Undocumented Constraint? publishing to topic *from* pubsub trigger - google-cloud-pubsub

Question

I don't know if I'm going crazy, or if this is a limitation that just isn't documented (I've scoured the GCP API docs):
Is it possible to have a cloud function with a pubsub trigger on 'topic A', and inside that cloud function, publish a message to 'topic B'.
I've tried all the other triggers with identical code running (cloud functions as HTTP triggers, Cloud Storage Triggers, Firebase Triggers), and they all successfully publish to topics.
But the moment I (almost literally) copy-paste my code into a pubsub trigger, after consuming the message, when it attempts to publish it's own message to the next topic, it just hangs. The function just times-out when attempting to publish.
So to recap, is the following possible in GCP?
PubSub Topic A --> Cloud Function --> Pubsub Topic B
Thanks in advance for any clarifications! This is all in Java 11. Here's the code:
...<bunch of imports>
public class SignedURLGenerator implements BackgroundFunction<PubSubMessage> {
private static final String PROJECT_ID = System.getenv("GOOGLE_CLOUD_PROJECT");
private static final Logger logger = Logger.getLogger(SignedURLGenerator.class.getName());
/**
* Handle the incoming PubsubMessage
**/
#Override
public void accept(PubSubMessage message, Context context) throws IOException, InterruptedException {
String data = new String(Base64.getDecoder().decode(message.data));
System.out.println("The input message is: " + data.toString());
//Do a bunch of other stuff not relevant to the issue at hand...
publishSignedURL(url.toString());
}
//Here's the interesting part
public static void publishSignedURL(String message) throws IOException, InterruptedException {
String topicName = "url-ready-notifier";
String responseMessage;
Publisher publisher = null;
try {
// Create the PubsubMessage object
ByteString byteStr = ByteString.copyFrom(message, StandardCharsets.UTF_8);
PubsubMessage pubsubApiMessage = PubsubMessage.newBuilder().setData(byteStr).build();
System.out.println("Message Constructed:" + message);
//This part works fine, the message gets constructed
publisher = Publisher.newBuilder(ProjectTopicName.of(PROJECT_ID, topicName)).build();
System.out.println("Publisher Created.");
//This part also works fine, the publisher gets created
publisher.publish(pubsubApiMessage).get();
responseMessage = "Message published.";
//The code NEVER GETS HERE. The message is never published. And eventually the cloud function time's out :(
} catch (InterruptedException | ExecutionException e) {
System.out.println("Something went wrong with publishing: " + e.getMessage());
}
System.out.println("Everything wrapped up.");
}
Edit
As requested, this is my current POM
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>cloudfunctions</groupId>
<artifactId>pubsub-function</artifactId>
<version>1.0-SNAPSHOT</version>
<properties>
<maven.compiler.target>11</maven.compiler.target>
<maven.compiler.source>11</maven.compiler.source>
</properties>
<dependencies>
<dependency>
<groupId>com.google.cloud</groupId>
<artifactId>libraries-bom</artifactId>
<version>20.6.0</version>
<type>pom</type>
<scope>import</scope>
</dependency>
<dependency>
<groupId>com.google.cloud.functions</groupId>
<artifactId>functions-framework-api</artifactId>
<version>1.0.1</version>
<type>jar</type>
</dependency>
<dependency>
<groupId>com.google.cloud</groupId>
<artifactId>google-cloud-storage</artifactId>
<version>1.117.1</version>
</dependency>
<dependency>
<groupId>com.google.cloud</groupId>
<artifactId>google-cloud-pubsub</artifactId>
<version>1.113.4</version>
</dependency>
<dependency>
<groupId>com.google.api</groupId>
<artifactId>gax</artifactId>
<version>1.66.0</version>
</dependency>
<dependency>
<groupId>com.google.api</groupId>
<artifactId>gax-grpc</artifactId>
<version>1.66.0</version>
</dependency>
<dependency>
<groupId>org.threeten</groupId>
<artifactId>threetenbp</artifactId>
<version>0.7.2</version>
</dependency>
</dependencies>
</project>

Answer 1

Can you try to explicitly set the flow control param in your publisher client? like that
publisher = Publisher.newBuilder(ProjectTopicName.of(PROJECT_ID, topicName)).setBatchingSettings(BatchingSettings.newBuilder()
.setDelayThreshold(Duration.of(10, ChronoUnit.SECONDS))
.setElementCountThreshold(1L)
.setIsEnabled(true)
.build()).build();
I don't know what happens, maybe a default and global configuration of PubSub. If it's not that, I will delete this answer.
EDIT 1
Here a screen capture of the builder class on a Publisher parent classe
You have all the default value of the library. However, the behavior that you observe isn't normal. The default must stay the default even if you are in a PubSub trigger. I will open an issue and forward it to the team directly.