Quic busy write loop detector

Summary:
This diff adds a DebugState in the QuicConnectionState to track the
reason we schedule transport WriteLooper to run, the reason we end up not
writing and the number of times such write happens consecutively. And when it
reaches a predefined limit, we trigger a callback on a loop detector interface.

Reviewed By: kvtsoy

Differential Revision: D15433881

fbshipit-source-id: d903342c00bc4dccf8d7320726368d23295bec66

quic/QuicConstants.cpp

@@ -37,4 +37,50 @@ std::vector<QuicVersion> filterSupportedVersions(
       });
   return filteredVersions;
 }
+
+std::string writeDataReasonString(WriteDataReason reason) {
+  switch (reason) {
+    case WriteDataReason::PROBES:
+      return "Probes";
+    case WriteDataReason::ACK:
+      return "Ack";
+    case WriteDataReason::CRYPTO_STREAM:
+      return "Crypto";
+    case WriteDataReason::STREAM:
+      return "Stream";
+    case WriteDataReason::LOSS:
+      return "Loss";
+    case WriteDataReason::BLOCKED:
+      return "Blocked";
+    case WriteDataReason::STREAM_WINDOW_UPDATE:
+      return "StreamWindowUpdate";
+    case WriteDataReason::CONN_WINDOW_UPDATE:
+      return "ConnWindowUpdate";
+    case WriteDataReason::SIMPLE:
+      return "Simple";
+    case WriteDataReason::RESET:
+      return "Reset";
+    case WriteDataReason::PATHCHALLENGE:
+      return "PathChallenge";
+    case WriteDataReason::NO_WRITE:
+      return "NoWrite";
+  }
+  folly::assume_unreachable();
+}
+
+std::string writeNoWriteReasonString(NoWriteReason reason) {
+  switch (reason) {
+    case NoWriteReason::WRITE_OK:
+      return "WriteOk";
+    case NoWriteReason::EMPTY_SCHEDULER:
+      return "EmptyScheduler";
+    case NoWriteReason::NO_FRAME:
+      return "NoFrame";
+    case NoWriteReason::NO_BODY:
+      return "NoBody";
+    case NoWriteReason::SOCKET_FAILURE:
+      return "SocketFailure";
+  }
+  folly::assume_unreachable();
+}
 } // namespace quic

quic/QuicConstants.h

@@ -391,6 +391,32 @@ inline std::ostream& operator<<(std::ostream& os, const QuicVersion& v) {
   return os;
 }
 
+enum class WriteDataReason {
+  NO_WRITE,
+  PROBES,
+  ACK,
+  CRYPTO_STREAM,
+  STREAM,
+  LOSS,
+  BLOCKED,
+  STREAM_WINDOW_UPDATE,
+  CONN_WINDOW_UPDATE,
+  SIMPLE,
+  RESET,
+  PATHCHALLENGE,
+};
+
+enum class NoWriteReason {
+  WRITE_OK,
+  EMPTY_SCHEDULER,
+  NO_FRAME,
+  NO_BODY,
+  SOCKET_FAILURE,
+};
+
+std::string writeDataReasonString(WriteDataReason reason);
+std::string writeNoWriteReasonString(NoWriteReason reason);
+
 /**
  * Filter the versions that are currently supported.
  */

quic/api/LoopDetectorCallback.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
+
+#pragma once
+
+#include <quic/QuicConstants.h>
+
+namespace quic {
+
+class LoopDetectorCallback {
+ public:
+  virtual ~LoopDetectorCallback() = default;
+  virtual void onSuspiciousLoops(
+      uint64_t emptyLoopCount,
+      WriteDataReason writeReason,
+      NoWriteReason noWriteReason,
+      const std::string& scheduler) = 0;
+};
+
+} // namespace quic

