poorbarcode commented on code in PR #24945:
URL: https://github.com/apache/pulsar/pull/24945#discussion_r2510742548


##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/transaction/buffer/impl/TopicTransactionBuffer.java:
##########
@@ -267,47 +257,142 @@ public long getCommittedTxnCount() {
         return this.txnCommittedCounter.sum();
     }
 
+    private record PendingAppendingTxnBufferTask(TxnID txnId, long sequenceId, 
ByteBuf buffer,
+                                         CompletableFuture<Position> 
pendingPublishFuture) {}
+
     @Override
     public CompletableFuture<Position> appendBufferToTxn(TxnID txnId, long 
sequenceId, ByteBuf buffer) {
-        // Method `takeAbortedTxnsSnapshot` will be executed in the different 
thread.
-        // So we need to retain the buffer in this thread. It will be released 
after message persistent.
-        buffer.retain();
-        CompletableFuture<Position> future = 
getPublishFuture().thenCompose(ignore -> {
-            if (checkIfNoSnapshot()) {
-                CompletableFuture<Void> completableFuture = new 
CompletableFuture<>();
-                // `publishFuture` will be completed after message persistent, 
so there will not be two threads
-                // writing snapshots at the same time.
-                
snapshotAbortedTxnProcessor.takeAbortedTxnsSnapshot(maxReadPosition).thenRun(() 
-> {
-                    if (changeToReadyStateFromNoSnapshot()) {
-                        timer.newTimeout(TopicTransactionBuffer.this,
-                                takeSnapshotIntervalTime, 
TimeUnit.MILLISECONDS);
-                        completableFuture.complete(null);
-                    } else {
-                        log.error("[{}]Failed to change state of transaction 
buffer to Ready from NoSnapshot",
-                                topic.getName());
-                        completableFuture.completeExceptionally(new 
BrokerServiceException.ServiceUnitNotReadyException(
-                                "Transaction Buffer take first snapshot 
failed, the current state is: " + getState()));
-                    }
-                }).exceptionally(exception -> {
-                    log.error("Topic {} failed to take snapshot", 
this.topic.getName());
-                    completableFuture.completeExceptionally(exception);
-                    return null;
-                });
-                return completableFuture.thenCompose(__ -> 
internalAppendBufferToTxn(txnId, buffer));
-            } else if (checkIfReady()) {
-                return internalAppendBufferToTxn(txnId, buffer);
-            } else {
-                // `publishFuture` will be completed after transaction buffer 
recover completely
-                // during initializing, so this case should not happen.
+        synchronized (pendingAppendingTxnBufferTasks) {
+            // The first snapshot is in progress, the following publish tasks 
will be pending.
+            if (!pendingAppendingTxnBufferTasks.isEmpty()) {
+                CompletableFuture<Position> res = new CompletableFuture<>();
+                buffer.retain();
+                pendingAppendingTxnBufferTasks.offer(new 
PendingAppendingTxnBufferTask(txnId, sequenceId, buffer, res));
+                return res;
+            }
+
+            // `publishFuture` will be completed after transaction buffer 
recover completely
+            // during initializing, so this case should not happen.
+            if (!checkIfReady() && !checkIfNoSnapshot() && 
!checkIfFirstSnapshotting() && !checkIfInitializing()) {
+                log.error("[{}] unexpected state: {} when try to take the 
first transaction buffer snapshot",
+                        topic.getName(), getState());
                 return FutureUtil.failedFuture(new 
BrokerServiceException.ServiceUnitNotReadyException(
                         "Transaction Buffer recover failed, the current state 
is: " + getState()));
             }
-        }).whenComplete(((position, throwable) -> buffer.release()));
-        setPublishFuture(future);
-        return future;
+
+            // The transaction buffer is ready to write.
+            if (checkIfReady()) {
+                return internalAppendBufferToTxn(txnId, buffer, sequenceId);
+            }
+
+            // Pending the current publishing and trigger new snapshot if 
needed.
+            CompletableFuture<Position> res = new CompletableFuture<>();
+            buffer.retain();
+            pendingAppendingTxnBufferTasks.offer(new 
PendingAppendingTxnBufferTask(txnId, sequenceId, buffer, res));
+
+            final java.util.function.Consumer<Throwable> failPendingTasks = 
throwable -> {
+                synchronized (pendingAppendingTxnBufferTasks) {
+                    PendingAppendingTxnBufferTask pendingTask = null;
+                    while ((pendingTask = 
pendingAppendingTxnBufferTasks.poll()) != null) {
+                        pendingTask.buffer.release();
+                        
pendingTask.pendingPublishFuture.completeExceptionally(throwable);
+                    }
+                }
+            };
+
+            final Runnable flushPendingTasks = () -> {
+                PendingAppendingTxnBufferTask pendingTask = null;
+                try {
+                    synchronized (pendingAppendingTxnBufferTasks) {
+                        while ((pendingTask = 
pendingAppendingTxnBufferTasks.poll()) != null) {
+                            final ByteBuf data = pendingTask.buffer;
+                            final CompletableFuture<Position> pendingFuture =
+                                    pendingTask.pendingPublishFuture;
+                            internalAppendBufferToTxn(pendingTask.txnId, 
pendingTask.buffer,
+                                    pendingTask.sequenceId)
+                                    .whenComplete((positionAdded, ex3) -> {
+                                        data.release();
+                                        if (ex3 != null) {
+                                            
pendingFuture.completeExceptionally(ex3);
+                                            return;
+                                        }
+                                        pendingFuture.complete(positionAdded);
+                                    });
+                        }
+                    }
+                } catch (Exception e) {
+                    // If there are some error when adding entries or caching 
entries, this log will be printed.
+                    log.error("[{}] Failed to flush pending publishing 
requests after taking the first"
+                                    + " snapshot.",
+                            topic.getName(), e);
+                    if (pendingTask != null) {
+                        pendingTask.buffer.release();
+                        
pendingTask.pendingPublishFuture.completeExceptionally(e);

Review Comment:
   Added



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