pw_transfer: Fix offset receive transfer startup

Receive offset transfer handshake sends a start ack conf that has no
offset, triggering the ignore previous data code path on processing.
This results in the parameters never being initiated so that data can be
sent. Exclude start ack conf packets from that error check.

Bugs: b/368620868
Change-Id: Icaa8632bbea861972d50b2365e2bde9322411e79
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/237095
Docs-Not-Needed: Jordan Brauer <jtbrauer@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Jordan Brauer <jtbrauer@google.com>

pw_transfer/context.cc

@@ -883,11 +883,14 @@ void Context::HandleReceiveChunk(const Chunk& chunk) {
 
 void Context::HandleReceivedData(const Chunk& chunk) {
   if (chunk.offset() != offset_) {
-    if (chunk.offset() + chunk.payload().size() <= offset_) {
+    if (chunk.offset() + chunk.payload().size() <= offset_ &&
+        chunk.type() != Chunk::Type::kStartAckConfirmation) {
       // If the chunk's data has already been received, don't go through a full
       // recovery cycle to avoid shrinking the window size and potentially
       // thrashing. The expected data may already be in-flight, so just allow
       // the transmitter to keep going with a CONTINUE parameters chunk.
+      // Start ack confs do not come with an offset set, so it can get stuck
+      // here if we are doing an offset transfer.
       PW_LOG_DEBUG("Transfer %u received duplicate chunk with offset %u",
                    id_for_log(),
                    static_cast<unsigned>(chunk.offset()));