quic/api/QuicTransportBase.cpp

@@ -9,6 +9,7 @@
 #include <quic/api/QuicTransportBase.h>
 
 #include <folly/ScopeGuard.h>
+#include <quic/api/LoopDetectorCallback.h>
 #include <quic/api/QuicTransportFunctions.h>
 #include <quic/common/TimeUtil.h>
 #include <quic/logging/QLoggerConstants.h>
@@ -1117,16 +1118,22 @@ void QuicTransportBase::updateWriteLooper(bool thisIteration) {
   }
   // TODO: Also listens to write event from libevent. Only schedule write when
   // the socket itself is writable.
-  if (shouldWriteData(*conn_)) {
+  auto writeDataReason = shouldWriteData(*conn_);
+  if (writeDataReason != WriteDataReason::NO_WRITE) {
     VLOG(10) << nodeToString(conn_->nodeType)
              << " running write looper thisIteration=" << thisIteration << " "
              << *this;
     writeLooper_->run(thisIteration);
+    conn_->debugState.needsWriteLoopDetect =
+        (conn_->loopDetectorCallback != nullptr);
   } else {
     VLOG(10) << nodeToString(conn_->nodeType) << " stopping write looper "
              << *this;
     writeLooper_->stop();
+    conn_->debugState.needsWriteLoopDetect = false;
+    conn_->debugState.currentEmptyLoopCount = 0;
   }
+  conn_->debugState.writeDataReason = writeDataReason;
 }
 
 void QuicTransportBase::cancelDeliveryCallbacksForStream(StreamId streamId) {
@@ -2181,7 +2188,23 @@ void QuicTransportBase::writeSocketData() {
     if (closeState_ != CloseState::CLOSED) {
       setLossDetectionAlarm(*conn_, *this);
       auto packetsAfter = conn_->outstandingPackets.size();
-      // If we sent a new packet and the new packet was either the first packet
+      bool packetWritten = (packetsAfter > packetsBefore);
+      if (packetWritten) {
+        conn_->debugState.currentEmptyLoopCount = 0;
+      } else if (
+          conn_->debugState.needsWriteLoopDetect &&
+          conn_->loopDetectorCallback) {
+        // TODO: Currently we will to get some stats first. Then we may filter
+        // out some errors here. For example, socket fail to write might be a
+        // legit case to filter out.
+        conn_->loopDetectorCallback->onSuspiciousLoops(
+            ++conn_->debugState.currentEmptyLoopCount,
+            conn_->debugState.writeDataReason,
+            conn_->debugState.noWriteReason,
+            conn_->debugState.schedulerName);
+      }
+      // If we sent a new packet and the new packet was either the first
+      // packet
       // after quiescence or after receiving a new packet.
       if (packetsAfter > packetsBefore &&
           (packetsBefore == 0 || conn_->receivedNewPacketBeforeWrite)) {

quic/api/QuicTransportBase.h

@@ -441,6 +441,10 @@ class QuicTransportBase : public QuicSocket {
     conn_->qLogger = std::move(qLogger);
   }
 
+  void setLoopDetectorCallback(std::shared_ptr<LoopDetectorCallback> callback) {
+    conn_->loopDetectorCallback = std::move(callback);
+  }
+
   virtual void cancelAllAppCallbacks(
       std::pair<QuicErrorCode, std::string> error) noexcept;
 

quic/api/QuicTransportFunctions.cpp

@@ -909,6 +909,11 @@ uint64_t writeConnectionDataToSocket(
   ioBufBatch.setContinueOnNetworkUnreachable(
       connection.transportSettings.continueOnNetworkUnreachable);
 
