diff -r d3100c987a39 -r f90a7cfe2480 test/test_buffer.cpp
--- a/test/test_buffer.cpp	Mon Jul 25 14:16:49 2022 +0200
+++ b/test/test_buffer.cpp	Mon Jul 25 15:29:56 2022 +0200
@@ -351,6 +351,23 @@
     }
 };
 
+static size_t mock_write_limited_rate(
+        void const *ptr,
+        size_t size,
+        __attribute__((unused)) size_t nitems,
+        CxBuffer *buffer
+) {
+    // simulate limited target drain capacity
+    static bool full = false;
+    if (full) {
+        full = false;
+        return 0;
+    } else {
+        full = true;
+        return cxBufferWrite(ptr, size, nitems > 2 ? 2 : nitems, buffer);
+    }
+}
+
 TEST_F(BufferWrite, SizeOneFit) {
     const char *data = "test";
     ASSERT_EQ(buf.capacity, 8);
@@ -595,6 +612,33 @@
     EXPECT_EQ(memcmp(target.space, "prepfoobarhello", 15), 0);
 }
 
+TEST_F(BufferWrite, FlushRateLimited) {
+    enableFlushing();
+    // limit the rate of the flush function and the capacity of the target
+    target.capacity = 16;
+    target.flags &= ~CX_BUFFER_AUTO_EXTEND;
+    buf.flush_func = (cx_write_func) mock_write_limited_rate;
+    ASSERT_EQ(buf.capacity, 8);
+    ASSERT_EQ(buf.pos, 4);
+    size_t written = cxBufferWrite("foo", 1, 3, &buf);
+    EXPECT_EQ(written, 3);
+    ASSERT_EQ(buf.pos, 7);
+    ASSERT_EQ(buf.size, 7);
+    ASSERT_EQ(target.pos, 0);
+    ASSERT_EQ(target.size, 0);
+    written = cxBufferWrite("hello, world!", 1, 13, &buf);
+    // " world!" fits into this buffer, the remaining stuff is flushed out
+    EXPECT_EQ(written, 13);
+    EXPECT_EQ(buf.pos, 7);
+    EXPECT_EQ(buf.size, 7);
+    EXPECT_EQ(buf.capacity, 8);
+    EXPECT_EQ(memcmp(buf.space, " world!", 7), 0);
+    EXPECT_EQ(target.pos, 13);
+    ASSERT_EQ(target.size, 13);
+    EXPECT_EQ(target.capacity, 16);
+    EXPECT_EQ(memcmp(target.space, "prepfoohello,", 13), 0);
+}
+
 class BufferSeek : public BufferFixture {
 };