PubsubMessageWithAttributesCoder.decode, check for null and create an empty Map before instantiating PubsubMessage
   - Allow attributes to be null for  PubsubMessage constructor, but create an empty Map if it is (similar to above, but handle it in PubsubMessage)
   - Allow PubsubMessage.attributes to be nullable, and indicate it as such
   
   
   Imported from Jira [BEAM-8085](https://issues.apache.org/jira/browse/BEAM-8085). Original Jira may contain additional context.
   Reported by: chadrik.


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