+  connection.debugState.schedulerName = scheduler.name();
+  connection.debugState.noWriteReason = NoWriteReason::WRITE_OK;
+  if (!scheduler.hasData()) {
+    connection.debugState.noWriteReason = NoWriteReason::EMPTY_SCHEDULER;
+  }
   while (scheduler.hasData() && ioBufBatch.getPktSent() < packetLimit) {
     auto packetNum = getNextPacketNum(connection, pnSpace);
     auto header = builder(
@@ -935,11 +940,13 @@ uint64_t writeConnectionDataToSocket(
     auto& packet = result.second;
     if (!packet || packet->packet.frames.empty()) {
       ioBufBatch.flush();
+      connection.debugState.noWriteReason = NoWriteReason::NO_FRAME;
       return ioBufBatch.getPktSent();
     }
     if (!packet->body) {
       // No more space remaining.
       ioBufBatch.flush();
+      connection.debugState.noWriteReason = NoWriteReason::NO_BODY;
       return ioBufBatch.getPktSent();
     }
     auto body =
@@ -973,6 +980,7 @@ uint64_t writeConnectionDataToSocket(
     // if ioBufBatch.write returns false
     // it is because a flush() call failed
     if (!ret) {
+      connection.debugState.noWriteReason = NoWriteReason::SOCKET_FAILURE;
       return ioBufBatch.getPktSent();
     }
   }
@@ -1014,16 +1022,16 @@ uint64_t writeProbingDataToSocket(
   return written;
 }
 
-bool shouldWriteData(const QuicConnectionStateBase& conn) {
+WriteDataReason shouldWriteData(const QuicConnectionStateBase& conn) {
   if (conn.pendingEvents.numProbePackets) {
     VLOG(10) << nodeToString(conn.nodeType) << " needs write because of PTO"
              << conn;
-    return true;
+    return WriteDataReason::PROBES;
   }
   if (hasAckDataToWrite(conn)) {
     VLOG(10) << nodeToString(conn.nodeType) << " needs write because of ACKs "
              << conn;
-    return true;
+    return WriteDataReason::ACK;
   }
   const size_t minimumDataSize = std::max(
       kLongHeaderHeaderSize + kCipherOverheadHeuristic, sizeof(Sample));
@@ -1032,12 +1040,12 @@ bool shouldWriteData(const QuicConnectionStateBase& conn) {
        *conn.writableBytesLimit - conn.lossState.totalBytesSent <=
            minimumDataSize)) {
     QUIC_STATS(conn.infoCallback, onCwndBlocked);
-    return false;
+    return WriteDataReason::NO_WRITE;
   }
   if (conn.congestionController &&
       conn.congestionController->getWritableBytes() <= minimumDataSize) {
     QUIC_STATS(conn.infoCallback, onCwndBlocked);
-    return false;
+    return WriteDataReason::NO_WRITE;
   }
   return hasNonAckDataToWrite(conn);
 }
@@ -1061,28 +1069,43 @@ bool hasAckDataToWrite(const QuicConnectionStateBase& conn) {
   return writeAcks;
 }
 
