From 6e62c9a12b07c05d8c7b4e0997568e8c3aa90260 Mon Sep 17 00:00:00 2001
From: Abtin Keshavarzian <abtink@google.com>
Date: Wed, 17 Mar 2021 10:13:01 -0700
Subject: [PATCH] [indirect-sender] skip over `HandleSentFrameToChild()` for an
 empty frame (#6293)

This commit updates `IndirectSender` to check exit quickly from the
`HandleSentFrameToChild()` callback if the sent frame is an empty
frame. `PrepareFrameForChild()` prepares an empty frame when there is
no indirect message in the send queue for the child. This can happen
in the (not common) situation where the radio platform does not
support the "source address match" feature and always include "frame
pending" flag in acks to data poll frames. The change in this commit
address a potential issue in this situation and ensures that the "send
done" is not incorrectly processed for any newly added indirect
message after preparing the empty frame (which would cause the message
to be assumed sent and removed without actual tx).
---
 src/core/thread/indirect_sender.cpp | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/core/thread/indirect_sender.cpp b/src/core/thread/indirect_sender.cpp
index b5c1d6550..bea98179a 100644
--- a/src/core/thread/indirect_sender.cpp
+++ b/src/core/thread/indirect_sender.cpp
@@ -338,6 +338,7 @@ Error IndirectSender::PrepareFrameForChild(Mac::TxFrame &aFrame, FrameContext &a
     if (message == nullptr)
     {
         PrepareEmptyFrame(aFrame, aChild, /* aAckRequest */ true);
+        aContext.mMessageNextOffset = 0;
         ExitNow();
     }
 
@@ -422,10 +423,28 @@ void IndirectSender::HandleSentFrameToChild(const Mac::TxFrame &aFrame,
 
     VerifyOrExit(mEnabled);
 
+    if (aError == kErrorNone)
+    {
+        Get<Utils::ChildSupervisor>().UpdateOnSend(aChild);
+    }
+
+    // A zero `nextOffset` indicates that the sent frame is an empty
+    // frame generated by `PrepareFrameForChild()` when there was no
+    // indirect message in the send queue for the child. This can happen
+    // in the (not common) case where the radio platform does not
+    // support the "source address match" feature and always includes
+    // "frame pending" flag in acks to data poll frames. In such a case,
+    // `IndirectSender` prepares and sends an empty frame to the child
+    // after it sends a data poll. Here in `HandleSentFrameToChild()` we
+    // exit quickly if we detect the "send done" is for the empty frame
+    // to ensure we do not update any newly added indirect message after
+    // preparing the empty frame.
+
+    VerifyOrExit(nextOffset != 0);
+
     switch (aError)
     {
     case kErrorNone:
-        Get<Utils::ChildSupervisor>().UpdateOnSend(aChild);
         break;
 
     case kErrorNoAck: