Bigquery Job's `isDone()` method returns true even if Job is not found in server

[lqiu96]

Discovered in https://github.com/googleapis/java-bigquery/pull/4039/changes#r2632323385

Issue: #3672, found that we are making a network call to check for the job status from the server even if the status is "done".

However, we noticed that when we do make the network call to try and check the job's status, we return true even if the server can't find the original job.

This may be due to jobless queries or may be a bug in our library. This ticket is to figure out more context behind this logic. If we can't change this without a behavioral change, then we will keep this and address this in a future major version bump.

[SivamuruganP]

​Hi team,
​I investigated the source code in Job.java regarding this behavior.
​It appears that returning true for non-existent jobs is the intended design, explicitly documented in the Javadoc and codified in the isDone() method:

// Job.java

/**
* ...
* @return {@code true} if this job is in {@link JobStatus.State#DONE} state or if it does not
* exist, {@code false} if the state is not {@link JobStatus.State#DONE}
*/
public boolean isDone() {
    // ... tracing setup ...
    try (Scope isDoneScope = isDone != null ? isDone.makeCurrent() : null) {
        Job job = bigquery.getJob(getJobId(), JobOption.fields(BigQuery.JobField.STATUS));
        // Explicitly returns true if job is null (not found)
        return job == null || job.hasDoneState();
    } 
    // ...
}

Context on the design:
This logic seems critical for preventing infinite loops in standard polling patterns. If users are doing:

while(!job.isDone()){
    Thread.sleep(1000L);
}

If isDone() were to return false for a missing job (e.g., if a job ID was invalid or the job was deleted), this loop would hang forever. The current behavior ensures the loop breaks, allowing the program to proceed (likely failing later with a specific error when trying to fetch results, which is easier to debug than a hang).

​Recommendation:
If we change this to return false, it would likely be a breaking change causing existing applications to hang. If the goal is to inform the user that the job is missing, perhaps isDone() should throw a specific BigQueryException (404) instead of returning a boolean, but that would also require a major version update.
​Could you clarify if the intention is to change this contract despite the risk of infinite loops?