-bool hasNonAckDataToWrite(const QuicConnectionStateBase& conn) {
+WriteDataReason hasNonAckDataToWrite(const QuicConnectionStateBase& conn) {
   if (cryptoHasWritableData(conn)) {
     VLOG(10) << nodeToString(conn.nodeType)
              << " needs write because of crypto stream"
              << " " << conn;
-    return true;
+    return WriteDataReason::CRYPTO_STREAM;
   }
   if (!conn.oneRttWriteCipher && !conn.zeroRttWriteCipher) {
     // All the rest of the types of data need either a 1-rtt or 0-rtt cipher to
     // be written.
-    return false;
+    return WriteDataReason::NO_WRITE;
   }
-  bool hasStreamData = getSendConnFlowControlBytesWire(conn) != 0 &&
-      conn.streamManager->hasWritable();
-  bool hasLoss = conn.streamManager->hasLoss();
-  bool hasBlocked = conn.streamManager->hasBlocked();
-  bool hasStreamWindowUpdates = conn.streamManager->hasWindowUpdates();
-  bool hasConnWindowUpdate = conn.pendingEvents.connWindowUpdate;
-  bool hasSimple = !conn.pendingEvents.frames.empty();
-  bool hasResets = !conn.pendingEvents.resets.empty();
-  bool hasPathChallenge = (conn.pendingEvents.pathChallenge != folly::none);
-  return hasStreamData || hasLoss || hasBlocked || hasStreamWindowUpdates ||
-      hasConnWindowUpdate || hasResets || hasSimple || hasPathChallenge;
+  if (!conn.pendingEvents.resets.empty()) {
+    return WriteDataReason::RESET;
+  }
+  if (conn.streamManager->hasWindowUpdates()) {
+    return WriteDataReason::STREAM_WINDOW_UPDATE;
+  }
+  if (conn.pendingEvents.connWindowUpdate) {
+    return WriteDataReason::CONN_WINDOW_UPDATE;
+  }
+  if (conn.streamManager->hasBlocked()) {
+    return WriteDataReason::BLOCKED;
+  }
+  if (conn.streamManager->hasLoss()) {
+    return WriteDataReason::LOSS;
+  }
+  if (getSendConnFlowControlBytesWire(conn) != 0 &&
+      conn.streamManager->hasWritable()) {
+    return WriteDataReason::STREAM;
+  }
+  if (!conn.pendingEvents.frames.empty()) {
+    return WriteDataReason::SIMPLE;
+  }
+  if ((conn.pendingEvents.pathChallenge != folly::none)) {
+    return WriteDataReason::PATHCHALLENGE;
+  }
+  return WriteDataReason::NO_WRITE;
 }
 } // namespace quic

quic/api/QuicTransportFunctions.h

@@ -95,9 +95,9 @@ uint64_t writeZeroRttDataToSocket(
  *
  * TODO: We should probably split "should" and "can" into two APIs.
  */
-bool shouldWriteData(const QuicConnectionStateBase& conn);
+WriteDataReason shouldWriteData(const QuicConnectionStateBase& conn);
 bool hasAckDataToWrite(const QuicConnectionStateBase& conn);
-bool hasNonAckDataToWrite(const QuicConnectionStateBase& conn);
+WriteDataReason hasNonAckDataToWrite(const QuicConnectionStateBase& conn);
 
 /**
  * Invoked when the written stream data was new stream data.

quic/api/test/Mocks.h

@@ -12,6 +12,7 @@
 
 #include <folly/io/async/EventBase.h>
 #include <quic/QuicException.h>
+#include <quic/api/LoopDetectorCallback.h>
 #include <quic/api/QuicSocket.h>
 #include <quic/codec/QuicConnectionId.h>
 #include <quic/common/Timers.h>
@@ -266,6 +267,14 @@ class MockQuicTransport : public QuicServerTransport {
   GMOCK_METHOD1_(, noexcept, , setConnectionIdAlgo, void(ConnectionIdAlgo*));
 };
 
+class MockLoopDetectorCallback : public LoopDetectorCallback {
+ public:
+  ~MockLoopDetectorCallback() override = default;
+  MOCK_METHOD4(
+      onSuspiciousLoops,
+      void(uint64_t, WriteDataReason, NoWriteReason, const std::string&));
+};
+
 inline std::ostream& operator<<(std::ostream& os, const MockQuicTransport&) {
   return os;
 }

