From af1caa88fd1f2da58ed6b6fb67f0a64da7e4f347 Mon Sep 17 00:00:00 2001
From: Sergei <sergej.shirokov@gmail.com>
Date: Thu, 22 Jan 2026 11:22:23 +0200
Subject: [PATCH] Fix for issue 1276 "Data corruption in the
 etl::bip_buffer_spsc_atomic" (#1277)

* Reproduce data corruption bug in the `etl::bip_buffer_spsc_atomic`.

* Fix data corruption bug in the `etl::bip_buffer_spsc_atomic`.
---
 include/etl/bip_buffer_spsc_atomic.h |   3 +
 test/test_bip_buffer_spsc_atomic.cpp | 119 +++++++++++++++++++++++++++
 2 files changed, 122 insertions(+)

diff --git a/include/etl/bip_buffer_spsc_atomic.h b/include/etl/bip_buffer_spsc_atomic.h
index 14d03ca2..9a557eae 100644
--- a/include/etl/bip_buffer_spsc_atomic.h
+++ b/include/etl/bip_buffer_spsc_atomic.h
@@ -281,6 +281,9 @@ namespace etl
         else
         {
           ETL_ASSERT_OR_RETURN((windex == 0) && ((wsize + 1) <= read_index), ETL_ERROR(bip_buffer_reserve_invalid));
+
+          // Correct wrapping point
+          last.store(write_index, etl::memory_order_release);
         }
         
         // Always update write index
diff --git a/test/test_bip_buffer_spsc_atomic.cpp b/test/test_bip_buffer_spsc_atomic.cpp
index 9711685d..c5f3aec5 100644
--- a/test/test_bip_buffer_spsc_atomic.cpp
+++ b/test/test_bip_buffer_spsc_atomic.cpp
@@ -28,6 +28,8 @@ SOFTWARE.
 
 #include "unit_test_framework.h"
 
+#include <iostream>
+#include <random>
 #include <thread>
 #include <chrono>
 #include <vector>
@@ -332,6 +334,123 @@ namespace
       CHECK(stream.empty());
     }
 
+    //*************************************************************************
+    TEST(test_optimal_write_issue_1276)
+    {
+      etl::bip_buffer_spsc_atomic<char, 5> stream;
+      etl::ibip_buffer_spsc_atomic<char>& istream = stream;
+
+      // 1. Make all `read`, `write` and `last` in the end of the buffer.
+      {
+        const auto writer = istream.write_reserve_optimal(5);
+        CHECK_EQUAL(5U, writer.size());
+        writer[0] = '0';
+        writer[1] = '1';
+        writer[2] = '2';
+        writer[3] = '3';
+        writer[4] = '4';
+        CHECK_NO_THROW(istream.write_commit(writer)); // [0 1 2 3 4]
+        const auto reader = istream.read_reserve();
+        CHECK_EQUAL(5U, reader.size());
+        CHECK_NO_THROW(istream.read_commit(reader));  //  * * * * *[]
+      }
+      // 2. Write & read 4 bytes.
+      {
+        const auto writer = istream.write_reserve_optimal(4);
+        CHECK_EQUAL(4U, writer.size());
+        writer[0] = '5';
+        writer[1] = '6';
+        writer[2] = '7';
+        writer[3] = '8';
+        CHECK_NO_THROW(istream.write_commit(writer.subspan(0U, 4U))); // 5 6 7 8] *[
+
+        const auto reader = istream.read_reserve();
+        CHECK_EQUAL(4U, reader.size());
+        CHECK_EQUAL('5', reader[0]);
+        CHECK_EQUAL('6', reader[1]);
+        CHECK_EQUAL('7', reader[2]);
+        CHECK_EQUAL('8', reader[3]);
+        CHECK_NO_THROW(istream.read_commit(reader));  // * * * *[]*
+      }
+      // 3. Write and read 2 bytes.
+      {
+        const auto writer = istream.write_reserve_optimal(2);
+        CHECK_EQUAL(3U, writer.size());
+        writer[0] = '9';
+        writer[1] = 'A';
+        CHECK_NO_THROW(istream.write_commit(writer.subspan(0, 2))); // 9 A]* *[*
+
+        const auto reader = istream.read_reserve();
+        CHECK_EQUAL(2U, reader.size());
+        CHECK_EQUAL('9', reader[0]);
+        CHECK_EQUAL('A', reader[1]);
+      }
+    }
+
+    //*************************************************************************
+    TEST(test_write_random_back_pressure)
+    {
+      // Deliberately seeded with fixed number, so that if it fails then always in the same way.
+      std::mt19937 mte(123);
+
+      constexpr size_t N = 256;
+      etl::bip_buffer_spsc_atomic<int, N> stream;
+      etl::ibip_buffer_spsc_atomic<int>& istream = stream;
+
+      auto makeRandomNumber = [&mte](const size_t n) -> size_t { return mte() % n; };
+
+      auto verifyIota = [&](const etl::span<int>& seq)
+      {
+        if (!seq.empty())
+        {
+          auto prev = seq[0];
+          for (auto i = 1U; i < seq.size(); ++i)
+          {
+            const auto curr = seq[i];
+            if (prev > curr)
+            {
+              CHECK(prev <= curr);
+              std::cerr << "prev(" << prev << ") should not be bigger than curr(" << curr << ")!\n";
+            }
+            prev = curr;
+          }
+        }
+      };
+
+      const auto all = istream.write_reserve(N);
+      CHECK_EQUAL(N, all.size());
+      std::fill_n(all.begin(), all.size(), 0);
+
+      int iota = 0;
+      // Loop writing a bit more than reading.
+      // 10K iterations is enough to detect 2 failures.
+      for (int i = 0; i < 10000; ++i)
+      {
+        // Write [0...N/16] chunks - on average N/32.
+        {
+          const size_t toWrite = makeRandomNumber(N / 16 + 1);
+          const auto reserve = istream.write_reserve(toWrite);
+          if (reserve.size() >= toWrite)
+          {
+            ++iota;
+            const auto written = makeRandomNumber(toWrite + 1);
+            std::fill_n(reserve.begin(), written, iota);
+            istream.write_commit(reserve.first(written));
+            verifyIota(reserve.first(written));
+          }
+        }
+
+        // Read a bit less [0...N/17] chunks - on average N/34.
+        if (const size_t toRead = makeRandomNumber(N / 17 + 1))
+        {
+          const auto reserve = istream.read_reserve(toRead);
+          verifyIota(reserve);
+          const auto read = makeRandomNumber(reserve.size() + 1);
+          istream.read_commit(reserve.first(read));
+        }
+      }
+    }
+
     //*************************************************************************
 #if REALTIME_TEST && defined(ETL_COMPILER_MICROSOFT)
     #if defined(ETL_TARGET_OS_WINDOWS) // Only Windows priority is currently supported