Fix a bug where valid data was invalidated in cases where the write would end exactly at the end of the buffer

Author: DNedic

lfbb/src/lfbb.c

@@ -115,11 +115,8 @@ void LFBB_WriteRelease(LFBB_Inst_Type *inst, const size_t written) {
         w = 0U;
     }
 
-    /* Increment the write index and wrap to 0 if needed */
+    /* Increment the write index */
     w += written;
-    if (w == inst->size) {
-        w = 0U;
-    }
 
     /* If we wrote over invalidated parts of the buffer move the invalidate
      * index
@@ -128,6 +125,11 @@ void LFBB_WriteRelease(LFBB_Inst_Type *inst, const size_t written) {
         i = w;
     }
 
+    /* Wrap the write index if we reached the end of the buffer */
+    if (w == inst->size) {
+        w = 0U;
+    }
+
     /* Store the indexes with adequate memory ordering */
     atomic_store_explicit(&inst->i, i, memory_order_release);
     atomic_store_explicit(&inst->w, w, memory_order_release);

tests/tests.cpp

@@ -194,3 +194,29 @@ TEST_CASE("Interleaved write and read without enough space",
     write_location = LFBB_WriteAcquire(&lfbb, sizeof(test_data2));
     REQUIRE(write_location == nullptr);
 }
+
+TEST_CASE("Write ends exactly at the end of the buffer",
+          "[write_ends_at_buffer_end]") {
+    const size_t write_size = 8;
+    uint8_t buf[write_size * 2];
+
+    LFBB_Inst_Type lfbb;
+    LFBB_Init(&lfbb, buf, sizeof(buf));
+
+    /* 1. Write the first half */
+    LFBB_WriteAcquire(&lfbb, write_size);
+    LFBB_WriteRelease(&lfbb, write_size);
+
+    /* 2. Read the first half */
+    size_t read_available;
+    LFBB_ReadAcquire(&lfbb, &read_available);
+    LFBB_ReadRelease(&lfbb, write_size);
+
+    /* 3. Write the second half */
+    uint8_t *second_half_write = LFBB_WriteAcquire(&lfbb, write_size);
+    LFBB_WriteRelease(&lfbb, write_size);
+
+    /* 4. Read the second half */
+    uint8_t *second_half_read = LFBB_ReadAcquire(&lfbb, &read_available);
+    REQUIRE(second_half_read == second_half_write);
+}