quic/api/test/QuicTransportFunctionsTest.cpp

@@ -1107,7 +1107,7 @@ TEST_F(
         writableBytes -= iobuf->computeChainDataLength();
         return iobuf->computeChainDataLength();
       }));
-  EXPECT_TRUE(shouldWriteData(*conn));
+  EXPECT_NE(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
   writeQuicDataToSocket(
       *rawSocket,
       *conn,
@@ -1117,7 +1117,7 @@ TEST_F(
       *headerCipher,
       getVersion(*conn),
       conn->transportSettings.writeConnectionDataPacketsLimit);
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, WriteQuicDataToSocketWithNoBytesForHeader) {
@@ -1542,16 +1542,16 @@ TEST_F(QuicTransportFunctionsTest, ShouldWriteDataTest) {
   CHECK(!conn->oneRttWriteCipher);
   conn->ackStates.appDataAckState.needsToSendAckImmediately = true;
   addAckStatesWithCurrentTimestamps(conn->ackStates.appDataAckState, 1, 20);
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 
   conn->oneRttWriteCipher = test::createNoOpAead();
   EXPECT_CALL(*transportInfoCb_, onCwndBlocked()).Times(0);
-  EXPECT_TRUE(shouldWriteData(*conn));
+  EXPECT_NE(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 
   auto stream1 = conn->streamManager->createNextBidirectionalStream().value();
   auto buf = IOBuf::copyBuffer("0123456789");
   writeDataToQuicStream(*stream1, buf->clone(), false);
-  EXPECT_TRUE(shouldWriteData(*conn));
+  EXPECT_NE(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 
   writeQuicDataToSocket(
       *rawSocket,
@@ -1562,14 +1562,14 @@ TEST_F(QuicTransportFunctionsTest, ShouldWriteDataTest) {
       *headerCipher,
       getVersion(*conn),
       conn->transportSettings.writeConnectionDataPacketsLimit);
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 
   // Congestion control
   EXPECT_CALL(*rawCongestionController, getWritableBytes())
       .WillRepeatedly(Return(0));
   EXPECT_CALL(*transportInfoCb_, onCwndBlocked());
   writeDataToQuicStream(*stream1, buf->clone(), true);
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 
   EXPECT_CALL(*transportInfoCb_, onCwndBlocked());
   writeQuicDataToSocket(
@@ -1581,7 +1581,7 @@ TEST_F(QuicTransportFunctionsTest, ShouldWriteDataTest) {
       *headerCipher,
       getVersion(*conn),
       conn->transportSettings.writeConnectionDataPacketsLimit);
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, ShouldWriteStreamsNoCipher) {
@@ -1595,7 +1595,7 @@ TEST_F(QuicTransportFunctionsTest, ShouldWriteStreamsNoCipher) {
   auto stream1 = conn->streamManager->createNextBidirectionalStream().value();
   auto buf = IOBuf::copyBuffer("0123456789");
   writeDataToQuicStream(*stream1, buf->clone(), false);
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, ShouldWritePureAcksNoCipher) {
@@ -1608,7 +1608,7 @@ TEST_F(QuicTransportFunctionsTest, ShouldWritePureAcksNoCipher) {
 
   conn->ackStates.appDataAckState.needsToSendAckImmediately = true;
   addAckStatesWithCurrentTimestamps(conn->ackStates.appDataAckState, 1, 20);
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, ShouldWriteDataNoConnFlowControl) {
@@ -1621,10 +1621,10 @@ TEST_F(QuicTransportFunctionsTest, ShouldWriteDataNoConnFlowControl) {
   auto stream1 = conn->streamManager->createNextBidirectionalStream().value();
   auto buf = IOBuf::copyBuffer("0123456789");
   writeDataToQuicStream(*stream1, buf->clone(), false);
-  EXPECT_TRUE(shouldWriteData(*conn));
+  EXPECT_NE(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
   // Artificially limit the connection flow control.
   conn->flowControlState.peerAdvertisedMaxOffset = 0;
-  EXPECT_FALSE(shouldWriteData(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, HasAckDataToWriteCipherAndAckStateMatch) {
@@ -1681,23 +1681,23 @@ TEST_F(QuicTransportFunctionsTest, HasCryptoDataToWrite) {
   auto conn = createConn();
   conn->cryptoState->initialStream.lossBuffer.emplace_back(
       folly::IOBuf::copyBuffer("Grab your coat and get your hat"), 0, false);
-  EXPECT_TRUE(hasNonAckDataToWrite(*conn));
+  EXPECT_EQ(WriteDataReason::CRYPTO_STREAM, hasNonAckDataToWrite(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, HasControlFramesToWrite) {
   auto conn = createConn();
   conn->streamManager->queueBlocked(1, 100);
-  EXPECT_FALSE(hasNonAckDataToWrite(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, hasNonAckDataToWrite(*conn));
 
