Keep incrementing sequence number when link prober is suspended and shutdown #55 (#65)

### Description of PR
Unable to cherry-pick the commit below cleanly from master branch: 
13f4879 Jing Zhang      Sun Apr 3 21:56:33 2022 -0700   Keep incrementing sequence number when link prober is suspended and shutdown  (#55)

Summary:
Fixes # (issue)
This PR is fix the issue when both sides' link prober is suspended, LinkManager still appears to be healthy. 
When HB suspends, TX sequence number is not updated either, hence link prober won't post an `unknown` event. 

sign-off: Jing Zhang [email protected]
### Type of change
- [x] Bug fix

### Approach
#### What is the motivation for this PR?
Make sure link prober is reporting `unknown` event when it's suspended or shutdown on both sides. 

#### How did you do it?
Increment sequence number even when HB is not sent. 

#### How did you verify/test it?
Tested on dualToR testbed.

Author: zjswhhh

src/link_prober/LinkProber.cpp

@@ -271,9 +271,10 @@ void LinkProber::sendHeartbeat()
 {
     MUXLOGTRACE(mMuxPortConfig.getPortName());
 
+    updateIcmpSequenceNo();
+    
     // check if suspend timer is running
     if ((!mSuspendTx) && (!mShutdownTx)) {
-        updateIcmpSequenceNo();
         boost::system::error_code errorCode;
         mStream.write_some(boost::asio::buffer(mTxBuffer.data(), mTxPacketSize), errorCode);
 

test/LinkProberTest.cpp

@@ -160,6 +160,17 @@ TEST_F(LinkProberTest, UpdateSequenceNo)
 
     EXPECT_TRUE(getRxSelfSeqNo() + 1 == ntohs(icmpHeader->un.echo.sequence));
     EXPECT_TRUE(getRxPeerSeqNo() + 1 == ntohs(icmpHeader->un.echo.sequence));
+
+    // sequence number should still be updated when heartbeat is suspended
+    EXPECT_EQ(getRxSelfSeqNo(), 0);
+    EXPECT_EQ(getRxPeerSeqNo(), 0);
+
+    handleSuspendTxProbes();
+    EXPECT_TRUE(getSuspendTx());
+    
+    handleSendHeartbeat();
+    EXPECT_EQ(getRxSelfSeqNo(), 1);
+    EXPECT_EQ(getRxPeerSeqNo(), 1);
 }
 
 TEST_F(LinkProberTest, GenerateGuid)

test/LinkProberTest.h

@@ -42,6 +42,8 @@ class LinkProberTest: public ::testing::Test
     void initializeSendBuffer() {mLinkProber.initializeSendBuffer();};
     void handleUpdateEthernetFrame() {mLinkProber.handleUpdateEthernetFrame();};
     void handleUpdateSequenceNumber() {mLinkProber.updateIcmpSequenceNo();};
+    void handleSuspendTxProbes() {mLinkProber.suspendTxProbes(300);};
+    void handleSendHeartbeat() {mLinkProber.sendHeartbeat();};
     void resetTxBufferTlv() {mLinkProber.resetTxBufferTlv();};
     size_t getTxPacketSize() {return mLinkProber.mTxPacketSize;};
     size_t appendTlvCommand(link_prober::Command commandType);
@@ -54,6 +56,7 @@ class LinkProberTest: public ::testing::Test
 
     uint16_t getRxSelfSeqNo() {return mLinkProber.mRxSelfSeqNo;};
     uint16_t getRxPeerSeqNo() {return mLinkProber.mRxPeerSeqNo;};
+    bool getSuspendTx() {return mLinkProber.mSuspendTx;};
 
     boost::asio::io_service mIoService;
     common::MuxConfig mMuxConfig;