Simplify logging

flink/v1.18/flink/src/main/java/org/apache/iceberg/flink/source/reader/IcebergSourceSplitReader.java

@@ -52,6 +52,9 @@ class IcebergSourceSplitReader<T> implements SplitReader<RecordAndPosition<T>, I
   private final int indexOfSubtask;
   private final Queue<IcebergSourceSplit> splits;
   private final Set<String> pausedSplits;
+  // The wakeUp flag which used to indicate that a wakeUp call has arrived, and we still need to
+  // handle it in the next
+  // fetch loop. Should be updated only when the synchronization lock is held.
   private final AtomicBoolean wakeUp = new AtomicBoolean(false);
 
   private CloseableIterator<RecordsWithSplitIds<RecordAndPosition<T>>> currentReader;
@@ -71,6 +74,25 @@ class IcebergSourceSplitReader<T> implements SplitReader<RecordAndPosition<T>, I
     this.pausedSplits = Sets.newHashSet();
   }
 
+  /**
+   * The method reads a batch of records from the assigned splits. If all the records from the
+   * current split are returned then it will emit a {@link ArrayBatchRecords#finishedSplit(String)}
+   * batch to signal this event. In the next fetch loop the reader will continue with the next split
+   * (if any).
+   *
+   * <p>FLIP-217 defines the API for enabling watermark alignment with limited buffering on Flink
+   * side, by allowing the {@link #fetch()} call to be blocked on a runaway reader. Since the
+   * Iceberg reader is single threaded, the reading is fully blocked if {@link
+   * #pauseOrResumeSplits(Collection, Collection)} pauses the current split. The {@link #wakeUp()}
+   * method is called by Flink to unblock a blocked reader. In this case after the {@link #fetch()}
+   * call returns, Flink will call {@link #handleSplitsChanges(SplitsChange)} or {@link
+   * #pauseOrResumeSplits(Collection, Collection)} to update the splits or splits state. After the
+   * state update Flink will call {@link #fetch()} again, which might be unblocked at this stage and
+   * continue reading.
+   *
+   * @return The fetched records
+   * @throws IOException If there is an error during reading
+   */
   @Override
   public RecordsWithSplitIds<RecordAndPosition<T>> fetch() throws IOException {
     metrics.incrementSplitReaderFetchCalls(1);
@@ -88,31 +110,23 @@ public RecordsWithSplitIds<RecordAndPosition<T>> fetch() throws IOException {
     }
 
     if (currentReader.hasNext()) {
-      if (pausedSplits.contains(currentSplitId)) {
+      while (pausedSplits.contains(currentSplitId)) {
+        // Needed for the implementation of FLIP-217
         // Wait until the reader is unblocked. Wake every second to catch any missed signal.
         // Return empty records if wakeUp is called, so pauseOrResumeSplits could be processed.
-        boolean first = true;
-
-        while (pausedSplits.contains(currentSplitId)) {
-          if (first) {
-            LOG.info("Reader {} paused reading split {}", indexOfSubtask, currentSplitId);
-            first = false;
-          } else {
-            LOG.trace("Reader {} is still paused reading split {}", indexOfSubtask, currentSplitId);
+        LOG.trace("Reader {} is still paused reading split {}", indexOfSubtask, currentSplitId);
+
+        synchronized (wakeUp) {
+          if (wakeUp.get()) {
+            wakeUp.set(false);
+            return new RecordsBySplits(Collections.emptyMap(), Collections.emptySet());
           }
 
-          synchronized (wakeUp) {
-            if (wakeUp.get()) {
-              wakeUp.set(false);
-              return new RecordsBySplits(Collections.emptyMap(), Collections.emptySet());
-            }
-
-            try {
-              wakeUp.wait(1000);
-            } catch (InterruptedException e) {
-              Thread.currentThread().interrupt();
-              throw new IOException("Interrupted while paused due to watermark alignment.", e);
-            }
+          try {
+            wakeUp.wait(1000);
+          } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new IOException("Interrupted while paused due to watermark alignment.", e);
           }
         }
       }