   conn->oneRttWriteCipher = test::createNoOpAead();
-  EXPECT_TRUE(hasNonAckDataToWrite(*conn));
+  EXPECT_EQ(WriteDataReason::BLOCKED, hasNonAckDataToWrite(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, FlowControlBlocked) {
   auto conn = createConn();
   conn->flowControlState.peerAdvertisedMaxOffset = 1000;
   conn->flowControlState.sumCurWriteOffset = 1000;
-  EXPECT_FALSE(hasNonAckDataToWrite(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, hasNonAckDataToWrite(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, HasAppDataToWrite) {
@@ -1705,10 +1705,10 @@ TEST_F(QuicTransportFunctionsTest, HasAppDataToWrite) {
   conn->flowControlState.peerAdvertisedMaxOffset = 1000;
   conn->flowControlState.sumCurWriteOffset = 800;
   conn->streamManager->addWritable(0);
-  EXPECT_FALSE(hasNonAckDataToWrite(*conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, hasNonAckDataToWrite(*conn));
 
   conn->oneRttWriteCipher = test::createNoOpAead();
-  EXPECT_TRUE(hasNonAckDataToWrite(*conn));
+  EXPECT_EQ(WriteDataReason::STREAM, hasNonAckDataToWrite(*conn));
 }
 
 TEST_F(QuicTransportFunctionsTest, UpdateConnectionCloneCounter) {

quic/api/test/QuicTransportTest.cpp

@@ -476,7 +476,7 @@ TEST_F(QuicTransportTest, WriteSmall) {
       conn.transportSettings.writeConnectionDataPacketsLimit);
 
   verifyCorrectness(conn, 0, stream, *buf);
-  EXPECT_FALSE(shouldWriteData(conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
 }
 
 TEST_F(QuicTransportTest, WriteLarge) {
@@ -511,7 +511,7 @@ TEST_F(QuicTransportTest, WriteLarge) {
       conn.transportSettings.writeConnectionDataPacketsLimit);
   EXPECT_EQ(NumFullPackets + 1, conn.outstandingPackets.size());
   verifyCorrectness(conn, 0, stream, *buf);
-  EXPECT_FALSE(shouldWriteData(conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
 }
 
 TEST_F(QuicTransportTest, WriteMultipleTimes) {
@@ -532,7 +532,7 @@ TEST_F(QuicTransportTest, WriteMultipleTimes) {
   transport_->writeChain(stream, buf->clone(), false, false);
   loopForWrites();
   verifyCorrectness(conn, originalWriteOffset, stream, *buf);
-  EXPECT_FALSE(shouldWriteData(conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
 }
 
 TEST_F(QuicTransportTest, WriteMultipleStreams) {
@@ -695,7 +695,7 @@ TEST_F(QuicTransportTest, WriteFin) {
       transport_->getVersion(),
       conn.transportSettings.writeConnectionDataPacketsLimit);
   verifyCorrectness(conn, 0, stream, *buf, true);
-  EXPECT_FALSE(shouldWriteData(conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
 }
 
 TEST_F(QuicTransportTest, WriteOnlyFin) {
@@ -723,7 +723,7 @@ TEST_F(QuicTransportTest, WriteOnlyFin) {
       transport_->getVersion(),
       conn.transportSettings.writeConnectionDataPacketsLimit);
   verifyCorrectness(conn, 0, stream, *buf, true);
-  EXPECT_FALSE(shouldWriteData(conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
 }
 
 TEST_F(QuicTransportTest, WriteDataWithRetransmission) {
@@ -744,7 +744,7 @@ TEST_F(QuicTransportTest, WriteDataWithRetransmission) {
   // lost data and new data
   buf->appendChain(std::move(buf2));
   verifyCorrectness(conn, 0, stream, *buf);
-  EXPECT_FALSE(shouldWriteData(conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
 }
 
