fix(spark): potential hang with skipped segments on overlapping decompression

client/src/main/java/org/apache/uniffle/client/impl/DecompressionWorker.java

@@ -154,10 +154,30 @@ public void add(int batchIndex, ShuffleDataResult shuffleDataResult) {
   }
 
   public DecompressedShuffleBlock get(int batchIndex, int segmentIndex) {
+    // guardedly safe to remove the previous batches if exist since the upstream will fetch the
+    // segments in order
+    for (int i = 0; i < batchIndex; i++) {
+      ConcurrentHashMap<Integer, DecompressedShuffleBlock> prevBlocks = tasks.remove(i);
+      if (prevBlocks != null) {
+        for (DecompressedShuffleBlock block : prevBlocks.values()) {
+          segmentPermits.ifPresent(x -> x.release());
+        }
+      }
+    }
+
     ConcurrentHashMap<Integer, DecompressedShuffleBlock> blocks = tasks.get(batchIndex);
     if (blocks == null) {
       return null;
     }
+
+    // guardedly safe to remove the previous segments if exist since the upstream will fetch the
+    // segments in order
+    for (int i = 0; i < segmentIndex; i++) {
+      if (blocks.remove(i) != null) {
+        segmentPermits.ifPresent(x -> x.release());
+      }
+    }
+
     DecompressedShuffleBlock block = blocks.remove(segmentIndex);
     // simplify the memory statistic logic here, just decrease the memory used when the block is
     // fetched, this is effective due to the upstream will use single-thread to get and release the

client/src/main/java/org/apache/uniffle/client/impl/ShuffleReadClientImpl.java

@@ -314,8 +314,16 @@ public ShuffleBlock readShuffleBlockData() {
         // mark block as processed
         processedBlockIds.add(bs.getBlockId());
         pendingBlockIds.removeLong(bs.getBlockId());
-        // update the segment index to skip the unnecessary block in overlapping decompression mode
-        segmentIndex += 1;
+
+        // update the segment index to skip the unnecessary block in overlapping decompression mode.
+        // In overlapping decompression mode, decompression tasks for the whole batch have already
+        // been submitted. If we skip a segment without removing the corresponding handler, the
+        // backpressure permits may never be released, which can block subsequent decompression.
+        if (decompressionWorker != null) {
+          decompressionWorker.get(batchIndex - 1, segmentIndex++);
+        } else {
+          segmentIndex += 1;
+        }
       }
 
       if (bs != null) {

client/src/test/java/org/apache/uniffle/client/impl/DecompressionWorkerTest.java

@@ -35,6 +35,7 @@
 import static org.apache.uniffle.common.config.RssClientConf.COMPRESSION_TYPE;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 public class DecompressionWorkerTest {
 
@@ -66,7 +67,9 @@ public void testBackpressure() throws Exception {
     }
     Thread.sleep(10);
     worker.get(0, maxSegments).getByteBuffer();
-    assertEquals(1024 * maxSegments, worker.getPeekMemoryUsed());
+    // Peak memory is a runtime metric and may include one additional segment due to thread timing.
+    assertTrue(worker.getPeekMemoryUsed() <= 1024L * (maxSegments + 1));
+    assertTrue(worker.getPeekMemoryUsed() >= 1024L * maxSegments);
     assertEquals(maxSegments, worker.getAvailablePermits());
   }
 

client/src/test/java/org/apache/uniffle/client/impl/ShuffleReadClientImplTest.java

@@ -52,6 +52,7 @@
 import org.apache.uniffle.storage.handler.impl.HadoopShuffleWriteHandler;
 import org.apache.uniffle.storage.util.StorageType;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
@@ -769,6 +770,47 @@ public void readTest16(Supplier<ShuffleClientFactory.ReadClientBuilder> builderS
     readClient.close();
   }
 
+  @ParameterizedTest
+  @MethodSource("clientBuilderProvider")
+  public void readTestSkipBlocksWithBackpressureDoesNotHang(
+      Supplier<ShuffleClientFactory.ReadClientBuilder> builderSupplier) throws Exception {
+    // This test is meaningful only when overlapping decompression is enabled.
+    // For non-overlapping mode, it should still pass and act as a regression guard.
+    String basePath = uniq(HDFS_URI + "clientReadTestSkipBlocksWithBackpressureDoesNotHang");
+    HadoopShuffleWriteHandler writeHandler =
+        new HadoopShuffleWriteHandler("appId", 0, 1, 1, basePath, ssi1.getId(), conf);
+
+    Map<Long, byte[]> expectedData = Maps.newHashMap();
+    Roaring64NavigableMap blockIdBitmap = Roaring64NavigableMap.bitmapOf();
+
+    // Write skipped blocks first to increase the chance of exhausting permits if permits are not
+    // released when skipping.
+    writeTestData(writeHandler, 20, 30, 1, 2, Maps.newHashMap(), blockIdBitmap);
+    writeTestData(writeHandler, 5, 30, 1, 0, expectedData, blockIdBitmap);
+
+    RssConf rssConf = new RssConf();
+    // Provide required base configs to avoid reader treating this as "prod mode" with empty values.
+    rssConf.set(RssClientConf.RSS_STORAGE_TYPE, StorageType.HDFS.name());
+    rssConf.setInteger(RssClientConf.RSS_READ_OVERLAPPING_DECOMPRESSION_FETCH_SECONDS_THRESHOLD, 1);
+    rssConf.setInteger(RssClientConf.RSS_READ_OVERLAPPING_DECOMPRESSION_MAX_CONCURRENT_SEGMENTS, 1);
+
+    // Expect only taskAttemptId=0 blocks; taskAttemptId=2 blocks will be skipped by the reader.
+    Roaring64NavigableMap taskIdBitmap = Roaring64NavigableMap.bitmapOf(0);
+    ShuffleReadClientImpl readClient =
+        builderSupplier
+            .get()
+            .partitionId(1)
+            .basePath(basePath)
+            .blockIdBitmap(blockIdBitmap)
+            .taskIdBitmap(taskIdBitmap)
+            .rssConf(rssConf)
+            .build();
+
+    assertDoesNotThrow(() -> TestUtils.validateResult(readClient, expectedData));
+    readClient.checkProcessedBlockIds();
+    readClient.close();
+  }
+
   private void writeTestData(
       HadoopShuffleWriteHandler writeHandler,
       int num,