Code review comment: ```suggestion             assertThat(cache.getJobRefCounters().get(jobId).references).isZero(); ```

Code under review:
void testPermanentJobReferences() throws IOException {

        JobID jobId = new JobID();

        Configuration config = new Configuration();
        config.setLong(
                BlobServerOptions.CLEANUP_INTERVAL,
                3_600_000L); 

        
        InetSocketAddress serverAddress = new InetSocketAddress("localhost", 12345);

        try (PermanentBlobCache cache =
                TestingBlobUtils.createPermanentCache(tempDir, config, serverAddress)) {

            
            cache.registerJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(1);
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            cache.registerJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(2);
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            cache.releaseJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(1);
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            long cleanupLowerBound =
                    System.currentTimeMillis() + config.getLong(BlobServerOptions.CLEANUP_INTERVAL);
            cache.releaseJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(0);
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil)
                    .isGreaterThanOrEqualTo(cleanupLowerBound);

            
            cache.registerJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(1);
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            cleanupLowerBound =
                    System.currentTimeMillis() + config.getLong(BlobServerOptions.CLEANUP_INTERVAL);
            cache.releaseJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(0);
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil)
                    .isGreaterThanOrEqualTo(cleanupLowerBound);
        }
    }

Line commented on: assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(0);

Revised code:
void testPermanentJobReferences() throws IOException {

        JobID jobId = new JobID();

        Configuration config = new Configuration();
        config.setLong(
                BlobServerOptions.CLEANUP_INTERVAL,
                3_600_000L); 

        
        InetSocketAddress serverAddress = new InetSocketAddress("localhost", 12345);

        try (PermanentBlobCache cache =
                TestingBlobUtils.createPermanentCache(tempDir, config, serverAddress)) {

            
            cache.registerJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isOne();
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            cache.registerJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isEqualTo(2);
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            cache.releaseJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isOne();
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            long cleanupLowerBound =
                    System.currentTimeMillis() + config.getLong(BlobServerOptions.CLEANUP_INTERVAL);
            cache.releaseJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isZero();
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil)
                    .isGreaterThanOrEqualTo(cleanupLowerBound);

            
            cache.registerJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isOne();
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil).isEqualTo(-1);

            
            cleanupLowerBound =
                    System.currentTimeMillis() + config.getLong(BlobServerOptions.CLEANUP_INTERVAL);
            cache.releaseJob(jobId);
            assertThat(cache.getJobRefCounters().get(jobId).references).isZero();
            assertThat(cache.getJobRefCounters().get(jobId).keepUntil)
                    .isGreaterThanOrEqualTo(cleanupLowerBound);
        }
    }