 TEST_F(QuicTransportTest, WriteImmediateAcks) {
@@ -780,7 +780,7 @@ TEST_F(QuicTransportTest, WriteImmediateAcks) {
   EXPECT_EQ(conn.ackStates.appDataAckState.largestAckScheduled, end);
   EXPECT_FALSE(conn.ackStates.appDataAckState.needsToSendAckImmediately);
   EXPECT_EQ(0, conn.ackStates.appDataAckState.numNonRxPacketsRecvd);
-  EXPECT_FALSE(shouldWriteData(conn));
+  EXPECT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
 }
 
 TEST_F(QuicTransportTest, NotWriteAcksIfNoData) {
@@ -1397,6 +1397,56 @@ TEST_F(QuicTransportTest, CloneNewConnectionIdFrame) {
   EXPECT_EQ(numNewConnIdPackets, 3);
 }
 
+TEST_F(QuicTransportTest, BusyWriteLoopDetection) {
+  auto& conn = transport_->getConnectionState();
+  conn.transportSettings.writeConnectionDataPacketsLimit = 1;
+  auto mockLoopDetectorCallback = std::make_unique<MockLoopDetectorCallback>();
+  auto rawLoopDetectorCallback = mockLoopDetectorCallback.get();
+  conn.loopDetectorCallback = std::move(mockLoopDetectorCallback);
+  ASSERT_FALSE(conn.debugState.needsWriteLoopDetect);
+  ASSERT_EQ(0, conn.debugState.currentEmptyLoopCount);
+  auto mockCongestionController = std::make_unique<MockCongestionController>();
+  auto rawCongestionController = mockCongestionController.get();
+  conn.congestionController = std::move(mockCongestionController);
+  EXPECT_CALL(*rawCongestionController, getWritableBytes())
+      .WillRepeatedly(Return(1000));
+
+  // There should be no data to send at this point
+  transport_->updateWriteLooper(true);
+  EXPECT_FALSE(conn.debugState.needsWriteLoopDetect);
+  EXPECT_EQ(WriteDataReason::NO_WRITE, conn.debugState.writeDataReason);
+  EXPECT_EQ(0, conn.debugState.currentEmptyLoopCount);
+  loopForWrites();
+
+  auto stream = transport_->createBidirectionalStream().value();
+  auto buf = buildRandomInputData(100);
+  transport_->writeChain(stream, buf->clone(), true, false);
+  transport_->updateWriteLooper(true);
+  EXPECT_TRUE(conn.debugState.needsWriteLoopDetect);
+  EXPECT_EQ(0, conn.debugState.currentEmptyLoopCount);
+  EXPECT_EQ(WriteDataReason::STREAM, conn.debugState.writeDataReason);
+  EXPECT_CALL(*socket_, write(_, _)).WillOnce(Return(1000));
+  loopForWrites();
+  EXPECT_EQ(1, conn.outstandingPackets.size());
+  EXPECT_EQ(0, conn.debugState.currentEmptyLoopCount);
+
+  // Queue a window update for a stream doesn't exist
+  conn.streamManager->queueWindowUpdate(stream + 1);
+  transport_->updateWriteLooper(true);
+  EXPECT_TRUE(
+      WriteDataReason::STREAM_WINDOW_UPDATE == conn.debugState.writeDataReason);
+  EXPECT_CALL(*socket_, write(_, _)).Times(0);
+  EXPECT_CALL(
+      *rawLoopDetectorCallback,
+      onSuspiciousLoops(1, WriteDataReason::STREAM_WINDOW_UPDATE, _, _))
+      .Times(1);
+  loopForWrites();
+  EXPECT_EQ(1, conn.outstandingPackets.size());
+  EXPECT_EQ(1, conn.debugState.currentEmptyLoopCount);
+
+  transport_->close(folly::none);
+}
+
 TEST_F(QuicTransportTest, ResendNewConnectionIdOnLoss) {
   auto& conn = transport_->getConnectionState();
 
@@ -2389,7 +2439,9 @@ TEST_F(QuicTransportTest, CloseWithDrainWillKeepSocketAround) {
 }
 
 TEST_F(QuicTransportTest, PacedWriteNoDataToWrite) {
-  ASSERT_FALSE(shouldWriteData(transport_->getConnectionState()));
+  ASSERT_EQ(
+      WriteDataReason::NO_WRITE,
+      shouldWriteData(transport_->getConnectionState()));
   EXPECT_CALL(*socket_, write(_, _)).Times(0);
   transport_->pacedWrite(true);
 }
@@ -2441,7 +2493,7 @@ TEST_F(QuicTransportTest, AlreadyScheduledPacingNoWrite) {
   // schedule a pacing timeout.
   loopForWrites();
 
-  ASSERT_TRUE(shouldWriteData(conn));
+  ASSERT_NE(WriteDataReason::NO_WRITE, shouldWriteData(conn));
   EXPECT_TRUE(transport_->isPacingScheduled());
   EXPECT_CALL(*socket_, write(_, _)).Times(0);
   transport_->pacedWrite(true);
@@ -2469,7 +2521,7 @@ TEST_F(QuicTransportTest, NoScheduleIfNoNewData) {
   // FunctionLooper won't schedule a pacing timeout.
   transport_->pacedWrite(true);
 
-  ASSERT_FALSE(shouldWriteData(conn));
+  ASSERT_EQ(WriteDataReason::NO_WRITE, shouldWriteData(conn));
   EXPECT_FALSE(transport_->isPacingScheduled());
 }
 

quic/loss/QuicLossFunctions.h

@@ -24,7 +24,7 @@ namespace quic {
 
 // Forward-declaration
 bool hasAckDataToWrite(const QuicConnectionStateBase& conn);
-bool hasNonAckDataToWrite(const QuicConnectionStateBase& conn);
+WriteDataReason hasNonAckDataToWrite(const QuicConnectionStateBase& conn);
 
 std::chrono::microseconds calculatePTO(const QuicConnectionStateBase& conn);
 
@@ -138,7 +138,8 @@ void setLossDetectionAlarm(QuicConnectionStateBase& conn, Timeout& timeout) {
    * in the buffers which is unsent or known to be lost. We should set a timer
    * in this case to be able to send this data on the next PTO.
    */
-  bool hasDataToWrite = hasAckDataToWrite(conn) || hasNonAckDataToWrite(conn);
+  bool hasDataToWrite = hasAckDataToWrite(conn) ||
+      (hasNonAckDataToWrite(conn) != WriteDataReason::NO_WRITE);
   auto totalPacketsOutstanding = conn.outstandingPackets.size();
   if (totalPacketsOutstanding == conn.outstandingPureAckPacketsCount) {
     VLOG(10) << __func__ << " unset alarm pure ack only"

quic/state/StateData.h

@@ -358,6 +358,7 @@ struct LossState {
 
 class Logger;
 class CongestionControllerFactory;
+class LoopDetectorCallback;
 
 struct QuicConnectionStateBase {
   virtual ~QuicConnectionStateBase() = default;
@@ -616,6 +617,20 @@ struct QuicConnectionStateBase {
 
   // Whether or not both ends agree to use partial reliability
   bool partialReliabilityEnabled{false};
+
+  // Debug information. Currently only used to debug busy loop of Transport
+  // WriteLooper.
+  struct DebugState {
+    bool needsWriteLoopDetect{false};
+    uint64_t currentEmptyLoopCount{0};
+    WriteDataReason writeDataReason{WriteDataReason::NO_WRITE};
+    NoWriteReason noWriteReason{NoWriteReason::WRITE_OK};
+    std::string schedulerName;
+  };
+
+  DebugState debugState;
+
+  std::shared_ptr<LoopDetectorCallback> loopDetectorCallback;
 };
 
 std::ostream& operator<<(std::ostream& os, const QuicConnectionStateBase& st);